Document Millcreek_doc_80d495faed

PREPARED FOR: NEFFS CREEK DEBRIS BASIN FEASIBILITY STUDY SEPTEMBER 2022 PREPARED BY: ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN FEASIBILITY STUDY Prepared by: Prepared for: September 2022 9/27/2022 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY i Table of Contents Contents: Page No. Executive Summary 1 Introduction 1 Hydrologic and Hydraulic Analyses 1 Debris Basin Alternatives 1 Supplemental 1 Alternatives Analysis 2 Coordination 3 Funding and Study Limitations 4 1 Introduction 8 1.1 Purpose of Project 8 1.2 Site Location 8 1.3 Background and History of Flooding 8 1.3.1 Previous Studies and Existing Data 11 1.4 Types of Flood Mitigation Alternatives to Be Evaluated 12 2 Hydrology Analysis 13 2.1 Introduction 13 2.1.1 General Hydrologic Design Requirements (FEMA vs Dam Safety) 13 2.2 Drainage Basin Characteristics 14 2.2.1 Subbasin Boundaries 14 2.3 Alternative Design Storm Criteria and Results 14 2.3.1 Precipitation Depth and Distribution 14 2.3.2 PMP and SEP for Dams 18 2.3.3 Design Storm Distribution 18 2.3.4 Development of Hydrologic Parameters 18 2.3.5 Design Storm Model Calibration 22 2.3.6 AMCIII 26 2.3.7 Snowmelt 26 2.4 Hydrologic Results and Recommendations 26 2.4.1 Results 26 2.4.2 Hydraulic Analysis 27 2.4.3 Conclusion and 29 3 Preliminary Geotechnical Analysis 30 3.1 Purpose of Analyses 30 3.2 Geologic Setting 30 3.3 Surficial Conditions/Topography 30 3.4 Potential Geologic/Geotechnical Hazards 30 3.4.1 Surface Fault Rupture / Tectonic Deformation Hazard 31 3.4.2 Ground Shaking 32 3.4.3 Liquefaction Hazard 32 3.4.4 Debris Flow / Flooding Hazard 32 3.4.5 Landslide 32 3.4.6 Rockfall Hazard 32 3.4.7 Problem Soil Hazard 33 3.5 Geologic Hazard Summary 33 3.6 Other Considerations 33 3.6.1 Reuse of Site materials 33 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY ii 3.6.2 Foundation Permeability 34 3.7 Recommendations for Additional Study 34 3.8 Limitations and Assumptions 34 4 Preliminary Debris and Sediment Volume Assessment 35 4.1 Purpose of Analyses 35 4.2 General Site Characteristics 35 4.3 Past Debris Flow Events 36 4.4 Field Investigation 36 4.5 Empirical Debris Flow Volume Estimates 36 4.6 Limitations and Assumptions For Debris Flow Analysis 37 5 Alternatives Analysis 38 5.1 Purpose of Alternatives Analyses 38 5.2 Basis of Design for Alternatives 38 5.3 Flood Mitigation Alternatives 39 5.3.1 Alternative 1: Dam/Debris Basin and New Storm Drain 40 5.3.2 Alternative 2: Dam/Debris Basin and Neffs Creek Channel Improvements... 41 5.3.3 Alternative 3: Below-Grade Debris Basin and New Storm Drain 43 5.3.4 Alternative 4: Below-Grade Debris Basin and Neffs Creek Channel Improvements 44 5.3.5 Alternative 5: Dam/Debris Basin or Below-Grade Debris Basin On USFS Wilderness Area LAND 45 5.3.6 Alternative 6: Do Nothing 45 5.4 Recommended Alternative 45 5.4.1 Floodplain Mapping – Recommended Alternative 46 5.5 Limitations and Assumptions 46 6 Engineer’s Opinion of Probable Construction Cost 53 7 Agency Coordination 54 7.1 Salt Lake County 54 7.2 FEMA/Utah Department of Emergency Management 54 7.3 U.S. Forest 55 7.4 Utah State Dam Safety 55 7.5 Others 55 8 Public Involvement 56 9 Potential Funding Sources 57 10 Limitations of Study 58 11 References 59 Appendix A – Previous Studies 60 Appendix B – Hydrologic Analysis TM 61 Appendix C – Geologic/Geotechnical Analysis TM 62 Appendix D – Debris and Sediment Volume AssessMent 63 Appendix E – Conceptual Cost Estimates 64 Appendix F – Conceptual Renderings 65 Appendix G – Conceptual Design 66 Appendix H – Public Meeting 67 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY iii TABLE OF CONTENTS (continued) Figures: Page No. Figure E-1. Project Figure E-2. Recommended Alternative: Below Grade Debris Basin and Figure E-3. Rendering of Recommended Debris Basin Figure 1-1: Project Location 10 Figure 2-1: Watershed Boundary 15 Figure 2-2: Design Storm Area 16 Figure 2-3. Hydrologic Soil Group Map 20 Figure 2-4. Hydrologic Model Existing Land Cover Map 21 Figure 2-5. Graphical Peak Discharge Comparison (Modified from 2007 BC&A Study) 25 Figure 2-6. Runoff hydrographs for Neffs Creek. 28 Figure 3-1. Geologic Hazards Map 31 Figure 4-1. Main Contributing Tributaries in Neffs Canyon 35 Figure 5-1. Example of Stepped Inflow Channel 41 Figure 5-2. Alternative 1 47 Figure 5-3. Alternative 2 48 Figure 5-4. Alternative 3 49 Figure 5-5. Alternative 4 49 Figure 5-6. Recommended Alternative 51 Figure 5-7. Rendering of Recommended Debris Basin Alternative 52 Tables: Page No. Table E-1. Summary of Recommended Flood Mitigation Improvements 3 Table 1-1. Study Data Sources 12 Table 2-1. Summary of Critical Storm Precipitation Inputs 13 Table 2-2. Standard Storm Depths 14 Table 2-3. Final Standard Storm Depths 17 Table 2-4. PMP and SEP Storm Precipitation Depths 18 Table 2-5. Curve Numbers for Hydrologic Soil-Cover Complexes 19 Table 2-6. Summary of Initial Drainage Basin Hydrologic Parameters 19 Table 2-7. Nearby Stream Gage Statistical Analysis Peak Flows (CRS 2020) 22 Table 2-8. Regression Analysis Peak Runoff Rates 23 Table 2-9. Flood Frequency Analysis and Regression Analysis Peak Runoff Rates for the 100-year Runoff Event (Taken from CRS 2020 Study) 24 Table 2-10. Summary of Final Calibrated Sub-basin Hydrologic Parameters 26 Table 2-11. Estimated Snow Melt Flow Rates 26 Table 2-12. Existing Conditions Model Peak Discharge Summary 27 Table 4-1. Summary of Debris Flow Volume Estimations 37 Table 5-1. Summary of Proposed Alternative 1 40 Table 5-2. Summary of Proposed Alternative 2 42 Table 5-3. Summary of Proposed Alternative 3 43 Table 5-4. Summary of Proposed Alternative 4 44 Table 6-1. Preliminary Conceptual Cost Estimate – Neffs Creek Alternative 3 Improvements 53 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 1 EXECUTIVE SUMMARY INTRODUCTION The purpose of the Neffs Creek Debris Basin Feasibility Study is to evaluate the feasibility of constructing a debris basin or other protective measure at the mouth of Neffs Canyon with the goal of eliminating the alluvial fan flood hazard that was recently developed by FEMA in this area. This study has been completed in accordance with FEMA, Utah Dam Safety Guidelines, and Utah State Rule R655-11 requirements. The funding for this feasibility study was provided through a FEMA Pre- Disaster Mitigation (PDM) grant. The principal objective of this study is to identify recommended flood control improvements and other actions that can mitigate the alluvial fan hazards on the Neffs Canyon alluvial fan. The results and recommendations of this study will be used to obtain funding for the future design and construction of the recommended alternative. An area map showing the project location is shown in Figure E-1. As Figure E-1 indicates, any feasible debris basin will need to be constructed on U.S. Forest Service land, between Federal Wilderness area and developed private property. HYDROLOGIC AND HYDRAULIC ANALYSES A detailed hydrologic analysis was performed as part of this study (as detailed in Section 2.0) to define the hydrologic design parameters for potential alternative improvements that include construction of a debris basin at the apex of the alluvial fan. This was an important part of the study because it was determined that the magnitude of the one-percent-annual-chance flood (commonly known as a 100-year flood) that was used to develop the alluvial fan flood hazards on the FEMA Flood Insurance Rate Map and adopted in November 2021 is almost three times higher than is justified. After coordinating with FEMA, Salt Lake County, Utah Division of Emergency Management, and Millcreek officials, it was determined that a request to FEMA to revise the magnitude of the 100-year flood numbers is justified. It is proposed to reduce the magnitude of the 100-year flood from 300 cubic feet per second (cfs) to 107 cfs. The magnitude of this discharge impacts the estimated volume of debris yield during a 100-year flood as well as the size of conveyance facilities needed of any recommended debris basin located near the apex of the alluvial fan. Hydraulic analyses were also performed to determine if the use of the reduced value of the 100-year flood could have a significant impact on the flood hazards shown on the new FEMA Flood Insurance Rate map for the Neffs Creek alluvial fan. It was determined that flow depths and velocities would be lower, but that the alluvial fan flood hazard boundaries and associated regulatory requirements would remain the same for properties in the newly mapped floodplain. DEBRIS BASIN ALTERNATIVES Two types of debris basins were considered in this study: an above-ground dam that would create a debris basin, and a below-grade storage facility. In either case, it was assumed that runoff from the 100-year flood would flow through the basin without storing any water. It should be noted that the regulatory design requirements for a debris dam at this location will be much more onerous than a below-grade debris basin. There is only one area where the construction of a debris basin was considered. It is on U.S. Forest Service land between Federal Wilderness Area and private developed land, as shown in Figure E-1. SUPPLEMENTAL STUDIES A Preliminary Geologic/Geotechnical Analysis was performed as part of this study to identify possible geologic and geotechnical hazards and issues at the proposed debris basin site and to provide information on what additional geologic and geotechnical work may be needed during the ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 2 final design stage of the project. No fatal flaws or issues that would prevent the construction of a debris basin in the preferred location were identified. The results of this analysis are summarized in Section 3 of the final report. A Preliminary Debris and Sediment Volume Assessment was performed to estimate the volume of sediment and debris that would be expected to mobilize during a 100-year flood event with normal watershed conditions. It was estimated that a 100-year flood could produce up to 8 acre-feet of debris and sediment. However, the debris volume estimate would increase substantially if a large portion of the Neffs Creek drainage basin were to be burned by a wildfire. The details of this analysis are summarized Section 4.0 of the final report. Ultimately, it was recommended that a debris basin be constructed to hold as much debris as is practicable based on the site constraints. ALTERNATIVES ANALYSIS Multiple alternatives to eliminate the mapped alluvial fan flood hazards were evaluated (as detailed in Section 5.0) using the results of the hydrologic, geologic and geotechnical, and debris volume analyses. The flood mitigation alternatives included constructing either a dam or below-grade debris basin to collect debris and to direct runoff from Neffs Canyon into a pipe or open channel that would ultimately convey the runoff to a point where it would discharge into the existing Neffs Creek storm drain pipe near Wasatch Blvd. Each alternative that was evaluated met the following design goals and project constraints: 1. Safely convey runoff from the 100-year flood through the debris basin and the project area 2. Avoid construction impacts in the Federal Wilderness Area 3. Do not impact existing homes and structures on private property 4. Meet U.S. Forest Service requirements for construction improvements on U.S. Forest Service land (Permitting, etc.) 5. Coordinate the construction of the Neffs Creek debris basin with planned improvements to the Neffs Canyon trailhead parking lot expansion project and desired associated wildfire fighting facilities that include a public restroom, helipad and heliwell 6. Maintain the existing active Neffs Creek channel through the neighborhood and limit peak discharges into that channel to about 15 cfs to limit flood potential 7. Address geologic/geotechnical issues during final design, including: • Wasatch Fault • Landslide Potential • Regulatory Issues (Dam Safety). 8. Obtain FEMA conditional approval that the revision to the magnitude of the 100-year flood is acceptable for the basis of design and that the recommended improvements, if constructed, will eliminate the alluvial fan flood hazard that is shown on the new flood insurance rate maps recently adopted by FEMA. The elements of the recommended alternative (Alternative are shown schematically in Figure E- 2 and summarized in Table E-1. This alternative, the most feasible and least-cost alternative, has a conceptual cost estimate of about twenty million dollars. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 3 Table E-1. Summary of Recommended Flood Mitigation Improvements No. Description of Improvement I Construct about a 9 acre-ft below-grade debris basin between the Federal Wilderness area boundary and the residential properties along Zarahemla Drive. II Construct a diversion structure on the existing active Neffs Creek channel to divert runoff in excess of 15 cfs into the new debris basin. III Construct an armored channel to convey runoff from the diversion from the existing Neffs Creek channel to the debris basin and to the debris basin outlet structure. IV Construct an armored channel to convey runoff from the historic Neffs Creek channel into the debris basin and to the debris basin outlet structure. V Construct an outlet/primary overflow structure on the debris basin that discharges into a pipe that will convey up to 228 cfs (the 500-year flood) to the intersection of Zarahemla Drive and Mathews Way. VI Construct an emergency spillway on the debris basin that discharges into the historic channel that currently terminates at Zarahemla Drive. VII Construct a new storm drain that will convey at least 107 cfs (the 100-year flood) from the intersection of Zarahemla Drive and Mathews Way through public streets in several Millcreek neighborhoods before discharging into the Neffs Creek storm drain near Wasatch Blvd. VIII Use material from the detention basin excavation to backfill the old reservoir east of the Neffs Canyon trailhead parking lot as part of the parking lot expansion project. IX Revegetate all areas disturbed during construction. A conceptual rendering of the recommended below-grade debris basin is included in Figure E-3. Implementing the recommended project improvements may require acquisition of some easements for the new pipeline that will convey runoff from the debris basin to Zarahemla Drive. COORDINATION Due to the location and complexity of designing, permitting, and constructing the recommended project improvements, multiple coordination meetings with different agencies were conducted during this study. A summary of agency coordination that has been and will be performed during final design and construction phases is summarized below. • U.S. Forest Service coordination and approval will be paramount on this project, as the recommended debris basin is located on public lands that they manage. Millcreek and design team members have coordinated with U.S. Forest Service personnel. The project seems feasible from their perspective. U.S. Forest Service staff stated that the proper NEPA process would need to be followed, some agreements and permits would need to be obtained, and coordination regarding the construction of the Neffs Canyon Trailhead improvements that include a public restroom, a helipad, and a heliwell will need to be coordinated. Construction and funding of the desired trailhead and wildfire fighting facilities will be a separate project. BRIC Grant funds are not being requested for these trailhead project improvements. • Salt Lake County coordination will also be required as Neffs Creek is a County flood control facility. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 4 • A submittal to FEMA will be required before final design to approve that the proposed design parameters of the recommended facilities are acceptable to accomplish the goal of eliminating the alluvial fan flood hazards in the area. • A post construction submittal to FEMA will be required to request that the alluvial fan flood hazards shown on the existing flood insurance rate map be eliminated. • A public meeting was held on December 15, 2021 to present the findings and recommendations of this study to determine if residents in the area are supportive of the recommended flood mitigation improvements. All that participated in the meeting expressed general support of the project and its objectives. Additional public involvement will also be required during final design and construction. FUNDING AND STUDY LIMITATIONS The final report includes a brief summary of potential funding sources that may be able to help fund the design and construction work associated with the recommended improvements. These include FEMA, the State of Utah, NRCS, and others. City staff are currently evaluating alternatives. Limitations of this feasibility study are discussed in Section 10 of the report. The biggest limitation is that the debris basin does not have capacity to store debris from a burned watershed condition. Also, it should be noted that this is a feasibility study that is fairly general in nature. Small revisions can be expected to some of the recommended design features as the project advances through the permitting and final design stages. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 5 Figure E-1: Project Location E-1 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 6 Figure 0-1 Figure 2: Recommended Alternative-Below-Grade Debris Basin and Pipe to Neffs Creek Storm Drain E-2 ---PAGE BREAK--- BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 7 Figure E-3: Rendering of Recommended Debris Basin Alternative ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 8 1 INTRODUCTION 1.1 PURPOSE OF PROJECT The purpose of the Neffs Creek Debris Basin Feasibility Study is to evaluate the feasibility of constructing a debris basin or other protective measure at the mouth of Neffs Canyon with the goal of eliminating the alluvial fan flood hazard that was recently developed by FEMA in this area. This study has been completed in accordance with FEMA, Utah Dam Safety Guidelines, and Utah State Rule R655-11 requirements. The funding for this feasibility study was provided through a FEMA Pre- Disaster Mitigation (PDM) grant. The principal objective of this study is to identify recommended flood control improvements and other actions that can mitigate the alluvial fan hazards on the Neffs Canyon alluvial fan. The report documents the results of hydrologic, geologic/geotechnical, and debris flow analyses associated with a site on U.S. Forest Service (USFS) Land, located at the apex of the alluvial fan, east of existing development, and west of the Federal Wilderness Area boundary. The results and recommendations of this study will be used to obtain funding for the future design and construction of the recommended alternative. 1.2 SITE LOCATION Millcreek, Utah is located on the east side of Salt Lake County with Neffs Canyon being located on the west face of the Wasatch Mountains. The proposed project site is located on USFS land between Federal Wilderness Area property and the residential properties in the surrounding Olympus Cove neighborhood. The project location is shown in Figure 1-1. 1.3 BACKGROUND AND HISTORY OF FLOODING The portion of Millcreek located near the mouth of Neffs Canyon has experienced flooding from Neffs Creek in the past due to runoff from intense rainfall or snowmelt events. In early May 2019, a spring runoff event resulted in shallow flooding on a large area below the mouth of Neffs Canyon. The flooding was caused by debris that accumulated on some logs that were placed across the main Neffs Creek channel by hikers. The debris created a flow restriction that diverted water out of the active main creek channel into an inactive historic channel. More than 50 years ago, the existing active main channel was constructed on the hillside, south of the natural historic channel located at the topographic low point of the drainage near the mouth of Neffs Canyon. From the diversion point the bottom of the newer channel is higher in elevation than the historic channel. The historic channel that is currently generally inactive traverses behind some homes and terminates at Zarahemla Drive where there are no storm drains or flood control facilities to collect and manage runoff from the natural watershed. The newer active Neffs Creek channel runs through yards and under roads in the Mount Olympus neighborhood and discharges into a storm drain that runs under I-15 and continues west through Holladay before discharging into Big Cottonwood Creek. During the May 2019 event, shallow flooding was experienced in yards and streets. The flooding was caused more by the water diverted out of the creek by the debris than by the peak discharge in the creek. However, during other recent significant runoff, water has also overtopped the north bank of the existing active main channel in this area and flowed into the lower historic channel that terminated at Zarahemla Drive. The area where water has left the current main channel is located on Federal Wilderness Area land that is managed by the U.S. Forest Service, thus it is difficult to access and maintain because no motorized equipment is allowed in a Wilderness Area. The area of the mouth of the canyon is mostly built out or developed and many of the roads no not have traditional curb and gutter or storm drain facilities. Many homes also have driveways that slope from the street to the house. Due to the lack of curb and gutter and the down- ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 9 sloping driveways in the area, the roadways do not have adequate storm water runoff conveyance capacity. Those homes and structures with down-sloping driveways and those without curb and gutter are more susceptible to shallow flooding. The active alluvial fan below the mouth of Neffs Canyon was recently studied by the Federal Emergency Management Agency (FEMA) to better define the alluvial fan flood hazards in the area. The updated Flood Insurance Rate Map (FIRM) Panels (49035C0316H and 49035C0317H) went into effect in November 2021. In the new FIRM maps, Neffs Canyon is categorized using three different flood designations for active alluvial fans: Zone A (higher risk), Zone AO (moderate risk), and Shaded Zone X (lower risk). The Zone A areas, predominantly located highest (uphill) on the fan, could experience the highest water levels (depths) and water velocities during a flood. The Zone AO areas are defined by flooding with lower water depths and velocities. The Shaded Zone X areas have the lowest risk for alluvial fan hazards. For properties in Zone A and Zone AO, homeowners will be required to purchase flood insurance if those structures are financed by federally backed mortgages. Although properties in the Shaded Zone X areas will not be subject to federal flood insurance requirements, flood insurance is strongly recommended for structures in these areas, as alluvial fan flooding is so unpredictable. In addition to the flood insurance requirements, several restrictive building requirements no walk out basements, remodels must be less than 50% of fair market value of the structure or the entire structure must be updated to meet current FEMA flood zone requirements, and new construction must meet FEMA building restrictions) result from being in an active alluvial fan. The new FIRM maps in association with National Flood Insurance Program requirements and current building codes have an impact that results in the United Fire Authority (UFA) Station 112 not being able to replace their fire station on its current site since it is currently located in an alluvial fan Zone AO FEMA designated Special Flood Hazard Area (SFHA). The only feasible way to remove the alluvial fan flood hazards from the FEMA FIRM is to design and construct a structural flood mitigation facility at the apex of the fan which will collect runoff and debris and remove the flood flow path uncertainties associated with alluvial fans. This can be done by conveying runoff from the canyon and flood mitigation facility across the alluvial fan in an enclosed storm drain pipeline. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 10 Figure 1-1: Project Location 1-1 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 11 1.3.1 PREVIOUS STUDIES AND EXISTING DATA 1.3.1.1 2007 Salt Lake County Study In 2007, Salt Lake County commissioned the “Neffs Canyon Creek Master Plan” by Hansen Allen and Luce (HAL). The purpose of that study was to develop conceptual mitigation options to help mitigate the alluvial fan flooding hazards in the area. Geotechnical, hydrological and hydraulic analyses were performed. The study concluded that the area sits on an alluvial fan and recommended that a debris basin be constructed at the mouth of Neffs Canyon to mitigate the alluvial fan flood hazards. A 100- year flood discharge estimate of 300 cfs was developed for a point at the mouth of the Neffs Canyon. The amount of potential debris, debris basin sizing, and channel improvements were also estimated. The recommendation for that conceptual design study was to design and construct a debris basin that would convey the 100-year flood through the basin with no detention and to construct facilities to convey the 100-year flood through the developed area. This study will be referred to as the 2007 County Study and is included in Appendix A. 1.3.1.2 2016 FEMA Study In 2016, the firm JE Fuller Hydrology and Geomorphology performed work associated with the Neffs Creek Flood Hazard Assessment for FEMA and developed alluvial fan floodplain mapping using a 2D model. They were directed to utilize the 100-year discharge developed in the 2007 County Study as the basis of the floodplain analysis. New FEMA Flood Insurance Rate Maps (FIRM) developed from that work became effective November of 2021. The new FIRM defines a larger alluvial fan floodplain in the area than existed previously, impacting a large number of existing structures in Millcreek. This study is included Appendix A. 1.3.1.3 2020 CRS Study In March of 2020, the firm CRS Engineers performed an independent engineering review of the 2016 FEMA study. The purpose of the review was to determine if the study followed the FEMA Guidance for Flood Risk Analysis and Mapping Alluvial Fans. The conclusions from the independent review were that the 2016 study followed all FEMA guidance standards except for the hydrology which if it did, would lower the discharge being used for the study area from 300 cfs to 107 cfs. This study is included in Appendix A. 1.3.1.4 Existing Data The hydrologic and alternatives analyses performed as part of this study utilized the data presented in Table 1-1. Additional data along with information from the geotechnical, geologic, and debris flow analyses were performed to supplement the existing data and provide more details than were provided as part of the previous studies. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 12 Table 1-1. Study Data Sources Data Source Description LiDAR Utah Automated Geographic Reference Center, (AGRC) 2013 1-meter resolution bare-earth digital terrain model (DTM) data set of Wasatch Front. Aerial Imagery Hexagon 30cm, via Utah AGRC, 2021 Aerial imagery was used for the background of the figures and drawings and to determine existing land uses for hydrologic models Soil Data NRCS Web Soil Survey Soil Survey Geographic Database (SSURGO) mapping data used to determine Hydrologic soil type for hydrologic models Hydrologic Analysis Utah Dam Safety, FEMA Guidance Hydrologic analysis of spillway, and as-builts of inlet structures and spillways. Used to evaluate the capacity of the channel and structures. Land Cover Type NLCD 2016 The National Land Cover Database (NLCD) provides nationwide data on land cover and land cover change at a 30m resolution with a 16-class legend based on a modified Anderson Level II classification system. Elevation Datum NAVD 88 For this report the elevations are based on NAVD 88. Previous Studies and Reports Millcreek Reports used to gather relevant information of the Neffs Canyon Drainage area: 2007 Salt Lake County Study, 2016 FEMA Study, and the 2020 CRS Study. 1.4 TYPES OF FLOOD MITIGATION ALTERNATIVES TO BE EVALUATED Multiple alternatives (see Section 5) were evaluated that could potentially mitigate the recurring flooding and existing debris flow hazards associated with the Neffs Creek alluvial fan. These alternatives would allow for the Neffs Creek alluvial fan flood hazards on the new FEMA FIRM to be removed. The alternatives considered include the construction of some type of dry detention/debris basins on U.S. Forest Service land to capture debris from a large runoff event. Both a below-grade debris basin and a dam debris basin were considered as alternatives as part of this analysis. In addition to the construction of a debris basin, multiple alternatives were also evaluated regarding how to convey stormwater safely through the developed neighborhood. One alternative included conveying runoff via a new storm drain pipeline under existing public streets before discharging back into Neffs Creek Channel near Wasatch Boulevard. Another included conveying the runoff through a storm drain pipeline part of the way and then utilizing an improved open channel to convey runoff to the existing Neffs Canyon storm drain pipe near Wasatch Boulevard. The 2007 County Study examined at a dam and associated open channel improvements and indicated that the existing Neffs Creek channel would need extensive improvements, such as culvert upsizing and channel enlargement to successfully convey the 100-year storm event safely. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 13 2 HYDROLOGY ANALYSIS 2.1 INTRODUCTION In accordance with Utah Dam Safety Guidelines (Rule R655-11 of the Utah Administrative Code) and FEMA guidelines, BC&A developed detailed hydrologic computer models of the Neffs Canyon drainage area for use in evaluating alternatives with various debris basin sizes, spillway discharges, and other mitigation options. The hydrologic analyses included developing several hyetographs for different storm durations, routing computed runoff hydrographs through potential detention or flood retarding facilities, and computing the Inflow Design Flood (IDF) for the project site. The models were developed using the Hydrologic Engineering Center Hydrologic Modeling System (HEC- HMS) computer software. This was an important part of the study because it determined that the magnitude of the one-percent- annual-chance flood (commonly known as a 100-year flood) that was used to develop the alluvial fan flood hazards on the FEMA Flood Insurance Rate Map and adopted in November 2021 is almost three times higher than is justified. The development of the various parameters and elements of the model are discussed in detail in the sections that follow. 2.1.1 GENERAL HYDROLOGIC DESIGN REQUIREMENTS (FEMA VS DAM SAFETY) If a debris basin is constructed at the mouth of Neffs Canyon, it could include a dam embankment to impound water and debris. Due to its proximity to homes and development, such a dam would likely be classified as a High Hazard Dam by the Utah State Engineer and would have to be designed to meet minimum State dam safety criteria for high hazard facilities. Per Utah Dam Safety and Rule R655-11 (requirements for the design, construction, and abandonment of dams), two theoretical probable maximum precipitation (PMP) events must be evaluated: a 6-hour local storm and a 72-hour general storm (see Section 2.4 for more information). A high hazard dam must be designed not to fail for the larger of those two events. Additionally, the rainfall events with return periods from 10- to 500-years based on 6-hour design storm criteria were also simulated to evaluate associated flood hazards on the alluvial fan. A 6-hour storm event was chosen to serve as the basis of developing a hydrograph for a 100-year flood, similar to what has been done in other streams in the area that have been studied by FEMA and Salt Lake County. It should be noted that the effective FEMA study 100-year flood discharges for nearby creeks (Mill Creek, Big Cottonwood Creek, and Little Cottonwood Creek) are based on 3-hour storm events. In total, eleven storm events (see Table 2-1) with various durations and rainfall temporal distributions were evaluated to meet various FEMA and Utah Dam Safety requirement associated with the design and construction of dams and flood control facilities. Table 2-1. Summary of Critical Storm Precipitation Inputs 6-Hour Storms 24-Hour Storms 1 Local PMP (AMC II) 9 100-year (AMC III)) 2 Local SEP (AMC II) 72-Hour Storms 3 10-year (AMCII) 10 General PMP (AMC II) 4 25-year (AMCII) 11 General SEP (AMC II) 5 50-year (AMCII) 6 100-year (AMCII) 7 100-year (AMC III) 8 500-year (AMCII) ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 14 2.2 DRAINAGE BASIN CHARACTERISTICS The Neffs Canyon drainage area at the canyon mouth contains about 2,300 acres (3.7 square miles), with an average basin elevation of 7,800 ft and an average basin slope of 60.5 percent based on information obtained using the USGS StreamStats online website. The ground cover in the Neffs Canyon watershed is composed of Juniper, Oak-Aspen, and herbaceous vegetation. The drainage area is also almost entirely comprised of public land managed by U.S. Forest Service (see Figure 1-1) and has two different flow paths. The current active Neffs Creek channel appears to be man-made, and was constructed on the hillside. The original historic Neffs Creek channel is located at the topographic low point of the canyon which conveys runoff from any flow breakouts from the current active creek channel to Zarahemla Drive where the historic channel, which is fairly small and ill-defined, currently terminates. 2.2.1 SUBBASIN BOUNDARIES Utilizing the data sources listed in Table 1-1, along with observations obtained from field investigations, the drainage boundaries of the study area were determined. The Neffs Canyon drainage basin (or watershed) boundary is shown on Figure 2-1. 2.3 ALTERNATIVE DESIGN STORM CRITERIA AND RESULTS 2.3.1 PRECIPITATION DEPTH AND DISTRIBUTION Design storm depths, except for the PMP, were obtained from the NOAA Precipitation Frequency Data Server (PFDS) which reports total storm depths for a specific geographic location based on NOAA Atlas 14 for various storm durations and recurrence intervals. For the purposes of this project, a large storm concurrently raining over the entire drainage basin was used for hydrologic modeling purposes. The extent of the considered design storm was approximated by the elliptical area shown on Figure 2-2, which has an area of 7.8 square miles. Storm depths were obtained from the NOAA precipitation grid data with the mean value for the watershed. Mean storm depth estimates for recurrence intervals from 10- to 500-years for a 6-hour storm event and 100- to 500-years for the 24-hour storm event are presented in Table 2-2. Table 2-2. Standard Storm Depths Description Recurrence Interval (year) 6 hour 24 hour 10 25 50 100 500 100 NOAA Atlas 14 Mean Depth (in) 1.68 2.01 5.29 2.60 3.70 4.07 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 15 Figure 2-1: Watershed Boundary 2-1 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 16 Figure 2-2: Design Storm Area 2-2 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 17 2.3.1.1 Areal Reduction The NOAA Atlas 2 (1973) recommends a storm-centered areal reduction (ARF) of 0 to 15 percent for 6-hour storm cells ranging from 0 to 100 square miles in area. These factors, however, are based on data from thunderstorms in the Midwest rather than those typical to the Salt Lake Valley. The results of a more locally pertinent depth-area precipitation analysis were taken from the Salt Lake City Hydrology Manual (1993). This method was used in this study to be consistent with the 2007 County Study and other recent FEMA studies in the county. The manual recommends the following precipitation depth-area relationship for a thunderstorm, with area in square miles: 6-hour Reduction Factor = 0.01*(100 – 3.5*Area^0.46) 1-day Reduction Factor = 0.01*(100 – 2.0*Area^0.46) This relationship is based on data from Project Cloudburst, a study completed by the U.S. Army Corps of Engineers in April 1979. That study involved collection of data from a network of rain gages in Salt Lake City and vicinity covering an area of roughly 350 square miles. The storm cell over the watershed is approximately 7.8 square miles which would have a ARF of 0.91 for the 6-hour design storm and 0.95 for the 24-hour design storm. The ARF rainfall depths are provided in Table 2-3 2.3.1.2 Seasonal Reduction Rainfall Depths in NOAA Atlas 14 include all precipitation events, including snow which does not typically generate immediate runoff. To better estimate a rain precipitation event, an additional seasonal adjustment was made to the areal adjusted precipitation depths. NOAA Atlas 2 (which was superseded by Atlas 14) includes separate point precipitation estimates for annual data and for a subset of seasonal data occurring between the months of May and October. For each storm recurrence interval, the ratio of the NOAA Atlas 2 seasonal depth to the annual depth was computed and used to convert the Atlas 14 derived, areal adjusted depths to a final estimated rainfall depth. The seasonal adjustment ratio and final adjusted rainfall depths are provided in Table 2-3. Calculations to determine the seasonal adjustment factor are provided in Appendix B. Table 2-3. Final Standard Storm Depths Description Recurrence Interval (year) 6 hour 24 hour 10 25 50 100 500 100 NOAA Atlas 14 Mean Depth (in) 1.68 2.01 5.29 2.60 3.70 4.07 Areal Adjusted Depth (in) 1.53 1.83 2.08 2.37 3.37 3.86 Seasonal Adjustment 89 90 91 92 94 94 Final Adjusted Design Depth (in) 1.36 1.65 1.90 2.18 3.16 3.63 Eq 2.2 Eq 2.1 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 18 2.3.2 PMP AND SEP FOR DAMS The Probable Maximum Precipitation (PMP) for the local 6-hour and general 72-hour storm depths along with their distributions are normally obtained by following the procedures from the Hydrometeorological Report 49 (HMR49) which covers the Colorado River and Great Basin Drainages and includes Utah and surrounding areas. The State of Utah has published two additional reports that modify the HMR49 values. Those two studies are titled “Probable Maximum Precipitation Estimates for Short-Duration, Small-Area Storms in Utah” (USUS) and “2002 Update for Probable Maximum Precipitation, Utah, 72-Hour Estimates, Area to 5,000 mi²” (USUL). The precipitation values developed from HMR49 and supplemented by USUS or USUL, are referred to as local and general Spillway Evaluation Precipitation (SEP) values and include an areal reduction. The developed PMP and SEP values for the 6-hour and 72-hour general and local storms are shown in Table 2-4. Once the critical SEP has been determined, it will be compared to the 100-year, 6-hour (for local storms) or the 100-year, 24-hour (for general storm). Table 2-4. PMP and SEP Storm Precipitation Depths Storm Duration Precipitation Depth (inches) PMP1 SEP2 6-hour3 8.6 8.0 72-hour 16.26 16.21 Notes: 1. The precipitation depths were generated using HMR49 only. 2. The precipitation depths were generated using HMR49 along with USUL (72HOUR) and USUS (6 HOUR). 2.3.3 DESIGN STORM DISTRIBUTION The selected design storm distribution for the standard 6-hour design storms were based on the Quartile 2, 50 Percentile Convective Storm Pattern found in the NOAA Atlas 14 Semi-Arid Region documents. The 6-hour local storm and 72-hour general storm were developed using the methods identified in HMR49 and State of Utah standards. These unit rainfall distributions were used with the storm depths adjusted for area and seasonality as described above and provided in Appendix B. 2.3.4 DEVELOPMENT OF HYDROLOGIC PARAMETERS 2.3.4.1 Curve Number Runoff Curve Numbers (CN) were estimated for the Neffs Canyon drainage basin based on soil type and land use/vegetative cover. Hydrologic soil group (HSG) maps were obtained from the NRCS Soil Survey Geographic (SSURGO) dataset and are shown on Figure 2-3. Land use or vegetative cover was determined by inspection of aerial imagery, conducting field observations, and using the USGS National Land Cover Dataset (NLCD) mapping data. Land use descriptions from the NLCD data were related to similar land cover descriptions in TR-55. A map of land uses and vegetative cover present within the study area are shown on Figure 2-4. Using GIS software, the composite CN representing the drainage basin was calculated on a weighted area basis. The CNs used for hydrologic soil-cover complexes were based on information from TR-55 and are summarized in Table 2-5 The initial calculated composite curve number for the sub-basin (before calibration) in the study area is provided in Table 2-6 with the calculations shown in Appendix B. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 19 Table 2-5. Curve Numbers for Hydrologic Soil-Cover Complexes Land Use/Vegetative Cover Hydrologic Soil Group A B C D Deciduous Forest, Oak-Aspen (Fair) 48 48 57 63 Evergreen Forest, Juniper (Fair). 58 58 73 80 Grassland/Herbaceous, Sagebrush (Fair) 51 51 63 70 Mixed Forest, Juniper (Poor) 75 75 85 89 Shrub/Scrub, Oak-Aspen (Poor) 66 66 74 79 2.3.4.2 Estimate of Initial Abstraction Initial abstraction is the fraction of the storm depth after which runoff begins and is governed by the following equation: 𝐼𝑎= 0.2𝑆 Where, 𝑰𝒂 = Initial abstraction (in.) 𝑺 = potential maximum soil moisture retention (in.) This relationship is based on empirical relationships between infiltration, surface depression storage, interception, and evapotranspiration. The potential maximum soil moisture retention, S, is related to CN (NRCS 2004) and can be estimated using the following equation: 𝑆= 1000 𝐶𝑁−10 The preceding equations were used to compute the starting (before calibration) initial abstraction value shown in Table 2-6. 2.3.4.3 Transform Method As was done in the 2007 County Study, the lag time was calculated using the regression equation from the M.J Simas and R.H. Hawskins “Lag Time Characteristics for Small Watersheds in the U.S.” study. This method requires the basin area, slope, and curve number characteristics: The flow length was estimated using GIS computer tools and the elevations were taken from the digital terrain model. The basin characteristics and resulting lag time for the single subbasin are provided in Table 2-6. Table 2-6. Summary of Initial Drainage Basin Hydrologic Parameters Drainage Area Watershed Length Max Elev (ft) Min Elev (ft) Composite Curve Number Lag Time (min) Initial Abstraction (inches) Sq. ft Sq. mi. 102,310,000 3.7 17,500 9675 5634 66 110 1.0 Eq 2.4 Eq 2.3 Eq 2.5 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 20 Figure 2-3. Hydrologic Soil Group Map 2-3 2-3 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 21 Figure 2-4. Hydrologic Model Existing Land Cover Map 2-4 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 22 2.3.5 DESIGN STORM MODEL CALIBRATION 2.3.5.1 Flood Frequency Analysis A flood frequency analysis is the preferred method for estimating the magnitude and return frequency of peak flows from gaged watersheds. However, no stream flow gage is present in Neffs Canyon. There are some stream gages in other nearby watersheds that can provide insight on reasonable ranges for peak 100-year flood discharges. In 2020, CRS Engineers performed a study to analyze the discharge in Neffs Creek and nearby gages were selected to run a flood frequency statistical analysis following the procedures in Bulletin 17C. Although the areas for the peak discharges of the three gages shown in Table 2-7 are larger than Neffs Creek (3.67 square miles), they provide insight on the range of reasonable values that can be expected for Neffs Creek. Additionally in 2007, BC&A completed a study for Alpine City that referenced published flow frequency values that were obtained using the methods outlined in the older Bulletin 17B document for the multiple stream gages throughout the Wasatch Front, including the three by CRS. The results from the 2007 BC&A study are graphically shown Figure 2-5 below and were updated to include the CRS study results. 2.3.5.2 Regression Analysis The USGS has developed regional regression equations for estimating peak discharge rates in un- gaged watersheds throughout Utah. The regression equations are based on peak flow frequency analyses conducted on gaged watersheds throughout the state. The USGS published equations in 1994 under the National Flood Frequency program (NFF) and again in 2008 under the National Streamflow Statistics program (NSS). Both the NFF (Region 4) and NSS (Region 2) equations were applied to the study drainage basin for the 100-year event and NSS (Region 2) for the 500-year event. A drainage area of 3.67 square miles and an elevation of 7,800 was used in the equations. A summary of the regression equation estimates for the flood events are provided in and graphically shown on Figure 2-5. The NSS equations were also applied to the same creeks with stream gages for comparison purposes and are shown in Table 2-9. Table 2-7. Nearby Stream Gage Statistical Analysis Peak Flows (CRS 2020) Stream Name Location Watershed Area (mi2) Years of Record 100-year Peak Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) Mill Creek Canyon Mouth 21.7 1899-Curent (107 Peaks) 150 134 171 Emigration Creek Canyon Mouth 18.4 1902- Current (92 Peaks) 160 135 202 Red Butte Creek Red Butte Reservoir, Fort Douglas 7.25 1964-2019 (56 Peaks) 114 90 157 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 23 Table 2-8. Regression Analysis Peak Runoff Rates Event Method Drainage Area Mean Basin Elev. 100-year Peak Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) 10-year NSS 3.67 7,800 55 29 84 25-year NSS 70 36 106 50-year NSS 91 46 137 100-year NSS 107 54 161 500-year NSS 156 81 237 100-year NFF 154 82 236 2.3.5.3 Calibration to Previous Studies Another option for calibrating model results or performing a reasonableness check in absence of stream gage data is to use results from previously published hydrologic studies. The most recent hydrology study completed on Neffs Creek is the 2007 County study as discussed previously. That study utilized a 24-hour 100-year storm event to estimate a 100-year peak discharge of 300 cfs at the mouth of the canyon. The engineer who performed the analyses associated with the 2007 hydrology study stated that the purpose of that study was to develop a conceptual design for a debris basin. The 100-year design storm developed as part of that study is considered to be conservative (for dam safety purposes) and no calibration work or comparisons to nearby watersheds was performed to determine if the number was reasonable for a 100-year flood. After reviewing the hydrologic methodology used in the 2007 County study and completing an analysis of nearby stream gages and regression discharges, BC&A came to the same conclusion as the 2020 CRS study which is: that following FEMA guidance on hydrologic studies used to estimate the peak magnitude of the 100-year flood, the estimated 100- year flood of 300 cfs developed in the 2007 County study is outside one standard deviation of the mean regression peak discharge and may be considered unreasonable as shown in Table 2-9. 2.3.5.4 Calibration Results Due to the lack of historic stream gage records on Neffs Creek, the 100-year flood discharge computed using the USGS regression equations was used to help calibrate the hydrologic model. The hydrologic parameters of CN, Initial Abstraction, and Lag Time in the computer model were adjusted until the computed peak discharge was within at least one standard deviation of the regression equation discharge estimate. The CN parameter is intended to simulate the runoff potential for a given area and is somewhat subjective. During the calibration process it was determined that each event from the 10-year to the 500-year would need to be calibrated separately to obtain the desired results. For the 100-year event and smaller events, the initial composite CN value shown in Table 2-6 was reduced by 3% to 64 to calibrate the model to the regression equation results. For the 500-year flood event, the CN value was reduced by 10% to 60 to calibrate the model to the regression equation results. The Initial Abstraction was initially calculated to be 1.1-inches using the standard equation for each event. The 1.1-inch value was determined to be an appropriate value for the 100-year event. Storm return periods less than the 100-year used a lower initial abstraction value, while the 500- ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 24 year event used a higher value. The final calibrated hydrologic parameters are provided in Table 2-10 along with the rainfall runoff discharges and a comparison to the USGS regression equations. The final calibrated hydrologic parameters resulted in rainfall runoff discharges for the 100-year (and smaller) events were very close to the discharge estimates obtained using the USGS regression equations. The 500-year event discharge is at the upper end of the one error standard deviation which is within FEMA’s requirements. Further calibration to fit the results of the regression equation more closely would require the use of unreasonable CN or initial abstraction values. BC&A feels that the higher 500-year discharge would also be conservative for the design of the debris basin or other alternatives. For this study, the PMP and State-defined Spillway Evaluation Precipitation (SEP) hydrologic scenarios for high hazard dams used the 500-year calibrated CN parameters as they represented the best calibration for larger events. Table 2-9. Flood Frequency Analysis and Regression Analysis Peak Runoff Rates for the 100-year Runoff Event (Taken from CRS 2020 Study) ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN ALTERNATIVES ANALYSIS BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 25 Figure 2-5. Graphical Peak Discharge Comparison (Modified from 2007 BC&A Study) ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 26 Table 2-10. Summary of Final Calibrated Sub-basin Hydrologic Parameters Event Composite Curve Number Initial Abstraction (inches) Rainfall- Runoff Discharge (cfs) NSS Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) 10-year 64 0.7 45 55 29 84 25-year 64 0.8 71 70 36 106 50-year 64 0.9 96 91 46 137 100-year 64 1.1 107 107 54 161 500-year 60 1.4 228 156 81 237 Note: The PMP and SEP dam scenarios will use the 500-year calibrated parameters 2.3.6 AMCIII As part of the Utah Dam Safety requirements for dams, a saturated soil condition (AMCIII) is required for the selected 100-year scenario (6-hour or 24-hour) to help size the spillway. To account for changes due to soil moisture, the NRCS provides a table for converting AMC II Curve Numbers to AMC III Curve Numbers. Using this conversion table, the composite 100-year AMC III CN for the Neffs Canyon drainage basin is 81. As no other events require the AMCIII, the conversions are not included. 2.3.7 SNOWMELT Historical snowmelt peak flows are not available for Neffs Canyon. The 2007 County study used regression equations developed by Gingery and Associates (see 2007 study) and provided the snowmelt flow rates shown in Table 2-11. It should be noted that Mill Creek and other large creeks in the area generally have annual peak discharges that are almost entirely based on spring runoff from snowmelt events. Table 2-11. Estimated Snow Melt Flow Rates Location Predicted Snowmelt Flow Rates (cfs) 10-year 50-year 100-year Mouth of Canyon 50 70 75 2.4 HYDROLOGIC RESULTS AND RECOMMENDATIONS 2.4.1 RESULTS An HEC-HMS computer model was developed for the 3.67 square mile Neffs Canyon Watershed based on the hydrologic parameters and inputs discussed above. Table 2-12 provides a summary of the peak discharges at the Neffs Canyon mouth and Figure 2-6 shows the computed runoff hydrographs for the various design storms. The results show that the magnitude of the 6-hour, 100-year flood computed by the calibrated model is 107 cfs (significantly less than the 300 cfs used in the FEMA ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 27 study to develop the alluvial fan flood hazards on the new FIRM). The results also show that if a debris basin dam is constructed at the mouth of Neffs Canyon, the emergency spillway may have to safely convey over 2,500 cfs over the dam from the SEF local storm to meet Dam Safety requirements. A meeting was held to review the results of the hydrologic analysis from this study with representatives from FEMA, Salt Lake County Flood Control/Engineering, Utah Division of Emergency Management, and Millcreek officials. In that meeting it was determined that a request to FEMA to reduce the magnitude of the 100-year discharge to about 107 cfs is justified. 2.4.2 HYDRAULIC ANALYSIS Hydraulic analyses were performed to determine if the use of the reduced value of the 100-year flood (107 cfs) could have a significant impact on the flood hazards shown on the new FEMA FIRM for the Neffs Creek alluvial fan. It was determined that flow depths and velocities would be lower, but that generally the alluvial fan flood hazard boundaries and associated regulatory requirements would remain the same for properties in the newly mapped floodplain and the only way to remove the alluvial floodplain would be with some type of mitigation option. Table 2-12. Existing Conditions Model Peak Discharge Summary Storm Event Peak Discharge(cfs) 06hour_10year_NOAA_AMCII 45 06hour_25year_NOAA_AMCII 71 06hour_50year_NOAA_AMCII 96 06hour_100year_NOAA_AMCII 107 06hour_100year_NOAA_AMCIII 445 06hour_500year_NOAA_AMCII 228 24hour_100year_NOAA_AMCII 207 24hour_100year_NOAA_AMCIII 453 06hour_PMP_Local_Storm 2,898 06hour_SEF_Local_Storm 2,542 72hour_PMP_General_Storm 1,460 72hour_SEF_General_Storm 1,460 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 28 Figure 2-6. Runoff hydrographs for Neffs Creek. 0 500 1000 1500 2000 2500 3000 3500 0 4 8 12 16 Discharge (cfs) Time (hr) 06hr_10yr_NOAA_AMCII 06hr_25yr_NOAA_AMCII 06hr_50yr_NOAA_AMCII 06hr_100yr_NOAA_AMCII 06hr_100yr_NOAA_AMCIII 06hr_500yr_NOAA_AMCII 06hr_PMP_Local_Storm 06hr_SEF_Local_Storm 0 200 400 [PHONE REDACTED] 1200 1400 1600 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 Discharge (cfs) Time (hr) 24hr_100yr_NOAA_AMCII 24hr_100yr_NOAA_AMCIII 72hr_PMP_General_Storm 72hr_SEF_General_Storm ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 29 2.4.3 CONCLUSION AND RECOMMENDATIONS Based on the results from the updated detailed hydrologic analysis discussed in the above section, the following conclusions and recommendations can be made: 1. The 100-year flood estimate of 300 cfs for Neffs Canyon is overly conservative. For the purposes of this study and for design of flood control facilities, a 100-year peak discharge of 107 cfs should be used. 2. Neither the existing active Neffs Creek channel nor the historic Neffs Creek channel have capacity to convey 107 cfs through the developed area on the alluvial fan. Breakout flows from the existing active channel are likely and still possible in channel segments on U.S. Forest Service land (upstream of the develop area) that would result in flood water being conveyed in the historic Neffs Creek channel, which would result in shallow flooding on the developed portion of the alluvial fan. 3. The sediment/debris volume estimate developed in the 2007 County study should be updated in this phase of the project. That updated estimate should be used in evaluating the size of any debris/detention basin. 4. A hydraulic analysis with a revised discharge of 107 cfs would lower depths and velocities, but not significantly change the FEMA floodplain and a mitigation measure would be required to remove the alluvial fan floodplain. 5. Based on field observations and capacity estimates of the existing active Neffs Creek channel through the developed neighborhood, it appears that it would also be prudent to construct an inlet and pipeline that will safely convey the 100-year flood of 107 cfs through the developed area. 6. A request for a hydrology-based Conditional Letter of Map Revision (CLOMR) should be submitted to FEMA to determine if FEMA will accept the lower discharge of 107 cfs as the design flow for facilities that would remove the alluvial fan flood hazard from the new FEMA FIRM. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 30 3 PRELIMINARY GEOTECHNICAL ANALYSIS 3.1 PURPOSE OF ANALYSES A preliminary geotechnical analysis was conducted by Gerhart Cole to identify possible geologic/geotechnical hazards, constraints for design, and to identify issues that may need additional evaluation if a debris basin is to be designed at the apex of the alluvial fan. This section summarized the results of their analysis. Their full report is included text in the Technical Memorandum (TM) provided in Appendix C. 3.2 GEOLOGIC SETTING The site of the Neffs Creek alluvial fan is located within the Basin and Range Physiographic Province, on the western slope of the Wasatch Mountain Range in the Salt Lake Valley. A sizable portion of the Basin and Range Province, including the proposed project area, is part of a system of watersheds topographically restricted from draining into the ocean. Instead, drainage and groundwater accumulate within terminal lakes and playas in the valley bottoms such as Utah and Great Salt Lake. Neff’s Creek and other adjacent drainages have flowed into the valley and deposited coarse materials (sands, gravels, cobbles, and boulders) in an alluvial fan at the mouth of the canyon. Surface morphology in the area of the Neffs Creek alluvial fan has been formed in part by these processes but has also been altered by surface faulting associated with the Wasatch Fault and slope movement processes. Some of the morphology of the area has also been altered by human disturbance associated with the construction of a small, abandoned reservoir, water storage and conveyance structures, and maintenance roadways. 3.3 SURFICIAL CONDITIONS/TOPOGRAPHY The apex if the alluvial fan is located in a flatter area at the mouth of Neff’s Canyon, above the parking area for the Neffs Canyon Trailhead. The ground slopes toward the west at about a 10 to 15 percent slope. A natural stream channel and diversion channel incise and create steep channel banks in the surrounding ground surface. The canyon walls rise steeply above the project location at slopes approaching 1H:1V in places. Materials exposed at the surface and in stream cuts in the project area consist mainly of coarse materials with a silty sand matrix. Abundant cobbles and boulders are exposed within the stream channel, including boulders up to 10 feet in diameter. 3.4 POTENTIAL GEOLOGIC/GEOTECHNICAL HAZARDS Based on the site data collected and evaluated during the geotechnical analysis, the following potential geological/geotechnical hazards were identified at the proposed basin and discussed below: • Surface Fault Rupture/Tectonic Deformation • Ground Shaking • Liquefaction • Debris Flow • Flooding hazards • Landslide • Rockfall • Problem soils, including collapsible and expansive soils ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 31 Figure 3-1. Geologic Hazards Map 3.4.1 SURFACE FAULT RUPTURE / TECTONIC DEFORMATION HAZARD The site of a potential debris basin is located within 600 feet of a mapped trace of the Salt Lake City Segment of the Wasatch Fault Zone (WFZ) and within a surface-fault-rupture hazard special study area identified by the Utah Geological Survey. The WFZ is considered active and capable of magnitude 7.1 events in the vicinity of the property (WGUEP, 2016). Mapped traces of the fault zone based on recent mapping by the Utah Geological Survey (UGS; see Hiscock and McKean, 2018) are provided in the full TM found in Appendix C and are shown in Figure 3-1. Despite the recent mapping performed by the UGS, the location of the WFZ in this area remains poorly constrained as a result of active alluvial fan deposition and human disturbance along the fault in this area. The proposed project location is also within a Utah Geologic Survey (UGS) surface-fault-rupture hazard special study area and construction of a debris basin with an above-ground dam embankment would be subject to Utah Dam Safety Rules R655-10 and R655-11, which precludes dam embankment construction on active faults. Based on the location of this special study area, the uncertainty in the location of the WFZ in this area, and Utah Dam Safety Rules, it is recommended that a site-specific surface-fault-rupture hazard study, such as trenching, be performed to evaluate the presence or absence of active faults at the proposed embankment location (if a dam is desired to be constructed). If a below-ground debris basin is constructed, the dam safety regulations may not apply. Further details on the tectonic hazards present at the proposed basin site and the potential subsurface study methods are provided in the full TM found in Appendix C. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 32 3.4.2 GROUND SHAKING The potential debris basin’s proximity to the Wasatch Fault, a major feature of the Intermountain Seismic Belt (ISB), suggests that a significant ground-shaking hazard exists at the site. A site-specific determination of seismic design parameters will be required to assess the performance of a dam embankment for design (if the dam alternative is selected). 3.4.3 LIQUEFACTION HAZARD Liquefaction is an earthquake-related phenomenon in which soils lose their shear strength when subjected to seismic shear waves. Previous mapping suggests a low liquefaction triggering susceptibility at the site, likely due to the nature of alluvial-fan deposits and potential for deep groundwater. However, this mapping was performed at a very large scale, and further analysis would be required to assess material properties at the site in more detail. 3.4.4 DEBRIS FLOW / FLOODING HAZARD The purpose of the potential debris basin is to mitigate potential debris flow and flooding hazards to the developed land below Neffs Canyon. It is anticipated that, depending on the expected sediment loads and basin capacity, routine maintenance of the debris basin will need to be completed to remove sediment that will accumulate as a result of debris flow events to maintain design storage and freeboard. Furthermore, the diverted channel, which currently conveys a majority of the Neffs Creek discharge, begins at a diversion point that has a small embankment between the natural channel and an excavated diversion channel. In its current configuration, either the diversion or the natural channel could be blocked in a debris flow event, which could change how runoff interacts with the current hydraulic conveyance infrastructure 3.4.5 LANDSLIDE HAZARD Landslide hazards consist of shallow and deep-seated movements of earth that manifest as either slumping (rotational sliding) or translational sliding. Based on the review of available data, several landslides have been mapped in the area, as shown in Figure 3-1, and at least one may have the potential to affect stability of a potential dam embankment in the area. The mapped landslide is just north of the potential basin footprints and is shown in the full TM attached in Appendix C. Given the potential for landslide hazards affecting the potential debris basin site, it is recommended that further site-specific studies and slope stability analyses be completed to characterize the subsurface geometry, type of landslide, and assess potential for reactivation. Excavation of the basin, likely to occur near the toe of the landslide, should be explicitly considered in the future assessment. 3.4.6 ROCKFALL HAZARD A rockfall hazard exists where one or more portions of an elevated rock mass detach and fall under the force of gravity. Based on the preliminary geotechnical analysis, rockfall potential/susceptibility has not been mapped to any significant degree near the proposed debris basin site. The proposed debris basin footprint(s) are located where most surficial geologic deposits are mapped as unconsolidated deposits, with bedrock exposed in the canyon upstream. Rockfall from these unconsolidated units would likely consist of single cobbles or boulders, which would be unlikely to cause damage to proposed embankments. Based on this information, it is not anticipated that any bedrock units are close enough to the proposed footprints to cause large rock mass failures that could affect the proposed embankment and generally anticipate the rockfall hazard to be low at the proposed basin locations. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 33 3.4.7 PROBLEM SOIL HAZARD Expansive and collapsible soils are subject to volumetric changes (expansion and collapse) when free water and/or new loads are introduced to an otherwise stable soil mass. While both are present in Utah, collapsible soils are expected to be a more likely concern at the project site than expansive soils due to the depositional environment. Collapsible soils tend to form in quickly placed alluvial fan or debris flow deposits. Geologic mapping at the property shows units generally associated with collapse, where proposed improvements may involve adding loads to the soil (such as a dam embankment). Further subsurface exploration and laboratory testing should be performed to assess collapse potential. 3.4.7.1 Internal Erosion and Piping Soils subject to collapse can also be subject to piping or internal erosion caused by seepage of groundwater through an erodible soil matrix. This geotechnical analysis study did not explicitly identify potential for this condition, but alluvial soils can have this potential. 3.5 GEOLOGIC HAZARD SUMMARY The hazards mentioned above would all likely require some form of subsurface studies to better understand how they could affect design of any potential debris basin at the apex of the alluvial fan. However, the most significant risks to the project appear to be related to fault rupture and the mapped landslide north of the proposed basins. Given these hazards and the challenges that exist with performing a fault hazard study at this particular site, the following options should be considered: 1. Relocating the debris basin further up the canyon, outside of the fault special study zone. (Which is not currently feasible because the site is on Federal Wilderness land). 2. Construction of a debris basin entirely below grade, precluding the need for a dam embankment, provided that a “natural dam” is not created. If this option is considered, we recommend producing conceptual drawings and meeting with Utah Dam Safety to discuss permitting issues. 3. Designing the dam embankment and demonstrating that the dam can withstand the anticipated fault offset if a dam alternative is pursued. 3.6 OTHER CONSIDERATIONS 3.6.1 REUSE OF SITE MATERIALS Sampling site soils was not performed as part of the geologic/geotechnical study, but given the depositional environment and design purpose of the proposed basin, it is expected that much of the onsite material can be made to be suitable for reuse as embankment fill with the following considerations: • The material is expected to be relatively coarse-grained and therefore more permeable than typical for embankment fill and may require special detailing to prevent excessive seepage or creation of piping conditions. • Shallower slopes may be required for stability if this material is used. • Very coarse material is likely, and this material can be used as riprap or would need to be crushed or otherwise processed prior to reuse. If advantageous to the design, placing surplus excavated material into the existing reservoir area as a way to waste it or to enlarge the parking area is considered feasible. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 34 3.6.2 FOUNDATION PERMEABILITY As discussed above, the site soils are anticipated to be relatively permeable. Under seepage is expected to be higher than typical for an embankment dam and would need to be considered in design if a dam alternative is pursued. 3.7 RECOMMENDATIONS FOR ADDITIONAL STUDY The following two lists of additional geologic/geotechnical analyses are recommended (one for each alternative) if the decision is made to proceed with the design and construction of a debris basin at the mouth of Neffs Canyon. These lists are not intended to be exhaustive but to provide a list of issues that may need to be addressed before final design of the recommended alternative begins. The following additional analyses should be considered before proceeding with the design and construction of the embankment dam alternative: 1. Surface fault rupture study, including trenching. 2. Site-specific seismic hazard analysis. 3. Liquefaction assessment. 4. Subsurface study and stability modeling related to the previously-mapped landslide. 5. Assessment of collapse potential of foundation soils. 6. Assessment of seepage/piping potential through embankment foundation and abutment soils. 7. Borrow study to identify materials suitable to build an embankment. 8. Embankment seepage and stability analyses. The following additional analyses should be considered before proceeding with the design and construction of the below-grade inline debris basin alternative (assuming the inline basin is not considered a dam by the Utah State Engineer’s Office: 1. Subsurface study and stability modeling related to the mapped landslide. 2. Assessment of seepage/piping potential through adjacent soils. 3. Evaluation of excavated soils for reuse elsewhere. 4. Stability assessment of basin side slopes. 5. Stability assessment of slopes depending on proximity to the basin. 6. A surface fault rupture study may still be required for the inline option. 3.8 LIMITATIONS AND ASSUMPTIONS The assessments and recommendations of the preliminary geotechnical analysis are based on information drawn from multiple sources, including the Utah Geological Survey and the United States Geological Survey as well as individual authors, researchers, consultants, and their firms. The location of hazards may be inferred using various means; however, confirmation of their presence will require additional, site-specific studies trenching is needed to positively or negatively determine the presence of faulting). Even then, the frequency of occurrence can remain imprecise due to both aleatory variability (the natural randomness of a process) and epistemic uncertainty (due to limited data and knowledge associated with infrequent events as measured by the length of human experience). It is important that a local geotechnical engineering engineer be consulted as additional actions are taken relative to the hazards and risks discussed in this report so that conclusions and recommendations contained in this document can be revised as additional information becomes available and/or as the scope of project work evolves. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 35 4 PRELIMINARY DEBRIS AND SEDIMENT VOLUME ASSESSMENT 4.1 PURPOSE OF ANALYSES A preliminary debris and sediment volume analysis was conducted by JE Fuller Hydrology and Geomorphology (JEF) to develop a concept-level debris volume estimate associated with the potential construction of a debris basin at the apex of the Neffs Creek alluvial fan. This section includes a summary of the findings and conclusions from the debris and sediment volume analysis. The full Technical Memorandum (TM) is included in Appendix D. 4.2 GENERAL SITE CHARACTERISTICS The Neffs Canyon watershed is characterized by steep channels, with slopes ranging from 4% to 16% in the lower reaches to greater than 50% in the upper reaches. This debris volume study focused on three main channels, denoted as Neffs Creek, Unnamed Tributary, and North Fork, shown in Figure 4-1. These channels have average slopes of 22%, 28%, and 38%, respectively. The existing vegetation in Neffs Canyon consists of scrub oak, maple brush, pine trees, and aspen trees. This vegetation is dense in areas but very sparse in the areas of the exposed slabs of steeply dipping, quartzite bedrock. Figure 4-1. Main Contributing Tributaries in Neffs Canyon ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 36 4.3 PAST DEBRIS FLOW EVENTS There is no known documentation for the debris flow history within Neffs Canyon or on the associated alluvial fan. However, the history of the neighboring areas is well documented and can be used to better understand the regional recurrence of debris flow events. Along the Wasatch Front, debris flows generally initiate on steep slopes or in channels, onset by intense rainfall (or “cloudburst events”) or rapid snowmelt. Historical records of debris flow events in Utah indicate that debris flows in this region can be highly variable in terms of their size, travel distance, and depositional behavior, which are all largely functions of the size, slope, vegetation cover, geology of their source basin, and intensity and duration of contributing precipitation events (Giraud, 2005). Several historic debris flow events along the Wasatch Front are described, for informational purposes, in the TM found in Appendix D. 4.4 FIELD INVESTIGATION JEF conducted a field investigation of the Neffs Canyon watershed in August 2021 to observe existing conditions of the three main channels. The channels rapidly increase in slope and debris loading of the channels was found to increase significantly in the upstream direction. The lower channel reaches exhibit minimal debris loading. The middle and upper reaches contain a substantial volume of debris loaded in the channel corridors, especially in the pool sequences. The heavy debris loading indicates Neffs Canyon has not experienced a large storm or rapid snowmelt event that produced a large discharge of the magnitude needed to mobilize and transport most of the available debris to the lower reaches, (or out) of the canyon. The three main criteria for mobilization of a debris flow event are 1) slope, 2) channel confinement, and 3) debris availability in the channels. Neffs Canyon exhibits all three criteria and is poised for a debris flow event(s) under a triggering meteorological condition, or in a post-wildfire watershed condition 4.5 EMPIRICAL DEBRIS FLOW VOLUME ESTIMATES Six empirical methods and equations were used to better understand and estimate the magnitude of debris flow volume that could be evacuated from Neffs Canyon in a high runoff event. Each of these empirical methods are described in detail in the TM included in Appendix D. The results of the 2007 County Study were also listed for comparison. The debris volume results using each of these methods are listed for comparison in Table 4-1. An estimate was also made using engineering judgment after completing onsite field reconnaissance in the Neffs Canyon drainage area. Table 4-1 presents a range of calculated potential debris flow volumes using the seven referenced methods. The predicted volumes of these methods range from 8 to 128 ac-ft. This equates to approximately 0.4 to 7 cubic yards of sediment per foot of channel length. Additionally, these volumes represent an event such that the entire Neffs Canyon drainage basin would contribute debris. It is possible that a localized, high-intensity storm event could occur on only a portion of the Neffs Canyon watershed, triggering debris mobilization from only a single channel. It should also be noted that estimating debris flow volumes is not an exact science. A reliable, reasonable estimate for Neffs Canyon must be obtained through comparing the results, understanding the science behind each method, familiarity with the drainage basin or watershed characteristics, and engineering judgement. The hydrologic assessment of the Neffs Canyon watershed summarized in Section 2 of this report estimated the total 100-year flood runoff volume associated with the 6-hour storm to be 33 ac-ft. A comparison of this runoff volume to the non-recurrence interval-based methods listed in Table 4-1 suggests that a recurrence interval event much higher than the 100-year flood would be required to mobilize the debris volume estimates in excess of 33 ac-ft. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 37 Given that the USACE (1992) methodology is recurrence interval storm-based, it is the most appropriate method to use with recurrence interval-based hydrologic analyses. The results of the USACE methodology indicate a 100-year storm on existing watershed conditions could produce up to 8 ac-ft of debris volume, and a 500-year storm could produce up to 11 ac-ft of debris volume. Table 4-1. Summary of Debris Flow Volume Estimations Summary of Volume Estimates Method Sediment Type Volume – Lower Limit (ac-ft) Volume – upper limit (ac-ft) Recurrence Interval USACE, 1992 - LA District Method, Equation 2 Debris flow volume 8 11 100-Year I500-Year Hungr et al., 1984 - Method 1 Debris flow volume 36 73 N/A Hungr et al., 1984 - Method 2 Debris flow volume 64 128 N/A AGEC, 2005 – Method A Debris flow volume 87 N/A N/A AGEC, 2005 – Method B Debris flow volume 84 N/A N/A Hansen, Allen & Luce (2007) Debris flow volume 21 N/A N/A Average 50 71 Median 50 73 BUREC, 1987 - Design of Small Dams Non debris flow sediment yield per year 4 N/A N/A 4.6 LIMITATIONS AND ASSUMPTIONS FOR DEBRIS FLOW ANALYSIS This debris flow analysis considered existing watershed conditions. Neffs Canyon is susceptible to wildfire risk. In a post-wildfire condition, debris flow risk to life and property of the canyon mouth increases significantly. Recent wildfires in the Wasatch Range have resulted in debris flow and mudflows that have adversely impacted communities. It is recommended that the design of any future debris basin located on the Neffs Canyon alluvial fan consider a post-wildfire watershed condition. A more detailed debris volume analysis is recommended as part of the final design phase of any future debris basin. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 38 5 ALTERNATIVES ANALYSIS 5.1 PURPOSE OF ALTERNATIVES ANALYSES The principal objective of this study is to identify a feasible debris basin or other flood control improvements that can be designed and constructed to mitigate the alluvial fan hazards on the Neffs Canyon alluvial fan. The results and recommendations of this study may be used to obtain future funding for construction of the recommended alternative. 5.2 BASIS OF DESIGN FOR ALTERNATIVES Several important factors were considered when identifying and evaluating potential flood mitigation alternatives for the flood hazards on the Neffs Canyon alluvial fan. Alternatives were evaluated based on their ability to eliminate the alluvial fan flood hazard and to minimize impacts to the residents. The design parameters considered when evaluating potential alternatives are described below. 1. The selected alternative must safely convey runoff from the 100-year flood through the residential neighborhood and eliminate the alluvial fan flood hazards defined on FEMA flood insurance rate maps. 2. No improvements can be constructed on the USFS Wilderness Area land. The current wilderness boundary was obtained from a GIS file on the Utah Geospatial Resource Center (UGRC) website and was used in the evaluation of the feasibility of alternative improvements. 3. The alternative must avoid impacting the existing homes and structures of the proposed debris basin site. The current residents also wish to minimize the visual impact of the alternative in order to maintain the natural landscape at the mouth of the canyon. 4. Since the proposed site is on USFS property, the selected flood mitigation alternative must meet USFS requirements, including any permitting or design requirements requested. It should be noted that the only available site for a potential debris basin is adjacent to the Neffs Canyon Trailhead parking lot. That parking lot is in poor condition and needs to be repaved. Millcreek recently received approval from the U.S. Forest Service to improve the trailhead facilities by expanding the parking lot, constructing a public restroom, and constructing wildfire fighting facilities that include a helipad and a heliwell, as the trailhead is also located on land managed by the U.S. Forest Service. Construction and funding of the desired trailhead and wildfire fighting facilities will be a separate project. BRIC Grant funds are not being requested for these trailhead project improvements. 5. Low flows from runoff in the existing Neffs Creek channel will be maintained (up to 15 cfs) by constructing a diversion structure on the channel on the south side of the proposed debris basin facility. Discharges in excess of about 15 cfs would be diverted into the debris basin. If residents desire, the low flow channel below the proposed debris basin could actually be abandoned. It was assumed that residents would like to keep the channel functional. This diversion would prevent flooding along the existing Neffs Creek channel due to capacity issues of the channel and existing culverts. 6. Any debris basin that is evaluated would not detain, or store, runoff from a 100-year flood. The debris basin facility would be designed primarily to collect debris and water from the canyon and convey it to new storm drain facilities and eliminate the uncertainty associated with flood flow paths on the alluvial fan. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 39 7. Several geologic/geotechnical issues (the presence of the Wasatch Fault, potential landslide issues, and regulatory issues such as Dam Safety) are present on the site and were considered when evaluating potential alternatives. 5.3 FLOOD MITIGATION ALTERNATIVES EVALUATED The following alternatives were evaluated with the goal of eliminating the alluvial fan flood hazards shown on the new FEMA FIRM: 1. Construct an above-ground dam/debris basin at the mouth of Neffs Canyon to capture debris and release canyon runoff into a new storm drain system that will convey it safely through the developed neighborhood to a point where it can discharge into the existing Neffs Creek storm drain near Wasatch Boulevard. 2. Construct an above-ground dam/debris basin at the mouth of Neffs Canyon to capture debris and release canyon runoff into shorter section of new storm drain pipeline that runs under neighborhood roads and outfalls into the active Neffs Creek channel. This alternative would also require significant channel improvements to the existing Neffs Creek channel below the new storm drain outfall to mitigate flood hazards along the channel. 3. Construct a below-grade debris basin at the mouth of Neffs Canyon to capture debris and release canyon runoff into a new storm drain system that will convey it safely through the developed neighborhood to a point where it can discharge into the existing Neffs Creek storm drain near Wasatch Boulevard. 4. Construct a below-grade debris basin at the mouth of Neffs Canyon to capture debris and release canyon runoff into a shorter section of a new storm drain pipeline that runs under neighborhood roads and outfalls into the existing Neffs Creek channel. This alternative would also require significant channel improvements to the existing Neffs Creek channel below the new storm drain outfall to mitigate flood hazards along the channel. 5. Construct a below-grade basin or dam/debris basin within the Federal Wilderness Area to provide for more debris storage volume and associated runoff conveyance facilities that will safely convey canyon runoff to the Neffs Creek storm drain near Wasatch Boulevard. 6. Do nothing. Each of these alternatives are shown in Figure 5-2-Figure 5-5 at the end of the section along with summaries of each alternative in Table 5-1 - Table 5-4. Conceptual costs to plan, permit, design and construct the alternatives evaluated (in 2022 dollars) are included in Appendix E. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 40 5.3.1 ALTERNATIVE 1: DAM/DEBRIS BASIN AND NEW STORM DRAIN The first alternative evaluated includes construction of a dam/debris basin at the mouth of Neffs Canyon, between the Federal Wilderness Area boundary and the residential properties along Zarahemla Drive, as shown on Figure 5-2. The purpose of this debris basin would be to collect runoff and debris generated in Neffs Canyon to a point such that the runoff can be conveyed into a new storm drain facility, reducing flooding hazards. The dam/debris basin would outfall into a new storm drain that begins near the north dam abutment. This storm drain would run behind the houses along Zarahemla Drive before discharging into a new storm drain at the intersection of Zarahemla Drive and Mathews Way. The storm drain would run east to west under several of the Millcreek neighborhood roads before discharging into Neffs Creek near Wasatch Boulevard. The major elements associated with Alternative 1 are summarized in Table 5-1. 5.3.1.1 Challenges and Requirements The following challenges or requirements were identified with this alternative: • Safely conveying the 100-year discharge through the developed area • Designing a dam that has a spillway to safely discharge the PMF discharge and does not direct water towards homes to increase flood hazards • Obtaining permission from USFS to build a dam on USFS property • Working with Utah Dam Safety to develop an acceptable design near a major seismic fault • Obtaining easements for the pipeline between the debris basin and Zarahemla Drive. 5.3.1.2 Conceptual Cost Estimate The conceptual construction cost estimate for the dam/debris basin with new storm drain alternative is included in Appendix E. The estimated cost for Alternative 1 is $21,665,000. Table 5-1. Summary of Proposed Alternative 1 No. Description of Improvement I Construct about a 14.5 ac-ft dam/debris basin between the Federal Wilderness area boundary and the residential properties along Zarahemla Drive. II Construct a diversion on the active Neffs Creek channel to convey runoff in excess of 15 cfs into a stepped concrete inflow channel into the debris basin (similar to what is shown in Figure 5-1) III Construct a new storm drain pipeline that conveys runoff from the dam/debris basin to the intersection of Zarahemla Drive and Mathews Way and has capacity to convey the 500-year flood. IV Construct a storm drain that runs from the intersection of Zarahemla Drive and Mathews Way under several Millcreek neighborhood roads before discharging into the Neffs Creek storm drain near Wasatch Boulevard. This storm drain would have capacity to convey the 100-year flood. V Construct a stepped concrete inflow channel to convey runoff in the historic Neffs Creek channel into the debris basin similar to what is shown in Figure 5-1. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 41 Figure 5-1. Example of Stepped Inflow Channel 5.3.2 ALTERNATIVE 2: DAM/DEBRIS BASIN AND NEFFS CREEK CHANNEL IMPROVEMENTS Similar to Alternative 1, the second alternative evaluated also includes construction of a dam/debris basin at the mouth of Neffs Canyon, between the Federal Wilderness Area boundary and the residential properties along Zarahemla Drive, as shown on Figure 5-3. The storm drain would run behind the houses between the debris basin and Zarahemla Drive before discharging into another new storm drain pipeline at the intersection of Zarahemla Drive and Mathews Way. The storm drain would then continue north along Zarahemla Drive, west along S 4260 E, and then south along Parkview Drive, before outfalling into the Neffs Creek channel (as shown in Figure 5-3). As part of this alternative, significant improvements would need to be made to the existing Neffs Creek channel to accommodate the increased flood flows. According to the 2007 County study, the existing Neffs Creek channel capacity is believed to be insufficient to remove the surrounding homes from the alluvial fan flood hazard designation. To improve the condition and capacity of the existing channel, significant improvements will need to be made to Neffs Creek. Bank stabilization would need to be added to the banks of the reconstructed channel to mitigate erosion potential. Since slopes in the channel are very steep and velocities in the channel are high, riprap would be needed to armor the channel from erosion hazards. Existing slopes along the channel are from 8 to 10 percent. In order to reduce the slope along Neffs Creek to 8 percent it is likely that several grade control structures would be needed. The major elements associated with Alternative 2 are summarized in Table 5-2. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 42 Table 5-2. Summary of Proposed Alternative 2 No. Description of Improvement I Construct about a 14.5 ac-ft dam/debris basin between the Federal Wilderness area boundary and the residential properties along Zarahemla Drive. II Construct a diversion on the active Neffs Creek channel to convey runoff in excess of 15 cfs into a stepped concrete inflow channel into the debris basin (similar to what is shown in Figure 5-1) III Construct a new storm drain that connects the dam/debris basin outfall to the intersection of Zarahemla Drive and Mathews Way and has capacity to convey the 500-year flood. IV Construct a storm drain that runs from the intersection of Zarahemla Drive and Mathews Way, then north along Zarahemla Drive, west along S 4260 E, and then south along Parkview Drive, before outfalling into the existing active Neffs Creek channel. This storm drain would have capacity to convey the 100- year flood. V Construct channel improvements to the existing Neffs Creek channel, such as upsizing the existing culverts, to increase the conveyance capacity and making other channel improvements located in backyards to accommodate the runoff from the proposed dam/debris basin and storm drain pipeline. VI Construct a stepped concrete inflow channel to convey runoff in the historic Neffs Creek channel into the debris basin similar to what is shown in Figure 5-1. 5.3.2.1 Challenges and Requirements The following challenges or requirements were identified with this alternative: • Safely conveying the 100-year discharge through the developed area • Designing a dam that has a spillway to safely discharge the PMF discharge and does not direct water towards homes to increase flood hazards • Obtaining permission from USFS to build a dam on USFS land • Working with Utah Dam Safety to develop an acceptable design near a major seismic fault • Obtaining easements for the pipeline between the debris basin and Zarahemla Drive. 5.3.2.2 Conceptual Cost Estimate The conceptual construction cost estimate for the dam/debris basin with the Neffs Creek channel improvements is included in Appendix E. The estimated cost for Alternative 2 is $24,839,000. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 43 5.3.3 ALTERNATIVE 3: BELOW-GRADE DEBRIS BASIN AND NEW STORM DRAIN The third alternative evaluated includes construction of a below-grade debris basin at the mouth of Neffs Canyon between the Federal Wilderness Area boundary and the residential properties along Zarahelma Drive as shown on Figure 5-4. The purpose of this below-grade debris basin would be to collect runoff and debris generated in Neffs Canyon such that the runoff can be conveyed into a new storm drain facility, reducing flooding hazards. The below-grade debris basin would outfall into a new storm drain pipeline that begins in the northwest corner of the below-grade debris basin. This storm drain follows the same alignment as Alternative 1. The major elements associated with Alternative 3 are summarized in Table 5-3. Table 5-3. Summary of Proposed Alternative 3 No. Description of Improvement I Construct about a 9 ac-ft below-grade debris basin between the Federal Wilderness area boundary and the residential properties along Zarahemla Drive. II Construct a diversion on the active Neffs Creek channel to convey runoff in excess of 15 cfs into a stepped concrete inflow channel into the debris basin (similar to what is shown in Figure 5-1) III Construct a new storm drain pipeline that conveys runoff from the below-grade basin to the intersection of Zarahemla Drive and Mathews Way and has capacity to convey the 500-year flood. IV Construct a storm drain that runs from the intersection of Zarahemla Drive and Mathews Way under several Millcreek neighborhood roads before discharging into the Neffs Creek storm drain near Wasatch Boulevard. This storm drain would have capacity to convey the 100-year flood. V Construct a stepped concrete inflow channel to convey runoff in the historic Neffs Creek channel into the debris basin similar to what is shown in Figure 5-1. 5.3.3.1 Challenges and Requirements The following challenges or requirements were identified with this alternative: • Safely conveying the 100-year discharge through the developed area • Confirming that the excavation for the below-grade debris basin will not destabilize the historic landslide in the vicinity • Obtaining permission and permits from USFS to construct a below-grade debris basin on USFS property • Transporting a large amount of excavated material through the Olympus Cove neighborhood • Obtaining easements for the pipeline between the debris basin and Zarahemla Drive. 5.3.3.2 Conceptual Cost Estimate The conceptual construction cost estimate for the below-grade debris basin and new storm drain alternative is included in Appendix E. The estimated cost for Alternative 3 is $19,748,000. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 44 5.3.4 ALTERNATIVE 4: BELOW-GRADE DEBRIS BASIN AND NEFFS CREEK CHANNEL IMPROVEMENTS Similar to Alternative 3, the fourth alternative evaluated also includes construction of a below-grade debris basin at the mouth of Neffs Canyon, between the Federal Wilderness Area boundary and the residential properties along Zarahemla Drive as shown on Figure 5-5. The below-grade debris basin would outfall into a new storm drain located in the northwest corner of the debris basin. This storm drain would utilize the same alignment as Alternative 2. The major elements associated with Alternative 4 are summarized in Table 5-4. 5.3.4.1 Challenges and Requirements The following challenges or requirements were identified with this alternative: • Safely conveying the 100-year discharge through the developed neighborhood • Confirming that the excavation for the below-grade debris basin will not destabilize the historic landslide in the vicinity • Obtaining permission from USFS to build a below-grade debris dam on USFS land • Transporting a large amount of excavated material through the Olympus Cove neighborhood • Obtaining easements for the pipeline between the debris basin and Zarahemla Drive and channel improvements through private developed property. 5.3.4.2 Conceptual Cost Estimate The conceptual construction cost estimate for the below-grade debris basin, storm drain improvements, and channel improvements is included in Appendix E. The estimated cost for Alternative 4 is $24,511,000. Table 5-4. Summary of Proposed Alternative 4 No. Description of Improvement I Construct about a 9 ac-ft below-grade debris basin between the Federal Wilderness area boundary and the residential properties along Zarahemla Drive. II Construct a diversion on the active Neffs Creek channel to convey runoff in excess of 15 cfs into a stepped concrete inflow channel into the debris basin (similar to what is shown in Figure 5-1) III Construct a new storm drain that connects the dam/debris basin outfall to the intersection of Zarahemla Drive and Mathews Way and has capacity to convey the 500-year flood. IV Construct a storm drain that runs from the intersection of Zarahemla Drive and Mathews Way, then north along Zarahemla Drive, west along S 4260 E, and then south along Parkview Drive, before outfalling into the existing active Neffs Creek channel. This storm drain would have capacity to convey the 100-year flood. V Construct channel improvements to the existing Neffs Creek channel, such as upsizing the existing culverts to increase the conveyance capacity and making other channel improvements located in backyards to accommodate the runoff from the proposed dam/debris basin and storm drain pipeline. VI Construct a stepped concrete inflow channel to convey runoff in the historic Neffs Creek channel into the debris basin similar to what is shown in Figure 5-1. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 45 5.3.5 ALTERNATIVE 5: DAM/DEBRIS BASIN OR BELOW-GRADE DEBRIS BASIN ON USFS WILDERNESS AREA LAND The fifth alternative evaluated includes construction of a dam/debris basin or a below-grade debris basin further up in the canyon, within the USFS Wilderness Area. This would greatly increase the storage of the dam/debris basin or below-grade debris basin since the basin could be much larger since less site constraints are present. Some type of mitigation option to safely convey the 100-year discharge through the developed area would also have to be designed. While the potential storage capacity of this alternative could be larger, construction of this facility within a Federal Wilderness Area would require approval from Congress. Gaining approval would likely be a process with no guarantee of success and therefore this alternative was not evaluated further. Construction of a debris basin on existing Wilderness Area land would also scar the hillside and be visible from a long distance, something that residents strongly oppose. 5.3.6 ALTERNATIVE 6: DO NOTHING The No Action Alternative consists of the City choosing to leave the Neff’s Canyon drainage basin “as- is” with no future improvements. Under this alternative the alluvial fan flood hazard developed by FEMA in this area would remain largely unchanged. By leaving the alluvial fan flood hazard in place, flood insurance requirements would remain as-is throughout the neighborhood. Restrictive building requirements would remain in place, remodels/rebuilds must meet the new building restrictions (i.e. no walk out basements, remodels must be less than 50% of fair market value of the structure or the entire structure must be brought up to current FEMA zone requirements), and the current fire station serving the Mount Olympus could not be replaced with a new building on the existing building site. 5.4 RECOMMENDED ALTERNATIVE Alternative 3 (constructing a new below-grade debris basin and conveying canyon runoff through a new storm drain) was selected as the preferred alternative after sending the draft feasibility study report to multiple agencies, the local community council, and holding a public meeting. That alternative will effectively utilize the available land between the Wilderness Area boundary east of the existing developed subdivision to the maximum extent practicable. Implementing Alternative 3 would be less impactful to the properties along the existing active creek channel through the developed neighborhood. It would also have significantly less visual impact to the hillside than an alternative that includes an above-grade dam. The proposed storm drain pipeline could also collect runoff generated on local streets and improve storm water management in the area. Construction of this recommended alternative would be coordinated with the previously approved project to improve the Neffs Canyon trailhead parking lot. Millcreek and the Contractor will coordinate with the U.S. Forest Service to obtain the needed agreements that would allow construction staging and placing some of the material excavated from the debris basin facility in a depressed area. This will raise the grade so that area can be used for expanding the trailhead parking lot. All other Trailhead Parking Lot Project improvement features that include parking lot paving, constructing a public restroom, and constructing a helipad and heliwell will be part of a separate project. The proposed debris basin outlet will include a large trash rack that extends from the bottom of the debris basin to the top. This will allow the outlet to function if lower portions become plugged with debris. It will also facilitate maintenance and cleaning debris from the top. A primary spillway will be located at the top of the outlet structure that will allow runoff up to the 500-year flood to discharge into the outlet pipe. The outlet pipe would decrease in size at the intersection of Zarahemla Drive and Mathews Way. The pipe of that point will be designed to convey the 100-year discharge. The junction at the point where the pipe capacities ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 46 change will be designed so that if the capacity of the pipe is exceeded, the excess water will surcharge in that structure and flow onto the ground surface and the roads in the area. This means that flood hazards associated with flood hazards larger than the 100-year storm will still exist and be very similar to those that existed before the project was constructed. An emergency spillway will be constructed where the new debris basin intercepts the existing historic Neffs Creek channel. That emergency spillway would discharge runoff from very extreme events back into the channel that has historically conveys large flows, so there should be no adverse flood impacts to properties. The recommended below-grade debris basin cannot be designed for a large post-fire debris flow. There is not enough available land to make the debris basin any larger. The proposed debris basin may be undersized to protect homes near the mouth of the canyon from a debris flow if a large fire were to occur in Neffs Canyon. Although the debris basin may be undersized for a large post-fire runoff event, the available area is being utilized to the maximum extent that is practicable. With a conceptual cost estimate of about $20 million, this alternative is the least cost of all of the evaluated alternatives. A zoomed-in version of the recommended alternative and a conceptual rendering are shown in Figure 5-6 and Figure 5-7, respectively. Additional conceptual renderings of the recommended alternative are included in Appendix F. Additionally, conceptual design drawings of the major recommended features are included in Appendix G. 5.4.1 FLOODPLAIN MAPPING – RECOMMENDED ALTERNATIVE The recommended improvements associated with Alternative 3 would mitigate potential flood hazards associated with the 100-year design storm and allow the existing alluvial fan flood hazards to be removed from the new FEMA FIRM. Runoff from 100-year design storm in Neffs Canyon would be contained in the new pipeline down the road. 5.5 LIMITATIONS AND ASSUMPTIONS This feasibility study is conceptual in nature. Key assumptions and limitations are listed below: 1. The recommended below-grade debris basin cannot be designed for a large post-fire debris flow. There is not enough available land to make the debris basin any larger. The proposed debris basin may be undersized to protect homes near the mouth of the canyon from a debris flow if a large fire were to occur in Neffs Canyon. Although the debris basin may be undersized for a large post-fire runoff event, the available area is being utilized to the maximum extent that is practicable. 2. More detailed geotechnical/geologic analysis is needed to determine the full limitations of the project site. The selected alternative assumes the site is suitable for extensive excavation based on the preliminary geotechnical/geologic analysis. The preliminary analyses did not passively identify any issues that would prevent the design of the below-grade debris basin from proceeding. If issues are identified in future studies or during the design, those issues should be able to be mitigated. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 47 Figure 5-2.. Alternative 1 5-2 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 48 Figure 5-3.. Alternative 2 5-3 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 49 Figure 5-4.. Alternative 3 Figure 5-5.. Alternative 4 5-4 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 50 5-5 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 51 Figure 5-6. Recommended Alternative 5-6 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 52 Figure 5-7. Rendering of Recommended Debris Basin Alternative ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 53 6 ENGINEER’S OPINION OF PROBABLE CONSTRUCTION COST A summary of the conceptual level construction costs for the recommended alternative (Alternative is shown in Table 6-1. The total estimated cost for Alternative #3 is approximately $19,748,000. The conceptual cost estimates developed for this study are based on the type of improvement that was evaluated, information on the preliminary design drawings (for the recommended alternative), information from recent construction bids on similar projects, information obtained from suppliers, and current cost estimating guidelines. Note that the conceptual cost estimate is in 2024 dollars and includes a 20-percent contingency to account for unknown details and factors that cannot be identified in a feasibility-level study. It should also be noted that with inflation and supply chain issues experienced over the last two years, construction costs have been very difficult for engineers to accurately estimate. Table 6-1. Preliminary Conceptual Cost Estimate – Neffs Creek Alternative 3 Improvements ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 54 7 AGENCY COORDINATION For this project to move forward there will need to be additional coordination required with multiple agencies. This section summarizes previous coordination with different agencies along with a brief description of coordination that will be required through final planning, permitting, and design phases. 7.1 SALT LAKE COUNTY Over 10 years ago, before Millcreek was an incorporated as a city, Salt Lake County commissioned a study for flood mitigation alternatives on the Neffs Creek alluvial fan. Neffs Creek is a Salt Lake County Flood Control Facility and any impact to Neffs Creek will require a Salt Lake County Flood Control permit. Alternative #3 includes a diversion to divert water into the below-grade debris basin and then conveys all canyon runoff to a County-owned storm drain near Wasatch Boulevard. As these improvements impact County-managed facilities, a County Flood Control Permit will be required before construction can begin. During the study, the County was included in multiple meetings that discussed the hydrology and different flood mitigation alternatives. They were also provided with a copy of the hydrology report and were allowed to provide feedback. Salt Lake County personnel should be included in future meetings related to permitting, planning, and design of the recommended facilities. 7.2 FEMA/UTAH DEPARTMENT OF EMERGENCY MANAGEMENT FEMA recently issued a new FIRM for the Neffs Creek alluvial fan floodplain. The new alluvial fan flood hazards cover significantly more area than did the previous map. Many more structures are now in the mapped floodplain. One of the main purposes of this feasibility study was to identify a flood mitigation option that, if implemented, would remove the FEMA-mapped alluvial fan flood hazard of the debris basin. For this to happen, a Conditional Letter of Map Revision (CLOMR) would need be to be reviewed and approved by FEMA before final design is completed followed by a post-construction Letter of Map Revision (LOMR) to eliminate the alluvial fan flood hazard defined in the new FIRM. During this feasibility study there were multiple meetings held where either representatives from FEMA Region 8, representatives from Utah Department of Emergency Management, or both have attended and provided feedback on the feasibility of revising the design hydrology and revising the FIRM after flood mitigation improvements are constructed. During those meetings, representatives from FEMA and Utah DEM were both supportive of the idea of using mitigation options to remove the alluvial fan flooding and resolve the many floodplain management challenges that are associated with development in the existing alluvial fan flood hazard area. The hydrologic analysis performed as part of this study resulted in a proposed 100-year design flood with a peak magnitude of 107 cfs. This value is lower than the 300 cfs used by FEMA to develop the new FIRM. As this study has proven that there is justification to revise the current-effective FEMA hydrology, a request for a CLOMR based on hydrology should be submitted to FEMA to verify they will approve the reduced discharge. This is important because the storm drain pipes will be sized to convey the 100-year discharge. After the revised hydrology has been approved by FEMA and the final design completed, it is recommended that that a second CLOMR be submitted to FEMA to review the proposed flood mitigation improvements (the debris basin, pipeline, and related facilities), obtain approval of the ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 55 proposed design criteria, and obtain concurrence that the alluvial fan flood hazards on the new FIRM can be eliminated if the recommended flood control facilities are constructed. After the second CLOMR is approved and mitigation options built, a LOMR will need to be submitted to FEMA to officially revise the floodplain maps. It may be possible to have the revised hydrology and the recommended improvements approved in one CLOMR application. It should be noted that the approval period for each CLOMR/LOMR can take six to nine months once submitted to FEMA. 7.3 U.S. FOREST SERVICE The location of the proposed debris basin is located in USFS land and near the Neffs Creek Trail Head parking lot, where there is a planned project to expand the parking lot and USFS land and construct a restroom, helipad, and heliwell to help fight forest fires in the area. Any planned construction work on USFS land requires coordination USFS personnel and obtaining the appropriate agreements and permits. During this study, personnel from Millcreek and the study team coordinated with USFS staff to discuss the objectives of this feasibility of this study and recommended improvements that would need to be constructed on USFS land. USFS service personnel indicated that with proper coordination, permitting, and environmental clearances, and coordinating on the funding and construction of the previously approved improvements to the Neffs Canyon Trailhead parking lot, there did not seem to be any USFS issues that would make the construction of the proposed flood control facilities on land that they manage not feasible. After funding is obtained to construct the proposed flood mitigation facilities, the detailed coordination and permitting process should immediately begin with the USFS. 7.4 UTAH STATE DAM SAFETY Utah Dam Safety oversees the design, construction, and inspection of all high hazard dams in the state. The current recommended mitigation option does not include what would be considered a dam and would minimize the amount of coordination required with Dam Safety. If the alternative changes to a dam during final design, then a significant amount of coordination with Dam Safety would be required as this would be considered a high hazard dam near a major fault. 7.5 OTHERS The recommended storm drain pipe between the recommended below-grade debris basin and Zarahemla Drive will need to cross private property and require construction of a large pipeline in a city street, coordination with utility companies and private property owners (easements) will be required during final design and construction. This needed coordination should be done early in final design phase. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 56 8 PUBLIC INVOLVEMENT Due to the location and complexity of planning, designing, permitting, and constructing flood mitigation measures that impact area residents, public lands, and public entities, multiple coordination meetings with the general public will be required during the planning and design of the project. A public meeting was held on December 15, 2021 to present the findings and recommendations of this feasibility study. The primary purpose of that meeting was to determine if residents in the area are supportive of the recommended flood mitigation improvements and the proposed approach to eliminate the alluvial fan flood hazards defined on the new FEMA FIRM. All that participated in the meeting expressed general support of the project and its objectives. They were asked to provide any questions or feedback to the City by February 15, 2022. Additional public involvement will also be required during final planning and design phases of the project. Appendix H contains a copy of the presentation and handout given at the public meeting. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 57 9 POTENTIAL FUNDING SOURCES This feasibility study identifies a recommended alternative to advance to final design and construction to mitigate the alluvial fan flood hazards on the Neffs Creek alluvial fan. Before final planning and design can proceed, Millcreek must find a way to fund the recommended project. Below is a summary of some potential funding sources that could be used to fund the final planning, design, and construction of the project. • FEMA – there are multiple grant opportunities available from FEMA to help pay for some of (not all) flood control projects. The City is current working with the State on applying for a Building Resilient Infrastructure and Communities (BRIC) grant. This funding can be used to undertake hazard mitigation projects that help reduce risks associated with natural disasters and natural hazards. • NRCS – The NRCS has grant funding options such as the “Watershed Protection and Flood Prevention Act” (PL-566) which helps participants solve natural resource and related economic problems on a watershed basis. Projects can include flood prevention and damage reduction among other things. • Federal Earmark – During the public meeting, the Millcreek Mayor mentioned that he has had discussions with elected federal representatives from Utah to explore the feasibility of obtaining federal funding to help pay for the project. Those efforts are in progress. • State Funding – City representatives are currently exploring the potential of obtaining some state funds to construct the recommended facilities. • County Funding – Salt Lake County may be willing serve as a funding partner on this project, as it impacts a County Flood Control Facility. However, it is not likely that they could fund a significant part of the project. • Special District – The City could set up a special district for those living in the alluvial fan flood hazard to help pay for the project. Most of these funding alternatives require a local match. It may be possible to obtain funds from multiple sources to minimize the financial impacts this project could have on Millcreek residents. It is recommended that City personnel use this feasibility study to request funding from all the potential sources listed above and proceed with design and construction as soon as possible. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 58 10 LIMITATIONS OF STUDY This document represents Bowen Collins & Associates’ professional judgement based on the information available at the time of its completion (existing conditions, not burned watershed), and as appropriate for the project scope of work. Services performed in developing the content of this document have been conducted in a manner consistent with that level and skill ordinarily exercised by members of the engineering profession currently practicing under similar conditions. No warranty, express or implied, is made. If the watershed were to burn, the proposed below grade basin may not be large enough to contain all of the debris that may come down during a large event. As this is a feasibility study, there are multiple things that will need to be worked out during final design and permitting which may have small revisions to the design. The following list contains some of the major items that will need to be completed during final design and construction: 1. CLOMR to get hydrology approved 2. Coordination with USFS, FEMA, Public, Dam Safety, and the County 3. Final design of debris basin, diversions, outlet works, and pipe improvements 4. Additional geotechnical analysis, borings/trenching, and locating fault 5. Additional debris flow analysis 6. Secure project funding. ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY 59 11 REFERENCES National Oceanic and Atmospheric Administration, 1984, Hydrometeorological Report No. 49, Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages. National Oceanic and Atmospheric Administration, 2006, NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume I, Version 4, Semiarid Southwest. State of Utah Administrative Code, December 2017, R655-11. Requirements for the Design, Construction and Abandonment of Dams. Utah State University, Utah Climate Center, March 2003, 2002 Update for Probable Maximum Precipitation, Utah, 72-Hour Estimates, Areas to 5,000 mi² (USUL). Utah State University, Utah Climate Center, March 2003, Probable Maximum Precipitation Estimates for Short-Duration, Small-Area Storms in Utah (USUS ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX A – PREVIOUS STUDIES ---PAGE BREAK--- 4246 S Riverboat Rd STE 200. Salt Lake City, UT 84123 o. [PHONE REDACTED]. f. [PHONE REDACTED]. crsengineers.com Ben Rood, PE CFM Manager, Water Resources c.[PHONE REDACTED] [EMAIL REDACTED] March 23, 2020 David Baird Mt. Olympus Community Council 4538 S Thousand Oaks Dr Millcreek, UT 84106 [EMAIL REDACTED] Re: Independent Engineering Review of Neffs Creek Flood Hazard Assessment TSDN Dear David: The purpose of this memorandum is to discuss the independent review findings for the engineering data, methodology, calculations, and determinations from the Neffs Creek Flood Hazard Assessment Technical Support Data Notebook prepared by JE Fuller, March 2016. The primary purpose for the review is to determine if the study followed the FEMA Guidance for Flood Risk Analysis and Mapping Alluvial Fans. Study Overview The Neffs Creek alluvial fan study was prepared for FEMA as part of the Risk Mapping, Assessment, and Planning (Risk MAP) program to provide residents of the Olympus Cove area with an understanding of the flood risk potential resulting from the Neffs Creek alluvial fan formation. The study was completed through a three-stage approach to evaluate the geological characteristics of the alluvial fan formation, make determinations regarding its extent and characteristics, and perform hydraulic analysis to predict the flood risk potential. The three stages are identified in the study as: • Stage 1: Recognizing and Characterizing Piedmont Landforms • Stage 2: Defining Active vs Inactive alluvial Fan Flooding • Stage 3: Defining the 100-Year Floodplain The independent technical review will follow the three-stage approach and provide an overview of each phase followed by a discussion and comments regarding findings. Considerations and Recommendations will be provided as part of the summary and conclusion of the technical review. Stage 1: Recognizing and Characterizing Piedmont Landforms The objective of stage one is to determine if the Olympus Cove area can be classified as an alluvial fan landform by evaluating the sediment composition, stream flow path morphology, vegetation, location, and extent. Each of these items was evaluated using information available from previous studies, NRCS soil maps, USGS geological maps, and field observations. Review Discussion Based on the FEMA guidelines and the available data collected and presented in the JE Fuller report, stage one of the delineation process was followed properly. While the soils mapping interpretations would limit the alluvial fan to a smaller area, the surficial geologic mapping provides further insight for a larger alluvial fan area. The evidence is clear that the ---PAGE BREAK--- Ben Rood, PE CFM Manager, Water Resources P a g e I 2 Olympus Cove area can be classified as an alluvial fan. The stage one data and findings are consistent with FEMA guidance and standards. Stage 2: Defining Active vs Inactive Alluvial Fan Flooding Defining active and inactive areas of the alluvial fan is done by evaluating the depositions, erosion, and unstable flow path potential on the alluvial fan. The determinations focus on the age of the fan formation, composition of deposits and evidence of flooding and flow path uncertainty over the past 1000 years. Review Discussion The USGS and NRCS soil maps suggest that the most recent alluvial fan depositions occurred during the late-Holocene period which ranges from 5,000 years ago to current day. The age of the alluvial fan deposits and the evidence of flow path uncertainty within historical photos clearly defines the alluvial fan as active. There is evidence in the areal images from 1930s and1950s that flow path uncertainty dominated the morphology characteristics of the alluvial fan and much of the fan shows evidence of avulsions with braided channels. Some minor adjustments to the active areas defined in the JE Fuller study could be made based on the soils mapping, but none of those adjustments would impact the actual areas included in the floodplain. The evidence suggest that the alluvial fan is active, and the flood risk analysis should consider the possibility of channel instability and flow path uncertainty. JE Fuller’s determination of active alluvial fan follows FEMA guidance and standards for stage two. Stage 3: Defining the 100-Year Floodplain The 100-year floodplain analysis is used to delineate the risk of flooding on the active alluvial fan landform. The primary components of analysis comprise of the Hydrology and Hydraulics. Hydrology The hydrologic analysis utilized in the JE Fuller study was developed from the Salt Lake County Neffs Canyon Creek Master Plan completed by Hansen Allen and Luce in 2007. The detailed hydrologic analysis uses the NRCS TR-55 methodology supplemented with site specific analyses and reduction factors. The analysis results in a 1-percent annual chance discharge of 300 cubic feet per second at the canyon mouth. The rainfall runoff model uses mathematical calculations to determine the amount of runoff that will result from a rainfall event of a specific magnitude. Rainfall runoff models are generally calibrated to known runoff events from stream gages and USGS regional regression equations to help improve the accuracy of the model. USGS regional regression equations are developed using many stream gage stations throughout the state to predict runoff probability for stream that do not have a gaging station. Review Discussion The hydrologic rainfall runoff analysis was compared to the FEMA guidance and standards for General Hydrologic Considerations and Hydrology: Rainfall-Runoff Analysis. The hydrology guidance and standards section 7.1 states, “The Mapping Partner reviewing hydrologic analyses based on rainfall-runoff models must compare the proposed base flood discharges to the flood discharges from USGS regional regression equations (if applicable); to flood discharges at gaging stations in the vicinity of the study; to the effective discharges; and to other hydrologic estimates as appropriate. If the rainfall-runoff model was calibrated to discharge-frequency relations (stream gages and/or regional regression equations), most of the hydrologic review has been completed. If not, the reviewing Mapping Partner must plot the flood discharge estimates from these sources against drainage areas on logarithmic paper to determine if the proposed base flood discharges are reasonable. The proposed base flood discharges from the rainfall-runoff model are considered reasonable if they are generally within one standard error (68-percent confidence interval) of the regression and gaging station estimates. Differences between the proposed and effective discharges must be documented in the hydrology report and an explanation given as to why they are different. If the proposed discharges are determined to be unreasonable, the model parameters should be reviewed to determine if they are within the range of engineering practice. The model parameters should either be revised to conform to engineering practice, or their values justified.” ---PAGE BREAK--- Ben Rood, PE CFM Manager, Water Resources P a g e I 3 The project study does not discuss the comparison of the peak flow rate (300 cfs) from the rainfall runoff model to the USGS regional regression equations or adjacent gage stations. CRS Engineers prepared a comparison of the peak flow rates with adjacent gages using Bulletin 17C methodology prescribed in the FEMA guidance and standards. CRS Engineers also compared the flows to the USGS regional regression equations as outlined in USGS SIR 2007-5158. The result of the CRS analysis show that the regression equations and nearby gaging stations do not provide a good comparison with the peak flow determined in the rainfall runoff model. FEMA did not provide justification for the flow rate discrepancies; however, some portion of these discrepancies can be attributed to watershed slope. The Neffs Canyon watershed is very steep and thus will produce higher peak discharge events than watersheds that have more mild canyon slopes. Table 1: Peak Discharge Comparison Stream Name Location Watershed Area (mi2) Method Years of Record 100-year Peak Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) Neffs Creek Canyon Mouth 4.2 Rainfall Runoff (HAL Study) N/A 300 Undetermined Undetermined Neffs Creek Canyon Mouth 3.67 Regression Equations (StreamStats) N/A 107 54 161 Mill Creek Canyon Mouth 21.7 Stream Gage Bulletin 17C 1899 – Current (107 Record Peaks) 150 134 171 Mill Creek Canyon Mouth 21.7 Regression Equations (USGS SIR 2007-5158) 63 143 73 214 Emigration Creek Canyon Mouth 18.4 Stream Gage Bulletin 17C 1902 – Current (92 Record Peaks) 160 135 202 Emigration Creek Canyon Mouth 18.4 Regression Equations (USGS SIR 2007-5158) 57 132 66 198 Red Butte Creek Red Butte Reservoir, Fort Douglas 7.25 Stream Gage Bulletin 17C 1964 – 2019 (56 Record Peaks) 114 90 157 Red Butte Creek Red Butte Reservoir, Fort Douglas 7.25 Regression Equations (USGS SIR 2007-5158) 42 113 57 170 The rainfall storm distribution used in the project hydrology study by Hansen Allen and Luce (HAL) is a proprietary storm distribution that was developed by Vaughn Hansen and Associates in 1985. Vaughn Hansen later became one of the founders of HAL. The 100-yr 24-hr storm distribution used in the analysis should not be confused with the Farmer-Fletcher distribution, which was based on 10-years of rainfall data from 1960-1970 and developed a 1-hour, 2-yr and 10-yr storm distribution. The controlling storm distribution for the 100-yr 24-hour event was developed by HAL using their company proprietary storm distribution. The HAL storm distribution from 1985 is cited in the text of the document as (VHA, 1985), however, there is no documentation in the references sections for the actual publication citation. FEMA guidance and standards for storm distributions states, “The choice of temporal storm distribution must be fully documented. If the source of the distribution is not a federal, state, or regional agency, the documentation must include a detailed description of the derivation of the distribution, including sources of data and the means of fitting those data to a particular distribution.“ ---PAGE BREAK--- Ben Rood, PE CFM Manager, Water Resources P a g e I 4 Documentation of the 100-yr 24-hour storm distribution is not provided in the project study, and the distribution was not developed by a federal, state, or regional agency. Additionally, the HAL distribution was not used for the remainder of the studies developed as part of the Salt Lake County Risk MAP project. The distribution used on the remainder of the studies in Salt Lake County was the NOAA Atlas 14 Temporal Storm Distribution. As part of the Salt Lake County Risk MAP project FEMA studied Midas, Bingham, and Rose creek that are located on the west side of Salt Lake County. FEMA did not approve the Southwest Canal and Creek Study as hydrology documentation for the west creeks, knowing it used a similar distribution as is being accepted in the HAL Study. There is inconsistency between what FEMA approved for Neffs Creek and what was approved for the other streams in the Risk MAP Study. CRS Engineers obtained a copy of the HAL hydrologic rainfall runoff model developed in the USACE HEC-HMS software and revised the storm distribution to determine the impacts of the distribution on the model results. The distribution used for this comparison was the NOAA Atlas 14 Temporal Storm Distribution with a 24-hour duration, second quartile, and 50% probability. This distribution is interpolated to match the model calculation time step of 3 minute and input into the model for the upper, middle, and lower basins for comparison at the canyon mouth. The storm distribution was the only parameter that was revised in the model which resulted in a decrease of 30 cfs with a peak flow rate of 270 cfs. CRS Engineers also compared the peak discharge using the SCS Type II distribution and found that the peak discharge at the canyon mouth increased 200 cfs to a peak discharge of 500 cfs. The comparisons of rainfall distribution shows that the HAL distribution used in the study compares well with the NOAA Atlas 14 rainfall distribution and suggests that the rainfall distribution used can be validated by distributions developed by federal agencies. Hydraulics Hydraulic modeling is used to predict how the peak flow rate will descend from the apex of the fan to the terminal point of the model which was selected as Wasatch Blvd. A 2-dimensional (2-D) hydraulic model was used to develop the depths and velocities of the flood event over the alluvial fan surface. Seven individual hydraulic scenarios were evaluated to determine the maximum flood depth and velocity over the alluvial fan and simulate the flow path uncertainty. The composite mapping was produced by compiling the maximum depths and velocities into one dataset. Review Discussion The 2-D hydraulic model developed for Neffs alluvial fan was compared to the FEMA guidance and standards for 2-D modeling and alluvial fan hydraulics. The study report steps through each of the parameters that were developed for the 2- D hydraulic modeling scenarios. This review found consistency between industry standard practice and the parameters that were developed for control of the model simulations. FEMA allows for composite methodology to be used in alluvial fan hydraulic modeling. The composite study simulates active alluvial fan flow path changes by adding levees that divert flow down various flow paths. The location, length, and direction of these levees was developed based on engineering judgement and know bifurcation location based on aerial imagery and topography. An evaluation of these levee diversion locations was done through a site visit to look at the potential for flow path uncertainty at each location. Additionally, the potential for debris accumulation at each location was evaluated to determine if the scenarios could realistically occur during the 100-yr flood event. The study indicates that 33,800 cubic yards of debris will be produced during this discharge event which will cause major avulsions and flow path uncertainty. Based on the quantity of material it seems feasible for the channels and flow paths to be cutoff and re-directed as modeled in the hydraulic scenarios. The engineering judgement for placement of the debris dams seems to be applied in a consistent manner to direct flow down known channel bifurcations to mimic what will happen in a large debris flow event. The floodplain mapping produced by the study is a compilation of maximum flow and velocities. Although this approach is conservative it provides a realistic representation of the risk for flooding down each of the historic flow paths. The depth and velocity maximums are consistent with FEMA guidance and standards and were developed from the composite study simulations. ---PAGE BREAK--- Ben Rood, PE CFM Manager, Water Resources P a g e I 5 Conclusions and Recommendations The Neff’s Creek alluvial fan independent review looked at each component of the study and compared them to the FEMA guidance and standards. The three-stage approach used by the study is in accordance with FEMA guidance and standards. The only deviations from standard found involved parameters used in the hydrologic study. Modifications to the hydrology for the study to bring it into conformance with FEMA guidelines would result in lower flow rates being modeled over the alluvial fan. The lower flow rates may result in a smaller flood zone and possibly reduce the number of houses that are in the flood zone, but it will not revise the designations of the flood hazard Zone AO and have the same restrictions. Recommendations CRS Engineers recommends that the hydrologic deviations from FEMA standard be provided to FEMA for review and comment to determine if they warrant an official appeal. The following questions should be asked to FEMA for a response. 1. The peak discharge comparison between the FEMA study, USGS regional regression, and stream gages shows that the determined flows do not fall within the confidence limits required in the FEMA guidance. Would FEMA accept a flow rate of 100 cfs based on calibration of the Hansen Allen and Luce hydrologic model? 2. Will FEMA provide documentation of their review of the hydrologic analysis that supports compliance with all FEMA guidance and standards? 3. The storm distribution used from the hydrologic study was not developed from a federal, state, or local agency. Will FEMA recommend a storm distribution that complies with FEMA guidance and is most appropriate for the Neffs Creek watershed? 4. Do the deviations from FEMA hydrology guidance and standards warrant an official community appeal? FEMA may provide additional data that supports their determinations for the hydrology and provide clarification for the deviations. A formal appeal to FEMA must be submitted within the 90-day appeal period which began on March 11th. The cost for producing a formal appeal would be equal to the cost for producing a letter of map revision (LOMR). CRS Engineers recommends making improvements on the alluvial fan including a debris basin, channel improvements, and culvert improvements. These improvements will help minimize the flood zone by providing capacity for the 100-year event and reduce the risk to community members. Upon completion of such improvements, a request for a LOMR should be submitted to FEMA to revise the flood delineation. Sincerely, CRS Engineers Ben Rood, PE, CFM Water Resources Manager cc John Miller Dan Drumiler Jeff Silvestrini 2020-0071 ---PAGE BREAK--- Neffs Creek Flood Hazard Assessment Technical Support Data Notebook March 2016 Prepared for I Utah Department of Emergency Management and AECOM Prepared byI 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 www.jefuller.com 03/02/2016 03/02/2016 ---PAGE BREAK--- i Table of Contents 1.0 INTRODUCTION 1 2.0 METHOD OVERVIEW 2 2.1. Stage 1 2 2.2. Stage 2 3 2.3. Stage 3 3 3.0 DATA SOURCES 4 3.1. NRCS Soils Mapping 4 3.2. Geologic Mapping 8 3.3. Aerial Photography 11 3.3.1. Modern Orthophotography 11 3.3.2. Historical Photography 11 3.4. Topographic Mapping 11 4.0 STAGE 1: RECOGNIZING AND CHARACTERIZING PIEDMONT LANDFORMS 15 4.1. Stage 1 Overview 15 4.2. Composition 15 4.2.1. Soils Mapping Interpretations 15 4.2.2. Surficial Geologic Mapping Interpretations 16 4.2.3. Field Observations 19 4.2.4. Summary 23 4.3. Morphology 23 4.3.1. Location 26 4.3.2. Boundaries 26 4.3.3. Previous Studies 26 4.3.4. Conclusion 27 5.0 STAGE 2: DEFINING ACTIVE VS. INACTIVE ALLUVIAL FAN FLOODING 29 5.1. Overview of Stage 2 Methodology Concepts 29 5.1.1. Age Relationships 30 5.2. Previous Studies 30 5.3. Summary of Stage 2 Analysis 30 6.0 SUMMARY AND RECOMMENDATIONS 32 7.0 STAGE 3 - DEFINING THE 100-YEAR FLOODPLAIN 33 7.1. Alluvial Fan Flood Hazard 33 7.2. Flowpath Uncertainty 33 7.3. Flowpath Uncertainty Modeling 34 7.4. Hydrologic Analysis 35 7.5. Hydraulic Analysis 36 7.5.1. FLO-2D Model Development 36 7.6. Floodplain Mapping 60 7.6.1. Development of Composite Velocities 60 7.7. Floodway Determination 62 7.8. Flood Hazard Profiles 62 8.0 64 ---PAGE BREAK--- ii List of Figures Figure 1. Vicinity map 1 Figure 2. NRCS soils mapping landforms 5 Figure 3. NRCS soils mapping 7 Figure 4. USGS Surficial Geologic Mapping (Van Horn, 1972) 9 Figure 5. USGS Surficial Geologic Mapping (Personius et al., 1992) 10 Figure 6. NAIP orthophotography 12 Figure 7. Historical aerial photograph comparison 13 Figure 8. Digital mapping data 14 Figure 9. Photographs of Neffs Creek diversion near the topographic apex 21 Figure 10. Flow bifurcations 22 Figure 11. Wasatch Range piedmont bajada within the project area 24 Figure 12. Piedmont percent slope 25 Figure 13. Stage 1 landform assessment 28 Figure 14. Stage 2 analysis map 31 Figure 15. FLO-2D model domain boundary 37 Figure 16. Manning's n-values used in the FLO-2D model 39 Figure 17. Typical vegetation density observed in the study area 40 Figure 18. Delineated building footprints 44 Figure 19. Inflow hydrograph location 45 Figure 20. Inflow hydrograph plot 46 Figure 21. Structure locations 47 Figure 22. Outflow grid locations 48 Figure 23. Flowpath uncertainty analysis virtual levees 49 Figure 24. Base condition FLO-2D model for maximum flow depth 51 Figure 25. Maximum flow depth results from the flowpath uncertainty scenario 1 model 52 Figure 26. Maximum flow depth results from the flowpath uncertainty scenario 2 model 53 Figure 27. Maximum flow depth results from the flowpath uncertainty scenario 3 model 54 Figure 28. Maximum flow depth results from the flowpath uncertainty scenario 4 model 55 Figure 29. Maximum flow depth results from the flowpath uncertainty scenario 5 model 56 Figure 30. Maximum flow depth results from the flowpath uncertainty scenario 6 model 57 Figure 31. Maximum flow depth results from the flowpath uncertainty scenario 7 model 58 Figure 32. Maximum flow depth results from the flowpath uncertainty composite model 59 Figure 33. Example of velocity raster clipped to floodplain boundary 61 Figure 34. Proposed revised floodplains 63 ---PAGE BREAK--- iii List of Tables Table 1. NRCS soil mapping descriptions 6 Table 2. Available USGS geologic maps 8 Table 3. USGS map unit descriptions 17 Table 4. Neffs Creek estimates discharges (from HAL, 2007) 35 Table 5. Manning's n-value assignments 38 Table 6. FLO-2D model simulation times 42 Table 7. Flowpath uncertainty scenario descriptions 50 Table 8. Summary of restudy and remapping efforts 60 Table 9. FEMA-based flood hazard designations associated with delineated floodplains 62 Appendices Appendix A. AGEC, 2005, Debris Flow Hazard Study Report, Neffs Canyon, Salt Lake County, Utah. Prepared for Hansen, Allen and Luce, Inc. Appendix B. Floodplain Workmaps Appendix C. Hansen, Allen & Luce (HAL), 2007, Neffs Canyon Creek Master Plan. Salt Lake County Appendix D. Annotated DFIRM Maps Appendix E. Digital Data Submittal ---PAGE BREAK--- 1 1.0 INTRODUCTION As part of the Jordan Watershed Risk MAP study for the Utah Division of Emergency Management (UDEM) and the Federal Emergency Management Agency (FEMA) Region VIII, JE Fuller/Hydrology & Geomorphology (JEF) was contracted by AECOM to conduct a geomorphic and flood hazard study for the Neffs Creek watershed located in Salt Lake County, Utah (Figure The effective FEMA regulatory floodplain for Neffs Creek inadequately depicts the flood hazards in this urban, highly developed area as evidenced by historical flooding. The purpose of this study was to assess the 100-year flooding hazard on Neffs Creek within the geomorphic context of the watershed landforms. Figure 1. Vicinity map ---PAGE BREAK--- 2 2.0 METHOD OVERVIEW The FEMA alluvial fan floodplain delineation methodology is based on a three stage process outlined in the National Research Council’s report, Alluvial Fan Flooding (NRC, 1996). The National Research Council (NRC) report describes a three stage method used to identify alluvial fan flood hazards, which was later adopted by FEMA and used in developing their Guidelines and Specifications for Flood Hazard Mapping Partners-Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping (FEMA, 2003), hereafter referred to as the FEMA Guidelines. The FEMA Guidelines describe the following three stage delineation process intended only for alluvial fan landforms:  Stage 1: Recognizing and Characterizing Alluvial Fan Landforms  Stage 2: Defining Active and Inactive Areas of Erosion and Deposition  Stage 3: Defining the 100-Year Floodplain (for Active Alluvial Fan Landforms) 2.1. Stage 1 Stage 1 of the FEMA alluvial fan methodology is the recognition and characterization of piedmont landforms. The intent of the Stage 1 analysis is to distinguish alluvial fan landforms from riverine, sheet flow, ponding, or coastal landforms.1 If the landform in question is identified as an alluvial fan landform, then the delineation may proceed using the FEMA Stage 2 and Stage 3 procedures. If the landform is not an alluvial fan landform, then other floodplain delineation procedures should be applied. The Stage 1 delineation relies on the following types of information:  Composition. Alluvial fans are composed of loose, unconsolidated materials transported by fluvial or debris flow processes “alluvium”).  Morphology. Alluvial fans have the shape of a partially or fully extended fan as observed on topographic maps or aerial photographs.  Location. Alluvial fans are usually found at a topographic break where stream channels become less confined than upstream of the break.  Boundaries. The boundary of an alluvial fan is called the “toe,” which is located at an axial stream, lake or landform not formed by alluvial fan flooding processes. The lateral boundaries of the fan are defined by a transition from alluvial fan flooding processes to riverine processes, although an alluvial fan may also coalesce into adjacent alluvial fans to form a bajada.2 1 FEMA Guidelines, p. G-6, 1st paragraph. 2 A bajada is a low-lying area of confluent pediment slopes and alluvial fans at the base of mountains around a desert. ---PAGE BREAK--- 3 Data sources for the Stage 1 assessment may include digital topography, National Resource Conservation Service (NRCS) soil surveys, geologic mapping, aerial photography, and hydrologic and hydraulic analyses. These data are used to differentiate piedmont landforms which may include mountains, alluvial fans, and riverine floodplains (both recent and geologically historical). Locations of topographic apices on the landform are also identified in Stage 1. The topographic apex is the extreme upstream extent of an alluvial fan landform, which is often located at the mountain front or within a mountain front embayment. Sudden expansion of flow at a topographic apex causes sediment deposition, uncertain flood flow paths, and uncertain flow distribution below the apex. The complex hydraulics associated with this flow expansion and sediment deposition create significant uncertainties (unpredictability) that "cannot be set aside in the realistic assessment of the flood hazard” (FEMA Guidelines), which is the defining characteristic for alluvial fan flooding. 2.2. Stage 2 Stage 2 of the FEMA alluvial fan methodology consists of defining active and inactive portions of an alluvial fan landform. The FEMA Guidelines define active areas as “that portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible”. Active areas on alluvial fans may experience active alluvial flooding defined by “flowpath uncertainty so great that the uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard” (FEMA Guidelines), or other types of flooding where uncertainty can be set aside in mitigating the hazard. Inactive alluvial fan areas are the portions of the alluvial fan where “flow paths with a higher degree of certainty in realistic assessments of flood risk or in the reliable mitigation of the hazard” (FEMA Guidelines) exist. According to the FEMA Guidelines, a Stage 2 delineation may be completed using a composite-based approach (integrate multiple methods into one result) if the alluvial fan has unique physical characteristics or varying levels of erosion and mitigation activity (Table G-1 in the FEMA Guidelines). The composite approach can utilize multiple methodologies (hydraulic analytical methods and geomorphic methods) to define the active and inactive areas of the fan landform. 2.3. Stage 3 Stage 3 of the FEMA alluvial fan methodology involves identifying the areas subject to flooding in a 100-year recurrence interval event. Stage 3 methodologies range from probabilistic models such as the FEMA FAN model, to a combination of deterministic models (e.g. two-dimensional hydraulic models) combined with geomorphic interpretations. For this study, a composite of hydraulic modeling and geomorphic methods were used of the topographic apex across the piedmont surface. ---PAGE BREAK--- 4 3.0 DATA SOURCES Using the geomorphic approach, surficial stability characteristics were compiled for this analysis and evaluated from the following sources:  Detailed Soils Mapping. Natural Resource Conservation Service (NRCS) soils maps describe soil composition, soil depth, as well as provide some degree of landform interpretation.  Surficial Geologic Mapping. The United States Geological Survey (USGS) completed surficial geologic mapping for project area between 1963 and 1965, prior to much of the present development. The USGS map indicates relative surface age and landform type.  Topographic Mapping. Digital Light Detection and Ranging (LiDAR) mapping (collected 2013) was provided by Salt Lake County and used to assess the surface profile, crenulation index (degree of incision), landform shape, and slope. Topography was also used to help define landform boundaries.  Vegetation. Vegetation patterns can be used to identify flow paths or areas of more frequent inundation (dense vegetation), sheet flow (uniform vegetation), the degree of soil development, soil material, surface age, and surface boundaries vegetation suites change with soil types and landform).  Drainage Pattern. Inactive fans tend to have tributary drainage patterns with well- defined divides. Active fans tend to have distributary drainage patterns with poorly defined divides and/or perched flow paths. 3.1. NRCS Soils Mapping The soils data used in this study were derived from the NRCS Soil Survey Geographic (SSURGO) digital soils database for the Salt Lake Area, UT (ut612) and Summit Area, UT (ut613). These detailed soil surveys were developed for use by land planners, farmers, ranchers, agronomists, rangeland managers, community officials, geologists, engineers, developers, builders, home buyers, and watershed and wildlife managers. Figure 2 shows the soil units found within the project area. Landform interpretation information was extracted from the NRCS database and is shown in Table 1. Using the NRCS soils landform information is a valuable first step in the Stage 1 analysis (differentiating alluvial fan landforms from non-fan landforms). Soil descriptions for all soils found within the project area are listed in Table 1 and shown in Figure 3. A more detailed discussion of the soils is included in the Stage 1 analysis (Section 4.0). ---PAGE BREAK--- 5 Figure 2. NRCS soils mapping landforms ---PAGE BREAK--- 6 Table 1. NRCS soil mapping descriptions Map Symbol Soil Description Landform Interpretation 101 Agassiz-Rock outcrop complex, 30 to 70 percent slopes Mountain 133 Fewkes-Hades complex, 30 to 60 percent slopes Mountain 136 Hades-Agassiz-Rock outcrop complex, 30 to 70 percent slopes Mountain 144 Horrocks-Cutoff complex, 15 to 30 percent slopes Mountain 179 Wanship-Kovich loams, 0 to 3 percent slopes Terrace BEG Bradshaw-Agassiz association, steep Mountain BhB Bingham gravelly loam, 3 to 6 percent slopes Lake Terrace EMG Emigration very cobbly loam, 40 to 70 percent slopes Mountain GGG Gappmayer-Wallsburg association, very steep Mountain HHF Harkers soils, 6 to 40 percent slopes Alluvial Fan HtF2 Hillfield-Taylorsville complex, 6 to 30 percent slopes Lake Terrace HWF Horrocks extremely stony loam, 5 to 50 percent slopes Mountain KnA Knutsen coarse sandy loam, 1 to 3 percent slopes Lake Terrace SC Sandy terrace escarpments Floodplain SP Stony terrace escarpments Lake Terrace St Stony alluvial land Floodplain ---PAGE BREAK--- 7 Figure 3. NRCS soils mapping ---PAGE BREAK--- 8 3.2. Geologic Mapping The USGS has published surficial and bedrock geologic mapping within the project area as listed in Table 2. Surficial mapping information is invaluable when conducting landform geomorphic investigations. Surficial mapping correlates relative ages of surfaces and helps identify the relative stability of surfaces with respect to flooding potential. Figure 4 shows a 1972 USGS surficial geologic map for the project area and Figure 5 shows a 1992 surficial geologic map. The geologic units in both maps are grouped by geologic composition (alluvium vs. bedrock) and landform type (stream deposit, lake deposit, etc.) which is relevant to the Stage 1 analysis (Section 4.0). Individual mapped unit descriptions are included in Section 4.0. The USGS mapping was the primary data source for determining the active vs. inactive alluvial fan surfaces (Stage As such, the geologic mapping is discussed in more detail in the Stage 2 analysis (Section 5.0). Table 2. Available USGS geologic maps Map Name Map Format Scale Year Author Surficial Geologic Map of the Sugar House Quadrangle, Salt Lake County, UT Raster 1:24,00 0 1972 Van Horn, R. Surficial Geologic Map of the Salt Lake City Segment and Parts of Adjacent Segments of the Wasatch Fault Zone, Davis, Salt Lake, and Utah Counties, Utah. Raster 1:50,00 0 1992 Personius, S.F., and W.E. Scott ---PAGE BREAK--- 9 Figure 4. USGS Surficial Geologic Mapping (Van Horn, 1972) ---PAGE BREAK--- 10 Figure 5. USGS Surficial Geologic Mapping (Personius et al., 1992) ---PAGE BREAK--- 11 3.3. Aerial Photography 3.3.1. Modern Orthophotography National Agriculture Imagery Program (NAIP) orthophotography was used for this analysis. NAIP acquires aerial imagery during the agricultural growing seasons in the continental U.S. A primary goal of the NAIP program is to make digital orthophotography available to governmental agencies and the public within a year of acquisition. This analysis used 2014 NAIP orthophotography at a resolution of 1-meter/pixel (Figure 3.3.2. Historical Photography Historical photographs from 1950 and 1962 were collected and semi-rectified using Geographic Information System (GIS) software tools (Figure The study area is highly urbanized which makes landform identification difficult. Historical photographs that pre-date major development are invaluable when conducting geomorphic investigations. 3.4. Topographic Mapping The primary mapping source used in this analysis was digital LiDAR data provided by Salt Lake County. The primary purpose of LiDAR data was to provide a source dataset for geospatial analysis and mapping, and the production of high resolution LiDAR derived products such as digital elevation models (DEMs). These classified LiDAR point cloud data were used to create 3D breaklines, hydro-flattened bare earth DEMs, and highest hit DEMs. The LiDAR was collected between November and December 2013. Figure 8 shows the mapping data as both a digital surface and as 10-foot contours. ---PAGE BREAK--- 12 Figure 6. NAIP orthophotography ---PAGE BREAK--- 13 Figure 7. Historical aerial photograph comparison ---PAGE BREAK--- 14 Figure 8. Digital mapping data ---PAGE BREAK--- 15 4.0 STAGE 1: RECOGNIZING AND CHARACTERIZING PIEDMONT LANDFORMS 4.1. Stage 1 Overview A Stage 1 alluvial fan delineation was performed for the Neffs Creek project area. Neffs Creek canyon is cut into the western slope of the Wasatch Range within the vicinity of the Mount Olympus Wilderness. The Neffs Canyon headwaters are at approximately 9,500 feet with the canyon mouth at approximately 5,600 feet. The transition from mountain to piedmont is abrupt which is common along much of the western Wasatch Range. Prior to the 1950s, only sparse agricultural development was present on the piedmont. By 1962 urbanization had begun to work its way up the slope toward the mountain front, and by the early 1970s the piedmont was entirely urbanized as it remains today (see Figure 6 and Figure 4.2. Composition One of the FEMA Guidelines criteria for defining an alluvial fan landform includes composition. Alluvial fans are composed of loose, unconsolidated materials transported by fluvial or debris flow processes “alluvium”). 4.2.1. Soils Mapping Interpretations Table 1 gives a list and description of the NRCS soil units within the project area and includes the landform classification as found within the soil unit description. The NRCS soils mapping indicates most of the piedmont is composed of unit HWF (Horrocks extremely stony loam) with the lateral limit areas composed of HHF (Harkers soils). The soil profile of HWF is cobbly clay loam to a depth of 20 inches, extremely stony sandy loam from 29 inches to 40 inches, and bedrock below 40 inches. It should be noted that this profile is not typical of active alluvial fan surfaces. By definition, alluvial fans are an aggrading landform and thus are generally composed of thick (10s of feet) layers of unconsolidated alluvium. The alluvial composition of active alluvial fans usually result in a soil profile that is characterized by moderate to high rates of precipitation infiltration. The NRCS has developed a series of Hydrologic Soil Groups (HSG) based soil runoff and infiltration characteristics. HSG-A has the highest infiltration rates while HSG-D has the lowest. HSG-D is generally characterized by high percentages of clay and less than 50% sand, which is atypical of active alluvial fans. Alluvial fans are generally classified as HSG-B or HSG-C. Unit HWF is classified by the NRCS as a mountain slope landform with HSG-D. The relatively thin soil profile of HWF (40 inches) and the HSG-D classification suggests a pediment landform rather than an alluvial fan landform. A pediment is defined as a broadly sloping erosional surface located at the base of a mountain front. The key difference between a pediment and an alluvial fan is a pediment is erosional and an alluvial fan is depositional. Both features are composed of alluvium which is the minimum standard in the FEMA Guidelines in defining an alluvial fan landform. ---PAGE BREAK--- 16 Unit HHF which is found along the lateral margins of the piedmont is characterized by the NRCS as an alluvial fan landform with a thick alluvial soil profile and a HSG-C classification. This soil description is more typical of what generally defines an alluvial fan landform. The HWF soil unit is truncated by unit SP (Stony terrace escarpments). This soil unit is composed of lacustrine sediments from Lake Bonneville. Lake Bonneville, a prehistoric pluvial lake3, covered much of northern Utah between approximately 32,000 years BP4 and 14,500 years BP. Lake Bonneville shoreline evidence is present within the piedmont area and is explained in more detail in the following section. Unit SP is composed of alluvium but is not deposited by alluvial fan flooding processes. The key fact derived from the NRCS soils mapping with respect to Stage 1 are that the piedmont area is underlain by alluvium that was derived from the Neffs Creek watershed. 4.2.2. Surficial Geologic Mapping Interpretations Figure 4 and Figure 5 show the USGS surficial geologic mapping for the study area. The figures show the entire piedmont project area is composed of alluvium of either Pleistocene or Holocene in age. Complete descriptions of the surficial geologic units that are provided on the maps are included in Table 3. The importance of the geologic mapping with respect to Stage 1 is to differentiate the alluvial (piedmont units) landform from the non-alluvial (bedrock) and riverine (floodplain) landforms. This differentiation separates the alluvial fan from the non-alluvial fan landforms. The USGS mapped units are described below in order of age youngest to oldest per map: 3 Pluvial Lake is defined as a closed basin that filled with water during times of glacial climatic conditions. 4 BP = before present ---PAGE BREAK--- 17 Table 3. USGS map unit descriptions Map Label Unit Description Unit Type Age Van Horn, 1972 fa Floodplain alluvium. Cobbly to silty sand, dark-gray at top grading downward to medium- to light-gray sandy to cobbly gravel and sand in lower part; locally bouldery near mountain front; more than 5 feet thick. Riverine Floodplain Late Holocene fg6-fg5 Bouldery to sandy silt at low altitudes and boulder to silty gravel and sand at high altitudes; stones angular to subrounded; dark gray to moderate brown; as much as 20 feet thick. Locally overlies, and at places grades laterally into, lake gravel. Relative age indicated by number. Undifferentiated fan deposits younger than the Bonneville shoreline. All units are subject to sudden and violent flash floods and mudflows. Fan Deposit Early to Mid- Holocene fg4 Fan Deposit Late Pleistocene fg3-fg1 Old undifferentiated alluvial fan deposits older than the Bonneville shoreline. Relative age indicated by number. Units fg2 and fg3 are subject to sudden and violent flash floods and mudflows. Fan Deposit Early to Mid- Pleistocene bs Sand, fine to coarse, silty, light- brown to light-gray, 5-10 feet thick. Deposited in a lake, probably near shore. Lacustrine Deposit (Lake Bonneville) Mid- to Late Pleistocene bgo Gravel and sand, locally cobbly, gray-to brownish-gray; rounded stones 5-20 feet thick. Locally has a weakly to moderately developed soil formed on it. Deposited as a lakeshore embankment at the Bonneville shoreline. Lacustrine Deposit (Lake Bonneville) Mid- to Late Pleistocene ag Gravel unit. Cobbly gravel and sand, medium- to light-bluish gray; rounded stones; more than 20 feet thick. Boulders commonly present near base. Upper 10-15 feet commonly moderately to weakly cemented by calcium carbonate. Deposited as a lakeshore embankment at about 5,130 feet above sea level. Lacustrine Deposit (Alpine Formation) Late Pleistocene to Early Holocene fgo Old undifferentiated alluvial fan that predates the Bonneville shoreline. Fan Deposit Pleistocene ---PAGE BREAK--- 18 Table 3. USGS map unit descriptions Map Label Unit Description Unit Type Age ldm Deposited at the mouth of Neffs Canyon by slow to rapid downslope movement of material forming the slope. Mudflow Deposit Quaternary r Bedrock Bedrock Jurassic to Precambrian Personius et al., 1992 al1 Stream alluvium. Sand, silt, and minor clay and gravel along Jordan River and lower reaches of its tributaries. Forms floodplain and terraces less than 5m above modern stream level. Stream Deposit Upper Holocene af2 Fan alluvium. Clast-supported pebble and cobble gravel, locally bouldery, in a matrix of sand and silty sand; poorly sorted; clasts subangular to round. Deposited in perennial and intermittent streams, debris flows, and debris floods graded to modern stream level. Fan Deposit Middle Holocene to Uppermost Pleistocene afb Fan alluvium related to transgressive phase. Clast-supported pebble and cobble gravel, locally bouldery, in a matrix of sand and silty sand; poorly sorted; clasts subangular to round. Deposited by streams graded to shorelines of the transgressive phase of the Bonneville lake cycle, and forms fans graded to theses shorelines. Fan Deposit Upper Pleistocene lbg Lacustrine sand and gravel related to transgressive phase. Clast-supported pebble, cobble, and rarely boulder gravel, in a matrix of sand and pebbly sand; locally includes interbedded silt and clay ranging from thin beds and lenses to lagoonal deposits as much as 10m thick. Deposited in beaches, bars, spits, and small deltas and lagoons. Commonly covered by deposits of hillslope colluvium, but typically forms wave-built bench at the Bonneville shoreline and at several less well developed beach berms between the Provo and Bonneville shorelines. Lacustrine Deposit (Lake Bonneville) Upper Pleistocene ---PAGE BREAK--- 19 Table 3. USGS map unit descriptions Map Label Unit Description Unit Type Age af4 Fan alluvium. Clast-supported pebble and cobble gravel, locally bouldery, in a matrix of sand and silty sand; poorly sorted; clasts subangular to round. Forms small fans and fan remnants topographically above or cut by the Bonneville shoreline. Correlative deposits probably underlie much of the map area and are buried by younger deposits downslope from the Bonneville shoreline. Fan Deposit Upper Middle Pleistocene cls Landslide deposits. Grain size and texture character of deposits in source area; usually unsorted, unstratified. Deposited as slides and on relatively steep slopes in mountains. Colluvial Deposit Holocene to Middle Pleistocene The surficial geologic mapping indicates four basic landform types are found within the vicinity of Neffs Creek: Piedmont (fan, mudflow, and colluvial deposits), Riverine (floodplain deposits), Lake (lacustrine deposits), and Bedrock. Units fg6 though fg1 (Van Horn, 1972) and units af2, af4, and afb (Personius et al., 1992) are identified specifically as alluvial fan deposits on their subsequent surficial geologic maps. 4.2.3. Field Observations A field visit was conducted on March 10, 2015 and consisted of walking and driving portions of the study area and collecting field photographs. A significant amount of time was spent within the area of the topographic apex to observe and interpret the existing conditions morphology. A man-made ditch was observed near the topographic apex that appeared to be constructed to divert low-flow from the creek (Figure There was no streamflow in either in the main channel or the ditch during the field visit. The ditch diverts flow away from the main channel which is topographically lower and steeper than the ditch. The right bank of the ditch is comprised of a boulder levee between two to three feet in height (Figure The flow capacity of the ditch is significantly less than the main channel. Field evidence indicated the boulder levee had been breached in the recent past (likely due to overtopping) near the diversion point. The ditch diverts flow away from the historical Neffs Creek channel and into a canal system that presently drains to the I-215 highway embankment. The 1950 historical aerial photograph shows the canal making a 90 degree bend near the present alignment of Fortuna Way and draining across a series of agricultural fields. This suggests the canal was constructed to divert Neffs Creek flows for irrigation. There are 10 present road crossings with culverts along the canal system. Each culvert was field verified and their openings were measured during the March 10th visit. That collected data was later used in the hydraulic modeling effort. ---PAGE BREAK--- 20 The historical photographs indicate many flow bifurcations of the topographic apex. The identification of bifurcations was challenging due to the dense vegetation present in the historical photographs. Although the landform has been substantially altered by the construction of roads and structures, many of the bifurcations are still active as was observed during the field visit. Roads and other structures have changed the relative distribution of flow across the surface, but the hydraulic modeling analysis (discussed later in this report) indicated many of the historical bifurcations are still active during large floods. Bifurcations identified from the 1950 aerial photograph are shown in Figure 10 (also plotted against the 2014 orthophotography). ---PAGE BREAK--- 21 Figure 9. Photographs of Neffs Creek diversion near the topographic apex Boulder levee breach Diversion ditch Diversion ditch Boulder levee ---PAGE BREAK--- 22 Figure 10. Flow bifurcations ---PAGE BREAK--- 23 4.2.4. Summary The NRCS soils mapping, USGS surficial geologic mapping, and field observations all report similar findings regarding the alluvial composition of the Wasatch Range piedmont within the vicinity of Neffs Creek. Therefore, it is concluded that the piedmont is composed of non- consolidated alluvium deposited by fluvial processes, which meets the composition criteria specified in the FEMA Guidelines to classify the surface as an alluvial fan landform. 4.3. Morphology According to the National Research Council definition (1996), “alluvial fans are landforms that have the shape of a fan, either partly or fully extended.” The Wasatch Range piedmont within the project area consists of a series of coalescing landforms each with the shape of a partially extended alluvial fan. These coalescing alluvial fans comprise a bajada which also shows a somewhat distorted, partially extended fan shape which is readily visible on the historical USGS topographic map that pre-dates most of the urbanization of the piedmont (map date: 1952). The USGS map shows smooth contour crenulations and radial lines indicating the degree of fan incision and channel confinement, but uniformly depict a fan shape (Figure 11). Contour radial lines that curve in the direction are indicative of alluvial fan landforms. Another morphologic feature which supports identifying the piedmont as an alluvial fan landform is the slope. Alluvial fan landforms represent the transition from the steep mountain slopes to the flatter axial valley streams. An analysis of the Wasatch Range piedmont slope indicates a sharp transition from very steep in the mountain to between 10% and 20% on the piedmont. The slope transition also indicates a general fan shape of the piedmont (Figure 12). The topographic break at the mountain-piedmont transition is the topographic apex of the alluvial fan. Based on the analysis of the topographic and morphologic data, it is concluded that the shape of the Neffs Creek piedmont meets the FEMA Guidelines definition of an alluvial fan landform. ---PAGE BREAK--- 24 Figure 11. Wasatch Range piedmont bajada within the project area ---PAGE BREAK--- 25 Figure 12. Piedmont percent slope ---PAGE BREAK--- 26 4.3.1. Location The NRC (1996) definition of an alluvial fan landform states that “alluvial fan landforms are located at a topographic break where long-term channel migration and sediment accumulation become markedly less confined than upstream of the break.” The piedmont abuts the steep mountain front of the Wasatch Range as indicated by the abrupt change in slope in Figure 12. The mountain front is deeply embayed, which reflects the age and long erosion history of the mountains and creates a linear upstream boundary at the topographic break. At the mountain front, the fluvial environment transitions to one of deposition as indicated by the contour lines (see Figure 11). 4.3.2. Boundaries The upstream and lateral limits of the piedmont within the project area are defined by the Wasatch Range mountain front, as indicated by the topographic break described previously. The limits of the piedmont were determined from examination of the following:  NRCS soils. Transition from NRCS interpreted landforms (mountain and alluvial fan to lake terrace).  USGS surficial geologic mapping. Transition from alluvial stream deposits to lake and river deposits.  Slope. Transition from steeper slopes (8%-10% = piedmont landform) to shallower slopes = lake and riverine floodplain landforms).  Aerial photograph interpretation 4.3.3. Previous Studies The Utah Geological and Mineral Survey (UGMS) conducted a study in 1974 titled Mt. Olympus Cove Environmental Geology Study. The primary purpose of the study was to address the concerns of the then Salt Lake County Planning Commission on the environmental factors that might have bearing on the future course of development in the Mt. Olympus Cove area. The study was also intended to provide a template for similar assessments in a broader context of the Wasatch Front. The following is an excerpt from the study: The cove itself is largely an alluvial fan. In the northeast the alluvium of the fan abuts against bedrock with a more or less clear break in slope at the contact, but in the southeast the break in slope is not well defined. The alluvium in the fan consists of intercalated muds, sands, and gravels of great thickness. A complex interfingering exists at depth with better sorted and stratified silts, sands, and gravels that were deposited in Lake Bonneville through the course of multiple regressions and incursions of its shoreline. p.3. The Mt. Olympus Cove area is referred to as an alluvial fan in several more instances within the 1974 study report. ---PAGE BREAK--- 27 4.3.4. Conclusion The NRCS soil mapping, USGS surficial geologic mapping, and field observations clearly show that the Wasatch Range piedmont within the vicinity of Neffs Creek is composed of sedimentary deposits (alluvium). The topographic mapping shows that the piedmont landform is located at the base of a mountain front and has the shape of a partially extended fan, has steep slopes, and radiating contours. Morphologic data, such as the drainage pattern, surface distribution, relief, and channel geometry, are also characteristic of an alluvial fan landform. Finally, the 1974 UGMS study described the Mt. Olympus Cover area as an alluvial fan. From these sources it can be concluded that the Wasatch Range piedmont is an alluvial fan landform and that the FEMA Guidelines for applicability of a Stage 2 assessment apply. Figure 13 shows the Stage 1 landform analysis map. ---PAGE BREAK--- 28 Figure 13. Stage 1 landform assessment ---PAGE BREAK--- 29 5.0 STAGE 2: DEFINING ACTIVE VS. INACTIVE ALLUVIAL FAN FLOODING Stage 2 of the FEMA alluvial fan methodology consists of defining active and inactive areas within specific portions of the Wasatch Range piedmont alluvial fan landform, as well as characterizing the nature and types of flooding throughout the landform. Active areas on an alluvial fan consist of those portions of the landform where deposition, erosion, and unstable flow paths are possible. Active areas can experience active alluvial fan flooding (flowpath uncertainty so great that the uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard), in addition to other types of flooding. Inactive alluvial fan areas are the portions of the alluvial fan landform where active fan processes do not occur, but are still subject to flooding hazards. 5.1. Overview of Stage 2 Methodology Concepts The physical characteristics of a landform provide clues as to its depositional history, existing level of stability, and future flood potential. If a portion of the landform becomes isolated from its original watershed and watercourse, it ceases to receive new deposits and its surface will begin to age and develop specific physical characteristics indicative of its age. Landform surfaces free from new deposition will also begin to erode due to direct rainfall and the ensuing runoff on the surface. As the surface erodes, new tributary channel networks develop which become more incised and integrated with time. The channels gradually deepen and widen, creating a greater degree of relief between the channel bottoms and the ridges which separate them. The degree of relief can be directly observed in the field or on aerial photographs, but can also be detected by examining the crenulation (curviness) of topographic map contours. The USGS surficial geology mapping, and to a lesser extent the NRCS soils mapping, differentiate surfaces based on the types of geomorphic characteristics discussed previously. Therefore, the map data also provide information about surface age, stability, and flood potential. Young surfaces are likely to continue to experience flood inundation, sediment deposition, and channel movement. Older surfaces are unlikely to experience such processes, or will experience such processes at a much lower magnitude. Older surfaces tend to be more stable because their soils are more resistant due to the cohesion provided by accumulations of clay, and calcium carbonate as well as due to containment of flow within defined, vegetation-lined channels. That is, the likelihood of the channel changing its location over time is greatly diminished. Conversely, areas with non-cohesive, coarse soil materials and little lateral relief are more susceptible to lateral changes in channel position. The USGS mapping indicates the Wasatch Range piedmont within the project area is composed of Pleistocene and Holocene-age surfaces. The surfaces increase in age moving south along the mountain front from Neffs Creek. The piedmont surface associated with Neffs Creek is the youngest unit (fg5) in the area and is early Holocene in age. The mapping also indicates the fg5 unit overlays the Bonneville Lake sediments indicating active deposition from Neffs Creek has occurred since the recession of the lake. The piedmont surface units south of Neffs Creek (fg3, fg2) are older than fg5, however their description indicates they ---PAGE BREAK--- 30 are subject to “sudden and violent flash floods and mudflows” which are characteristic of active alluvial fan flooding. 5.1.1. Age Relationships The surficial age of the areas of the Neffs Creek portion of the alluvial fan landform identified as active range between 1.8 million years before the present (Pleistocene) to the present (late-Holocene). The areas identified as inactive can be generally described as bedrock and range from 490 million years before present (Cambrian) to 248 million years before present (Permian). 5.2. Previous Studies A debris flow hazard study for Neffs Creek Canyon was conducted in 2005 (AGEC, 2005). The purpose of the study was to assess the debris flow hazard potential for Neffs Canyon as it related to existing development on the piedmont below the canyon mouth. The study included an assessment of aerial photographs to map the extent of the alluvial fan landform from the topographic apex, specifically looking for distinct debris flow indicators. Their study did not identify discrete debris flow lobes, but their interpretation was that the irregular extent of the distal boundary of the fan suggests a series of discrete flows with variable run-out distances. It was also noted that the fan surface overlies the Lake Bonneville deposits indicating deposition has occurred on the surface within the last 15,000 years. The overall conclusion of the study was that Neffs Canyon is subject to potential debris flows that could reach the alluvial fan. This conclusion suggests that the alluvial fan is potentially subject to active alluvial fan flooding processes. The AGEC report is included in Appendix A. 5.3. Summary of Stage 2 Analysis Figure 14 shows the limits of the Stage 2 analysis results within the study area. Analysis of all the pertinent data including soils mapping, geologic mapping, topographic mapping, aerial photography, field observations, and previous studies indicate the Neffs Creek piedmont within the study area can be classified as an active alluvial fan per the FEMA Guidelines. The active alluvial fan landform comprises the entire piedmont area that is composed of alluvial sediments derived from erosion of the upper canyon above the topographic apex. ---PAGE BREAK--- 31 Figure 14. Stage 2 analysis map ---PAGE BREAK--- 32 6.0 SUMMARY AND RECOMMENDATIONS The Neffs Creek study area is composed of two primary landforms, 1) Mountains; and 2) Piedmont. A FEMA Appendix G Guidelines assessment was conducted to determine whether the piedmont area could be characterized as an alluvial fan landform (Stage The results of the Stage 1 analysis concluded that the piedmont is an alluvial fan landform, thus necessitating a Stage 2 analysis. The Stage 2 analysis resulted in the findings that the piedmont is subject to active alluvial fan flooding. Based on the results of this analysis a Stage 3 (alluvial fan floodplain delineation) assessment is appropriate. ---PAGE BREAK--- 33 7.0 STAGE 3 - DEFINING THE 100-YEAR FLOODPLAIN The 100-year flood hazard assessment is an outgrowth of the information and results identified and generated in Stages 1 and 2. In Stage 1, portions of the project area were identified as part of an alluvial fan landform. In Stage 2, the active portions of the alluvial fan landform were identified. According to the FEMA Guidelines, “the delineated flood prone areas of Stage 2 should approximate the largest possible extent of the 100-year flood.” In Stage 3, floodplain limits for the 100-year annual chance) flood are delineated for the active alluvial fan areas characterized by:  Active Alluvial Fan Flooding. Flowpath uncertainty so great that the uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard. The floodplain in the areas with unstable flowpath flooding of the hydrographic apices were delineated using geomorphic data in conjunction with the Maximum flood hazard hydraulic modeling results. The Stage 3, 100-year floodplain delineations were incorporated into the Flood Insurance Rate Map (FIRM) Zone delineations described later in this report and are shown on the Floodplain Workmaps included in Appendix B. 7.1. Alluvial Fan Flood Hazard The FEMA Guidelines state that active alluvial fan flooding hazard is indicated by the following three criteria: 1. Flowpath uncertainty below the hydrographic apex. 2. Abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its ability to carry material eroded from a steeper, upstream source. 3. An environment where the combination of sediment availability, slope, and topography creates an ultrahazardous condition for which elevation on fill will not reliably mitigate the risk. The Neffs Creek piedmont exhibits these three criteria within limited portions of the active alluvial fan areas. One of the fundamental challenges with delineating a regulatory 1-percent annual chance floodplain on an active alluvial fan is addressing the actual hazard on any portion of the fan when, by definition, the landform is susceptible to changes both during and following a flood event. The most hazardous areas of an active alluvial fan are generally found near the apex with the flooding and sedimentation hazard generally decreasing in the direction. This is the situation found within the project area. 7.2. Flowpath Uncertainty An avulsion is the process by which flow is diverted out of an established channel into a new course on the adjacent floodplain (Slingerland & Smith, 2004). Avulsions divert flow from one channel into another, leading to a total or partial abandonment of the previous channel (Field, 2001; Bryant et. al., 1995), or may involve simple flowpath shifts in a braided or sheet flooding system (Slingerland & Smith, 2004). Avulsions are commonly associated with alluvial ---PAGE BREAK--- 34 fan flooding, but are also known to occur on riverine systems and river deltas (Slingerland & Smith, 2004). The occurrence of avulsions is what makes an alluvial fan “active.” Avulsions give the alluvial fan the ability to distribute water and sediment over the surface of the landform, which results in the radial “fan” shape. Avulsions influence flood hazards on an alluvial fan landform by changing the location, concentration and severity of flooding on the fan surface. That is, an area not previously inundated by flooding (or inundated only by shallow flow) may in a subsequent flood become the locus of flood inundation, sediment deposition, and/or erosion. If an alluvial fan has no risk of avulsion, flood hazard delineation and mitigation become much simpler engineering problems, consisting only of modeling two-dimensional flow and/or normal riverine hydraulic and sedimentation issues. The occurrence of major avulsions in an alluvial fan drainage system introduces the following complications into an engineering analysis of the flood hazard:  Uncertain and changing flowpath locations, during and between floods  Continually changing channel and overbank flowpath topography  Inundation and/or sedimentation hazards in previously un-flooded areas  Uncertain and changing flow rate distribution for areas of avulsions  Uncertain and changing watershed boundaries for areas of avulsions  Aggrading, net depositional land surfaces and channels with diminishing capacity  Unsteady, rapidly-varied flow conditions  High rates of infiltration and flow attenuation across the fan surface The flowpath uncertainty issue was addressed in this analysis by the use of the Maximum flood hazard two-dimensional hydraulic modeling results. Flowpath uncertainty is caused by abrupt channel avulsions that occur during flood events. The cause of the channel avulsion can vary from channel aggradation (sedimentation) causing a rapid lateral shift in channel position, to overbank flooding carving a new channel, to upstream headcutting resulting in channel piracy. Regardless of the cause, the resulting abrupt change in channel position is something that is generally unpredictable and uncertain. The flowpath uncertainty analysis methodology addresses the channel avulsion potential element of the hazard analysis. 7.3. Flowpath Uncertainty Modeling The overall objective of flowpath uncertainty modeling was to force flooding in directions that would simulate avulsions, and to estimate maximum depths and velocities over the whole radial width of the Neffs Creek active alluvial fan area by modeling a series of “virtual” levees. The number, geometry, and alignment of the virtual levees were selected to achieve those objectives. Each virtual levee scenario was optimized to direct flow from a bifurcation point to a different area across the width of the alluvial fan. Given the coalescing nature of the alluvial fans, there are multiple scenarios. ---PAGE BREAK--- 35 The following criteria were considered when developing the virtual levees for the Neffs Creek flowpath uncertainty analysis:  Levee Length. The virtual levee varied at each location. The were determined based on engineering judgment to achieve the objective of concentrating flows to various target locations  Number of Levee Scenarios. The number of virtual levee scenarios modeled were dependent on the surface morphology and the target objectives.  Alignment. The virtual levees were aligned at moderate angles to the fan axis so that they did not cause a significant “pile up” of flow in the model results.  Coding. The virtual levees were coded into the model as to not overtop or fail during the model simulations.  Model Iteration. Multiple modeling integrations were performed to meet the target area objectives. Several virtual levees scenarios can be run within the same hydraulic model if the model results indicate there is no hydrologic inter-mixing of the two scenario results of the virtual levees.  Final Hazard Delineation. The maximum depth and velocity at each model grid cell from the maximum flood hazard modeling results were used as the final regulatory flood depth and velocity. In other words, the maximum flow depth at each grid cell was computed using the highest depth value considering all the scenarios. This approach was applied to all the grid cells in the model. Additional details on this maximum “composite” approach to the hydraulic modeling results are discussed in the Model Development section of this report.  Conservative Results. The virtual levee scenario employed for this analysis produces conservative flood depth and velocity results, particularly given the (probable) low frequency of avulsion on fans in Utah, as well as the fact that actual avulsions do not completely divert the entire hydrograph along a particular alignment. 7.4. Hydrologic Analysis Hydrology used in the analysis was derived from a previous study commissioned and approved by Salt Lake County. The Neffs Canyon Creek Master Plan was completed in 2007 and included a complete hydrologic analysis for the 10- and 100-year recurrence interval storm events. Two concentration points were identified and summarized in the analysis: Table 4. Neffs Creek estimates discharges (from HAL, 2007) Location Predicted Rainstorm Runoff Flow Rates (cfs) 10-year 100-year Canyon Mouth 70 300 Wasatch Blvd. 90 350 ---PAGE BREAK--- 36 The discharge estimate at the Canyon Mouth is equivalent to the location of the topographic apex for this study. The hydrology for the 2007 study was the approved hydrology for Neffs Creek at the time of this study, thus was adopted as the inflow at the topographic apex. The 2007 study is included in entirety in Appendix C. 7.5. Hydraulic Analysis Hydraulic analyses performed for the Neffs Creek study was completed using the software package - FLO-2D. FLO-2D is a dynamic two-dimensional hydrologic and hydraulic model that conserves volume as it routes hydrographs over a system of square grid elements. The model routes runoff over the grid using the dynamic wave momentum equation and a central finite difference routing scheme. The floodwave progression is affected by the surface topography and roughness values (Manning’s n-values) associated with land use characteristics. The FLO- 2D version used for this study is Version 2009.06 Build No. 09-13.05.13, the executable is dated October 29, 2013. FLO-2D model development was based on the HAL (2007) 100-year hydrograph at the topographic apex. A total of seven FLO-2D models were run to estimate the 100-year composite flood hazard accounting for the flowpath uncertainty scenarios as described previously. The model naming convention is based on the flowpath uncertainty scenario number. 7.5.1. FLO-2D Model Development 7.5.1.1. Model Domain Development The scope of this project was to consider the flooding impact from Neffs Creek. Although areas outside of Neffs Creek flood inundation limits were mapped as part of the Stage 2 analysis (see Figure 14), the focus of the FLO-2D model development was the Neffs Creek flood inundation area only. The model domain boundary was selected so as to include the area of potential flow from Neffs Creek on the alluvial fan without including much excess area that would significantly increase the size of the model. The domain was developed iteratively by creating a model that was well in-excess of the Neffs Creek flood inundation area. That initial model domain was then modified to exclude significant areas that were not inundated by flows from Neffs Creek. The final selected domain boundary is approximately 0.80 square miles. The domain boundary is Wasatch Blvd. and was selected by a mutual decision between AECOM, JEF, and Salt Lake County. The model domain is shown in Figure 15. It should be noted that areas outside of the model domain may be subject to potential flood hazards from sources outside of Neffs Creek. ---PAGE BREAK--- 37 Figure 15. FLO-2D model domain boundary ---PAGE BREAK--- 38 7.5.1.2. Model Grid Size Development The watershed surface is represented in FLO-2D as a grid comprised of square elements. The size of the individual grid element is critical to the desired detail of model output and floodplain delineations – the smaller the grid element size, the more defined the model. For example, since the grid element elevation is averaged from the topographic data, large grid elements provide less topographic detail when compared to smaller grid elements. However, although a smaller grid element will provide more detail of the topographic surface data, model run time is significantly impacted by the number of grid elements. A practical grid element size should be selected to achieve the desired detail of the modeling effort while taking into consideration the model run time (number of grid elements). The grid element size selected for this study measures 10’x10’. The total number of grid elements for each FLO- 2D model is 223,343. 7.5.1.3. Model Grid Elevation Development Grid elements measuring 10’x10’ were considered detailed enough to capture the topographic relief and man-made features (roads, landscaping, etc.) found within the study area. Grid element elevations were estimated from the Salt Lake County LiDAR mapping data (see Section 3.4) using an ArcGIS (v.10.2.2) routine. The LiDAR was converted to a 10’x10’ pixel raster. The conversion procedure averages the elevation within each pixel to a single value assigned to the pixel. The raster was then clipped to the FLO-2D domain boundary. 7.5.1.4. Model Grid Roughness Development Grid element roughness values (roughness coefficients/Manning’s n-values/n-values) were assigned to each grid element based on surface characteristics aerial photograph interpretations, and field reconnaissance. The resulting interpretation was delineated into a GIS dataset (Figure 16) that became the basis for the grid element Manning’s n-value assignment. Table 5 lists the n-value assignment based on land cover type. Vegetation observed throughout most of the study area consisted of dense shrub and brush. Figure 17 shows typical vegetation density that was observed during the May 2015 field visit. The selection of Manning’s n values for the cover types were derived from the Table 1. In the FLO- 2D Reference Manual. Table 5. Manning's n-value assignments Cover Type Manning’s N-Value Assignment Roads 0.02 Structures 0.06 Vegetation 0.08 ---PAGE BREAK--- 39 Figure 16. Manning's n-values used in the FLO-2D model ---PAGE BREAK--- 40 Figure 17. Typical vegetation density observed in the study area ---PAGE BREAK--- 41 7.5.1.5. Model Grid Aerial Reduction Factor (ARF) Development An Area Reduction Factor (ARF) was applied to each grid element that had some percentage of area covered by a building structure. The factor reduces the area of a grid cell available for floodplain storage. Structure footprints were delineated based on interpretation of the 2014 orthophotography (Figure 18). Grid elements that were completely blocked by structures were assigned an ARF value of 1.0. All others were assigned an ARF value based on the percentage area of the grid being blocked by the structure. 7.5.1.1. Model Inflow Hydrograph Development No hydrology was computed in the FLO-2D model. The model was employed for hydraulic routing of flow from the topographic apex. Inflow for the FLO-2D model was extracted from the HAL (2007) study HEC-HMS hydrologic model (filename: NoDebBasin_KinematicU.hms). The outflow hydrograph for the 100-year, 24-hour storm at the Neffs Creek canyon mouth concentration point was used directly as the inflow hydrograph for the FLO-2D model. The inflow location was assigned to grid ID 12629 and is shown on Figure 19, and a plot of the inflow hydrograph is shown in Figure 20. Given the degree of development of the study area and the resulting extent of impervious area, it was determined that infiltration would not be used in the FLO-2D model. Rainfall- runoff modeling was also not included in the FLO-2D model. The purpose of the model was hydraulic routing only. 7.5.1.2. Model Time Step The FLO-2D model time step is computed automatically by the model but is limited by the Courant criteria defined in the TOLER.DAT input file. For this analysis, a Courant value of 0.60 was defined for the floodplain as recommended by the model input manual. The model result hydrographs and SUMMARY.OUT output file did not indicate model instability issues, which would justify altering the Courant value. 7.5.1.3. Model Surface Detention Surface detention is accounted in the model by setting the TOL value in the TOLER.DAT input file. A TOL value of 0.002 feet (0.024 inches) was used for all models. 7.5.1.4. Model Bulking Concentration Factor Bulking of inflow can be done in the model by adjusting the XCONC variable in the CONT.DAT input file. This is most commonly used to account for sediment load. No bulking factor was used in the modeling for this study. 7.5.1.5. Model Hydraulic Structures The diversion ditch discussed in Section 4.2.3 contains 11 culvert crossings that were incorporated into the model in the input file. Rating curves for each structure were developed using the HY-8 (v.7.2) software program. Culvert sizes were measured in the field and inlet and outlet elevation data was obtained from the LiDAR dataset (Section 3.4). Figure 21 shows the spatial location of the culverts. The HY-8 data files are included in the Appendix E digital data submittal. ---PAGE BREAK--- 42 Personal communication with Salt Lake County personnel indicated that no other significant drainage infrastructure is present within the study area. This was confirmed during the field investigation. 7.5.1.6. Model Outflow Boundary Conditions Outflow grids were assigned after selection of the final model domain. The project limit (Wasatch Blvd.) was determined through a mutual decision between AECOM, JEF, and Salt Lake County. Figure 22 shows the location of the outflow grids for the model. 7.5.1.7. Model Limiting Froude Number It is a standard of practice to set the limiting Froude to 0.9 or 0.95 in the CONT.DAT input file. A value of 0.9 was used for this study. FLO-2D adjusts the Manning’s n value for stability. To determine the total number of grid elements and the magnitude of change in n values, a shapefile was generated using data from the ROUGH.OUT output file for each model scenario. The results indicated that n values for 5,110 grid elements out of 223,343 were adjusted by the model. Most of those adjustments were for grid elements within the main flow corridors. The n value adjustments result in conservative flow depths. 7.5.1.8. Model Simulation Time Model simulation times are listed in Table 6. Table 6. FLO-2D model simulation times FLO-2D Model Simulation Time (hours) BASE 9.3 SCENARIO 1 7.0 SCENARIO 2 6.1 SCENARIO 3 8.5 SCENARIO 4 6.2 SCENARIO 5 5.0 SCENARIO 6 6.1 SCENARIO 7 5.2 7.5.1.9. Model Flowpath Uncertainty Development While a base conditions FLO-2D model depicts the existing, fixed-bed condition of an X-year flood hazard event, it does not predict the full flood hazard associated within the active alluvial fan flooding and should not be the only scenario used to compute flow depths. To account for flowpath uncertainty, avulsion scenarios were developed and simulated within the model to account for the possibility of avulsions that would adversely affect (increase the inflow discharge) The flowpath uncertainty scenarios were developed by reviewing existing flow bifurcations observed in aerial photography, topography, field reconnaissance, and the base FLO-2D model. In locations where avulsions appeared likely or evidence of prior avulsions was observed, avulsions were simulated by adding berm-like features to redirect flow along an avulsion path (Figure 23). ---PAGE BREAK--- 43 The flowpath uncertainty scenarios were modeled by redirecting flow with a hard barrier accomplished using the LEVEE.DAT input file within FLO-2D. These barriers were given an arbitrary height well-above the ground elevation to ensure no overtopping. The barriers essentially were aligned to direct all the flow in the avulsion direction. ---PAGE BREAK--- 44 Figure 18. Delineated building footprints ---PAGE BREAK--- 45 Figure 19. Inflow hydrograph location ---PAGE BREAK--- 46 Figure 20. Inflow hydrograph plot 0 50 100 150 200 250 300 350 0 2 4 6 8 10 12 14 16 18 20 22 24 Discharge (cfs) Time (hours) Inflow Hydrograph ---PAGE BREAK--- 47 Figure 21. Structure locations ---PAGE BREAK--- 48 Figure 22. Outflow grid locations ---PAGE BREAK--- 49 Figure 23. Flowpath uncertainty analysis virtual levees ---PAGE BREAK--- 50 7.5.1.10. FLO-2D Composite Flow Depth Modeling Results Given the immense density/quantity of output data associated with two-dimensional modeling (such as FLO-2D modeling), modeling results are best depicted graphically in figures, exhibits, maps, etc. Therefore, the composite (Maximum) flood hazard condition is depicted graphically for the 100-year storm event. Figure 25 through Figure 31 depicts all the maximum flow depth flowpath uncertainty scenarios modeled. Figure 24 depicts the maximum flow depth Base Condition FLO-2D model results for reference. Note that flow depths less than 0.5 feet (6 inches) are not displayed in the figures. Flow depth less than 1 foot are generally not regulated by FEMA and the National Flood Insurance Program (NFIP) since the inundation risk is low. It is important for the reader to distinguish, for the purpose of this study, the difference between flowpath uncertainty flood scenarios and composite flood hazard conditions. Each of the seven FLO-2D maximum flow depth models is considered a flowpath uncertainty flood scenario. Composite flood hazard conditions (maximum flow depth) were determined by compiling the flowpath uncertainty scenario rasters using ArcGIS software tools to extract the highest value for each pixel (combined maximum values), then convert those values to a single output raster grid. The output raster represent the potential composite flood hazard condition per model grid element. The maximum flow depth (composite flood hazard conditions) for the 100-year event is shown spatially below in Figure 32. A description of each flowpath uncertainty scenario is listed in Table 7. Table 7. Flowpath uncertainty scenario descriptions Flowpath Uncertainty Scenario Description Base Condition Existing conditions. No virtual levees were used. Scenario 1 Virtual levees were placed to direct flow toward the northern portion of the project area. Scenario 2 Virtual levees were places to direct flow toward the central portion of the project area. Scenario 3 Virtual levees were placed to split the flow between the northern and southern portions of the study area. Scenario 4 Virtual levees were places to direct flow toward the southern portion of the project area. Scenario 5 Virtual levees were places to direct all the flow into the diversion ditch channel. Scenario 6 Virtual levees were places to direct flow toward the southern fan apex area. Scenario 7 Virtual levees were places to direct flow toward the central fan apex area. ---PAGE BREAK--- 51 Figure 24. Base condition FLO-2D model for maximum flow depth ---PAGE BREAK--- 52 Figure 25. Maximum flow depth results from the flowpath uncertainty scenario 1 model ---PAGE BREAK--- 53 Figure 26. Maximum flow depth results from the flowpath uncertainty scenario 2 model ---PAGE BREAK--- 54 Figure 27. Maximum flow depth results from the flowpath uncertainty scenario 3 model ---PAGE BREAK--- 55 Figure 28. Maximum flow depth results from the flowpath uncertainty scenario 4 model ---PAGE BREAK--- 56 Figure 29. Maximum flow depth results from the flowpath uncertainty scenario 5 model ---PAGE BREAK--- 57 Figure 30. Maximum flow depth results from the flowpath uncertainty scenario 6 model ---PAGE BREAK--- 58 Figure 31. Maximum flow depth results from the flowpath uncertainty scenario 7 model ---PAGE BREAK--- 59 Figure 32. Maximum flow depth results from the flowpath uncertainty composite model ---PAGE BREAK--- 60 7.6. Floodplain Mapping The ultimate objective of this study was to remap the currently effective FEMA floodplain for Neffs Creek based on updated geomorphic and hydraulic analyses. At the time of this study the effective FEMA floodplain was designated as Zone A (Basic Study). A summary of the restudy and remapping efforts is provided in Table 8. Table 8. Summary of restudy and remapping efforts Zone A (Basic Study) Zone AO (Enhanced/Detailed Study) Shaded X (Enhanced/Detailed Study) Area of Currently Effective Floodplain 0.16 Sq. Mi. N/A N/A Approximate Area Updated Floodplain 0.04 Sq. Mi. 0.14 Sq. Mi. 0.19 Sq. Mi. For developed and undeveloped portions of the study area, proposed floodplain boundaries delineated as part of this restudy are based on 100-year maximum flow depths (according to composite flood hazard conditions) as discussed in Section 7.5.1. In addition to maximum flow depths, geomorphic and topographic characteristics of the flood source were considered in determining limits of inundation. Floodplain delineations are shown in Figure 34 and on the Floodplain Workmaps provided in Appendix B. Floodplain delineations are also shown on annotated DFIRM panels located in Appendix D. The annotated DFIRMs can be used to evaluate differences between effective floodplains (effective at the time of this study) and proposed delineations. FEMA-based flood hazard designations associated with the delineated floodplains are listed below in Table 9. Further discussion regarding typical selection of FEMA-based flood hazard designations is provided below. 7.6.1. Development of Composite Velocities The velocity designations for the FEMA Zones were developed using the same methodology as the composite flow depth analysis (Section 7.5.1.10). The maximum velocity rasters for each scenario were combined to create a composite maximum velocity raster for the entire study area. The composite maximum velocity raster was then clipped using the flood zone dataset. The average velocity value for each of the clipped raster segments was extracted and assigned to the corresponding flood zone. Figure 33 is an example of a velocity raster segment clipped to a flood zone boundary. ---PAGE BREAK--- 61 Figure 33. Example of velocity raster clipped to floodplain boundary ---PAGE BREAK--- 62 Table 9. FEMA-based flood hazard designations associated with delineated floodplains FEMA-Based Flood Hazard Designation Notes Zone X (shaded) 100-year flow depth between 0.5’ and 1.0’. Zone A Ultrahazardous zone near the alluvial fan topographic apex. Area subject to the highest degree of flowpath uncertainty. In other areas where the average flow depths are greater than 3 feet. Approximate 100-year floodplain. Zone AO2,1 100-year flow depth between 1.5 foot and 2.5 feet. Average flow velocities of 1 foot/second. Zone AO2,2 100-year flow depth between 1.5 feet and 2.5 feet. Average flow velocities of 2 feet/second. Zone AO2,3 100-year flow depth between 1.5 feet and 2.5 feet. Average flow velocities of 3 feet/second. Zone AO3,3 100-year flow depth between 2.5 feet and 3.0 feet. Average flow velocities of 3 feet/second. Zone AO3,4 100-year flow depth between 2.5 feet and 3.0 feet. Average flow velocities of 4 feet/second. 7.7. Floodway Determination No floodways were determined in this analysis. 7.8. Flood Hazard Profiles Given that the floodplains delineated for this study are designated as either Zone A or Zone AO, development of flood hazard profiles is not required. ---PAGE BREAK--- 63 Figure 34. Proposed revised floodplains ---PAGE BREAK--- 64 8.0 REFERENCES 1. AGEC, 2005, Debris Flow Hazard Study Report, Neffs Canyon, Salt Lake County, Utah. Prepared for Hansen, Allen and Luce, Inc. 2. Dohrenwend, J.C., 1987, Basin and Range, in Graf, W.L., ed., Geomorphic systems in North America: Geological Society of America, Centennial Special Volume 2, p. 303- 342. 3. FEMA, 2003, Guidelines and Specifications for Flood Hazard Mapping Partners – Appendix E: Guidance for Shallow Flooding Analyses and Mapping, April 2003. Available on-line at http://www.fema.gov/mit/tsd/FT_alfan.htm. 4. FEMA, 2003, Guidelines and Specifications for Flood Hazard Mapping Partners – Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping, April, 2003. Available on-line at http://www.fema.gov/mit/tsd/FT_alfan.htm. 5. Field, John, 2001, “Channel avulsion on alluvial fans in southern Arizona,” Geomorphology, Vol. 37, p. 93-104. 6. Hansen, Allen & Luce (HAL), 2007, Neffs Canyon Creek Master Plan. Salt Lake County. 7. Kalister, B.N., 1974, Mt. Olympus Cove Environmental Geology Study. Utah Geological and Mineral Survey Report of Investigation No. 86. 8. National Research Council, 1996, Alluvial Fan Flooding: Washington, D.C., National Academy Press, 172 p. 9. Personius, S.F, and W.E. Scott, 1992, Surficial Geologic Map of the Salt Lake City Segment and Parts of Adjacent Segments of the Wasatch Fault Zone, Davis, Salt Lake, and Utah Counties, Utah. U.S. Geological Survey. U.S. Department of the Interior. 10. Slingerland, R and Smith, N.D., 2004, River Avulsions and Their Deposits, Annual Review of Earth and Planetary Science, Vol. 32:257-285. 11. Van Horn, 1972, Surficial Geologic Map of the Sugar House Quadrangle, Salt Lake County, Utah. U.S. Geological Survey. U.S. Department of the Interior. ---PAGE BREAK--- APPENDIX A AGEC, 2005, Debris Flow Hazard Study Report, Neffs Canyon, Salt Lake County, Utah. Prepared for Hansen, Allen and Luce, Inc. ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- APPENDIX B Floodplain Workmaps ---PAGE BREAK--- Jupiter Dr E 3800 S Hale Dr DD Ln Oak Rim Way S Park vie w Dr Barbara Way Wasatch Dr Cove Point Dr Wasatch Blvd S 35 4 0 E E 3820 S E Millcreek Canyon Rd Parkview Dr Driveway M on za Dr S Millcrest Rd Cliff Dr Pluto Way S Wasatch Blvd E Millcreek Rd Upland Dr Jun o C ir E 3900 S St E a g le P o i nt Dr Quail Hollow Dr Phe asant Ridge Rd Driveway E Millcreek Canyon Rd C o v e Point Dr Driveway S Wasatch Blvd 4980 4990 4970 5000 5010 5020 5 0 3 0 5 0 4 0 5050 4 9 60 50 6 0 5070 5 0 80 4 9 5 0 5 0 9 0 5 10 0 4 9 4 0 5 1 1 0 5 1 2 0 4930 5100 5100 4970 5070 4 9 7 0 4950 5120 4950 4 9 6 0 5040 5030 4 9 4 0 4 9 50 50 4 0 4980 508 0 5070 4950 5100 5070 5 0 10 5040 4 9 6 0 49 90 5080 4950 50 20 502 0 5070 4 9 60 5 1 0 0 5010 5010 AO,2,1 AO,2,3 SHADED X Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 2 SHEET 1 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- Jupiter Dr Apollo Dr Ceres Dr Hermes Dr Achilles Dr Ruth Dr Lois Ln Wasatch Blvd Evelyn Dr Diana Way Hale Dr Adonis Dr Fortuna Way Mars Way Aurora Cir Hermes Cir Lois Ln Diana Way H ermes Dr Mars Way 5 06 0 5100 5130 5070 50 9 0 5010 5 0 8 0 5 1 2 0 5 0 0 0 5020 5110 5030 5 0 4 0 5 0 5 0 5140 4 9 9 0 4 9 8 0 5 1 50 5160 5 1 7 0 49 7 0 518 0 5 1 6 0 5 0 40 5070 5000 5 0 5 0 50 5 0 4 97 0 5160 505 0 5020 5070 5040 5050 4980 5070 4 9 80 4 980 50 9 0 51 6 0 5110 5 0 60 5070 5050 5 0 90 5050 5 0 00 5 15 0 4980 4 97 0 5150 4 970 5 0 3 0 5120 5070 5100 5130 5090 5010 5120 5020 5050 5 0 7 0 5030 A AO,2,2 AO,2,2 AO,2,2 AO,2,3 SHADED X SHADED X SHADED X SHADED X SHADED X SHADED X Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 4 SHEET 1 SHEET 3 SHEET 2 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- Parkview Dr Gary Rd Adonis Dr Mo unt Olym p us Wa y Ruth Dr Powers Cir Foubert A ve Evelyn Dr Nep tu ne D r 5210 5 20 0 5 2 2 0 5 2 3 0 5240 5 19 0 5 2 5 0 5 2 6 0 5 2 7 0 528 0 541 0 5 4 2 0 5290 5 400 5390 53 8 0 5 3 0 0 5370 5310 53 6 0 5 3 2 0 5 3 50 53 3 0 5 3 4 0 5 18 0 5 4 3 0 5 44 0 5 4 5 0 5 17 0 5 460 5 4 70 5 48 0 5 49 0 51 6 0 5 5 0 0 5 1 5 0 5 5 1 0 5 5 2 0 5 53 0 5540 5 5 5 0 5 56 0 55 7 0 55 8 0 5 5 9 0 5 60 0 51 4 0 5 610 5 62 0 563 0 5 64 0 5 6 5 0 56 60 5670 5680 5 2 60 5 1 90 5 1 7 0 5200 5 2 1 0 5190 5370 5280 5180 5330 5170 5270 5310 5 1 60 5180 5 1 50 51 5 0 5170 5280 52 4 0 52 3 0 5 280 5170 AO,2,2 AO,2,2 AO,2,2 AO,2,3 AO,2,3 AO,3,4 SHADED X SHADED X SHADED X SHADED X Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 5 SHEET 2 SHEET 3 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- Oakview Dr Jupiter Dr Fortuna Way Eastcliff Dr Diana Way Spruce Dr Eastoaks D r Spruc e Cir Ceres Dr Pin Oak St Brockbank Dr Neptune Dr Mars Way Mulholland St Loren Von Dr Crest Oak Dr Pa r k H i ll Dr Olympus View Dr Eastcliff Cir Achilles Dr Pax Cir Fortuna Cir Lares Way Viewcrest Dr 5 0 8 0 5 18 0 5200 5140 5150 5 1 70 5160 5 1 30 511 0 5070 5060 5190 5090 5 1 2 0 5100 5 2 1 0 5050 52 2 0 5 2 3 0 5 240 50 30 50 40 5 2 5 0 5 2 6 0 5 27 0 5020 5280 5 2 9 0 5 3 0 0 5 0 1 0 5310 53 2 0 5 0 0 0 5 3 3 0 5 0 30 5110 5240 5020 5 0 90 5 0 8 0 5 080 5 1 9 0 50 8 0 5 0 4 0 5110 514 0 5 1 40 51 1 0 5 0 9 0 5 0 5 0 5 0 8 0 5010 5160 52 10 5140 5120 5 1 0 0 5140 5110 5 0 50 5 0 5 0 5 0 2 0 5070 5230 5 1 2 0 5200 51 5 0 5030 5 0 40 5090 A AO,2,2 Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 2 SHEET 5 SHEET 4 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- Oakview Dr Parkview Dr Lares Way Adonis Dr Viewcrest Dr Mathews W a y Brockbank Dr Brockbank Way Abinadi Rd Cumorah Dr Covecrest Dr Mount Oly m p u s Way Lares Cir Park Terrace Dr Adonis Cir Parkview Dr Adonis Dr 5380 5390 5400 5420 5360 5370 5350 5 340 5430 5 3 30 5 4 10 5440 5 3 2 0 5310 5 45 0 5 4 6 0 5280 5300 5 4 70 5 2 70 5290 5480 5 25 0 5490 5 26 0 5 2 40 52 30 5 2 20 5 5 00 5 2 1 0 5 510 5 4 4 0 5 4 4 0 5 2 90 52 2 0 5230 5 4 3 0 5380 5280 5290 54 1 0 5390 5350 5 3 7 0 5 2 2 0 5 2 1 0 5460 5300 5210 5230 54 70 5 26 0 5420 5500 5240 5220 5420 52 40 A A AO,2,2 AO,2,3 AO,3,3 AO,3,4 SHADED X SHADED X SHADED X Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 3 SHEET 4 SHEET 6 SHEET 5 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- Zarahem la Dr W hite W ay Par k Terrace Dr Abinadi Rd Whi t e Way 55 2 0 5 5 3 0 5540 5 5 8 0 5 5 9 0 5 570 56 0 0 5550 5 6 7 0 5 61 0 5 5 6 0 5 6 6 0 5 6 2 0 5 63 0 56 5 0 5 6 4 0 5 51 0 5 6 8 0 56 9 0 5 7 0 0 5 71 0 5 72 0 5 73 0 55 0 0 5740 5750 54 8 0 5 4 7 0 5760 5770 5780 5490 5790 58 0 0 5 4 6 0 5 81 0 5820 5 8 3 0 58 40 58 5 0 5 8 6 0 5 45 0 5 8 7 0 588 0 5890 5 9 0 0 59 10 5 9 2 0 59 3 0 5 9 40 59 5 0 5 9 6 0 5 9 7 0 5 9 80 599 0 5 4 4 0 600 0 6 0 1 0 60 2 0 6 03 0 6040 56 0 0 5510 5480 5640 5 6 20 5 4 9 0 5 5 1 0 5 5 9 0 570 0 5 5 00 5540 5 59 0 5 6 0 0 5 6 90 5540 5590 5680 A AO,3,4 SHADED X Neffs Creek Flood Hazard Assessment LEGEND Limit of Study 2-Foot Contour Topography Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X 1110 State Office Building Salt Lake City, UT 84114-1710 0 100 200 50 Feet NOTES: 1. Base imagery: NAIP 2014 Orthophotography 2. 2-foot contour topography generated from Salt Lake County LiDAR mapping. 8400 South Kyrene Road, Suite 201 Tempe, AZ 85284 Phone: [PHONE REDACTED] µ SHEET 5 SHEET 6 OF 6 6 1 2 3 4 5 215¨§¦ 215 Copyright:© 2013 Esri Sheet Index FLOODPLAIN WORKMAP ---PAGE BREAK--- APPENDIX C Hansen, Allen & Luce (HAL), 2007, Neffs Canyon Creek Master Plan. Salt Lake County ---PAGE BREAK--- SALT LAKE COUNTY NEFFS CANYON CREEK MASTER PLAN (HAL Project No.: 014.10.100) FINAL REPORT December 2007 ---PAGE BREAK--- ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan i Successful completion of this study was made possible by the cooperation and assistance of many individuals, including the Salt Lake County Public Works Engineering, Flood Control Division , as shown below. We sincerely appreciate the cooperation and assistance provided by these individuals. Neil Stack Brent Beardall North Area: Jeff Silvestrini Judy Keane Darrel French Central Area: Ken Smith Warren Davis Shonnie Hayes South Area: Nick Powell Pat English Carol Morgan Tom Brown Merrill Ridd Gregory J. Poole, Principal-in-Charge David E. Hansen, Quality Assurance Ben Miner, Hydraulics Gordon Jones, Hydrology ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan ii INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 BACKGROUND AND PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 AUTHORIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1 HYDROLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 DRAINAGE BASIN CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 Subbasin Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 Hydrologic Soil Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 Percentage of Impervious Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 SCS Curve Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-2 Basin Lag Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-2 Conveyance System Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-2 MOUNTAIN AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-3 URBAN AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-3 DESIGN RAINSTORM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-4 Storm Duration Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-5 Storm Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-5 Aerial Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-6 Rainfall Adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-7 TRANSMISSION LOSSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-7 DESIGN FLOWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-7 SNOW MELT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-8 DEBRIS FLOW HAZARD STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-9 EXISTING CONVEYANCE SYSTEM DESCRIPTION AND CAPACITY . . . . . . . . . . . . . . . . . . . . . . . . IV-1 ALTERNATIVE EVALUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-1 DEBRIS FLOW MITIGATION ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-1 DEBRIS BASIN ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-1 Upper Debris Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-2 Lower Debris Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-2 URBAN AREA FLOOD CONVEYANCE SYSTEM ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . V-2 DESIGN FLOWS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI-1 APPENDIX A GLOSSARY AND ABBREVIATIONS B HYDROLOGY C HYDRAULICS D COST ESTIMATES COMPACT DISK (Debris Flow Hazard Study (AGEC), HEC-HMS files, and HEC-RAS files) ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan iii TABLE II-1 NEFFS CANYON SUBBASIN CHARACTERISTICS FOR MOUNTAIN AREAS . . . . . . . . . . II-3 TABLE II-2 NEFFS CANYON SUBBASIN CHARACTERISTICS FOR URBAN AREAS . . . . . . . . . . . . . II-4 TABLE II-3 COMPARISON OF TRC 1999 AND NOAA 14 RAINFALL DEPTHS . . . . . . . . . . . . . . . II-4 Table II-4 AREAL REDUCTION FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-6 Table II-5 ADJUSTED PRECIPITATION VALUES FOR 100-YEAR DURATION . . . . . . . . . . . . . . . . . II-7 Table II-6 NEFFS CANYON CREEK – DESIGN FLOW RATES . . . . . . . . . . . . . . . . . . . . . . . . . . . II-8 Table II-7 ESTIMATED SNOW MELT FLOW RATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-8 TABLE IV-I ESTIMATED CAPACITY OF EXISTING CULVERTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1 TABLE V-1 NEFFS CANYON CREEK CONVEYANCE ALTERNATIVES COMPARATIVE MATRIX . . . . V-3 Table VI-1 NEFFS CANYON CREEK – DESIGN FLOW RATES . . . . . . . . . . . . . . . . . . . . . . . . . . . VI-1 Table VI-2 ESTIMATED SNOW MELT FLOW RATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI-1 Figure II-1 DRAINAGE SUBBASIN BOUNDARIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-1 Figure IV-1 EXISTING NEFFS CREEK CAHNNEL ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . IV-1 Figure IV-2 CURRENT NEFFS CHANNEL AND CANYON THALWEG . . . . . . . . . . . . . . . . . . . . IV-1 Figure V-1 ALTERNATIVE DEBRIS BASIN LOCATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-1 Figure V-2 UPPER DEBRIS BASIN ALTERNATIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-2 Figure V-3 LOWER DEBRIS BASIN ALTERNATIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-2 Figure V-4 CONVEYANCE SYSTEM ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-2 Figure IV-2 NEFFS CREEK CONVEYANCE IMPROVEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . VI-2 ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan I-1 Neffs Creek is directly tributary to a residential development at the Canyon mouth. The 2002 Flood Insurance Study identified flooding associated with Neffs Creek affecting approximately 150 homes (see Flood Insurance Rate Map panels 49035C0316E and 49035C0317E). Currently normal Neffs Creek flows are conveyed to a storm drain system in Wasatch Boulevard. The Neffs Canyon conveyance system was constructed prior to the inception of the Federal Flood Insurance Program. A key purpose of Salt Lake County Flood Control is to plan drainage improvements to better protect County residents from flooding and bring the system up to the requirements of the Federal Flood Insurance Program. D Define the 100-year flood flows. D Evaluate debris flow hazard. D Identify means for flood and debris flow hazard mitigation. The scope of the Neffs Canyon Creek Master Plan included the following: D Documentation and review of the existing Neffs Canyon Creek conveyance system, D Hydrologic analyses to define design stream flows. D Debris flow hazard evaluation. D Develop alternatives for mitigating flood hazards to residences. D Participate in public meetings to receive public input on flood hazard mitigation alternatives. D Prepare Master Plan Document. The Neffs Canyon Creek Master Plan has been completed in accordance with a contract approved on April 7, 2005 between Salt Lake County and Hansen, Allen, & Luce, Inc. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-1 A drainage basin is an area where all precipitation that falls within it will collect to a common point. Another name for a drainage basin is watershed or catchment. Subbasins are located within a larger drainage basin. Drainage subbasin boundaries depend upon both the topography and the location of storm drainage facilities. The delineated Neffs Creek drainage basin and subbasin boundaries are shown on Figure II-1. Subbasin characteristics were developed based on field observations and the GIS mapping supplied by Salt Lake County. Important subbasin characteristics discussed in this report include: • Subbasin Area • Hydrologic Soil Group • Percentage of Impervious Area • SCS Curve Number • Basin Lag Time • Conveyance System Routing Subbasins were delineated within ArcView GIS using USGS Topographic Quadrangle maps and the locations of storm drainage facilities. Mountain watersheds were divided into subbasins where distinct vegetation, soil type and precipitation characteristics were found. Hydrologic soil group is a indication of the soil’s minimum infiltration rate. Soils are assigned a hydrologic group of A, B, C, or D by the Natural Resource Conservation Service (NRCS, formerly know as the Soil Conservation Service, SCS). Soils of hydrologic soil group A have the highest infiltration rate, and therefore produce the least amount of runoff. Soils of hydrologic soil group D have the lowest infiltration rate, and therefore produce the highest amount of runoff. Soil maps were obtained from the Natural Resources Conservation Service (NRCS) Web Soil Survey (http://websoilsurvey.nrcs.usda.gov/app/ Impervious areas within each urban subbasin were estimated using the GIS model. The impervious area was divided into two components: directly connected impervious areas and unconnected impervious areas. Directly connected impervious areas provide a direct path for runoff from the impervious area to a conveyance such as a pipe, gutter, or channel. Directly connected impervious areas include roadways, parking lots, driveways, and sometimes the roofs of buildings. Runoff from unconnected impervious areas include sidewalks that are not ---PAGE BREAK--- ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-2 adjacent to the curb, patios, sheds, and usually some portion of the roof of the house or structure. Unconnected impervious area is combined with the pervious area of a subbasin resulting in a weighted curve number for unconnected area. The SCS curve number methodology is described in the NRCS publication TR-55. A curve number is determined based on several factors described in the manual. These factors include: hydrologic soil group, cover type, treatment and hydrologic condition. The hydrologic soil groups were discussed earlier in the hydrologic soil group section. The cover type is the kind of vegetation prominent in that area. Urban areas were assumed to have a normal mix of grasses and shrubs common in residential yards. Vegetation cover types were delineated using aerial photography and the NRCS soils map. Vegetation cover types were verified through site reconnaissance. The mountain vegetation cover types are described following. This complex includes a mixture of grass, weeds, and low-growing brush, with brush being the minor element. This cover was found on the ridges and more exposed areas. This cover type includes pinyon, juniper or both with a grass understory. This vegatative cover consists of mountain brush mixture of oak brush, aspen, mountain mohogany, bitter brush, maple, and other brush. This is only found on the high north-facing slopes. The drainage subbasin composite curve numbers were calculated by an area weighting method. The basin lag time for mountain areas was calculated using the regression equation outlined in the article entitled “Lag Time Characteristics for Small Watersheds in the U.S.” by M.J. Simas and R.H. Hawkins. The equation relies on basin area, slope, and curve number characteristics. The regression equation follows: Tlag = .0051 x width.594 x slope-.15 x Snat .313 where width = Watershed Area / Watershed Length slope = Maximum Elevation difference / Longest Flow Path Snat = 1000/CN - 10 Mountain area runoff enters Neffs Canyon Creek via sheet flow, shallow concentrated flow and stream flow. In urban locations runoff is routed to Neff’s Creek through storm drain pipes or road ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-3 side drainage ditches. The shape and roughness of these conveyance systems were estimated based on site visits and engineering judgment. Subbasin hydrologic characteristics for the mountain area conditions are summarized in Table II-1. Required hydrologic characteristics for use in modeling storm water runoff with the Soil Conservation Service Curve Number (CN) and Unit Hydrograph technique include drainage area, Curve Number, and Lag Time. Upper Basin 723 63 1.32 Middle Basin 822 67 1.18 Lower Basin 840 66 1.25 SMB1 73 65 0.12 SMB2 235 65 0.16 TOTAL: 2693 Hydrologic characteristics for urban areas in the model are presented in Table II-2. Urban hydrologic characteristics for use in modeling storm water runoff with the SCS Curve Number and Unit Hydrograph technique include drainage area, percent of the subbasin which is covered by impervious area, percent of the subbasin which is directly connected impervious area, composite curve number representing the portion of the subbasin which includes the pervious area plus the impervious areas which are unconnected (that is runoff off these areas flows across pervious surfaces prior to entering the conveyance system), and time of concentration. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-4 Urb-1 31 32 14 65.6 42 Urb-2 81 35 17 66.0 43 Urb-3 24 38 19 66.6 18 Urb-4 18 38 19 66.5 17 Urb-5 13 32 16 64.8 18 Urb-6 30 45 29 66.0 28 Urb-7 10 42 25 66.3 15 Urb-8 21 53 36 68.0 16 TOTAL: 207 Precipitation depth-duration return period information provided in the”Rainfall Intensity Duration Analysis Salt Lake County, Utah” (TRC North American Weather Consultants, 1999) (hereinafter referred to as TRC 1999) and from National Oceanic and Atmospheric Administration Atlas 14 (NOAA 14) found on the website http://hdsc.nws.noaa.gov/hdsc/pfds were compared. The TRC 1999 depth-duration return period maps cover the urban portion of the study area. The following table provides a comparison between the predicted 100-year rainfall depths for the urban area taken from the two sources. RETURN PERIOD - DURATION TRC 1999 NOAA 14 100-YEAR 30-MINUTE 1.24 1.49 100-YEAR 1-HOUR 1.62 1.84 100-YEAR 6-HOUR 2.38 2.33 100-YEAR 24-HOUR 3.46 3.53 ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-5 Because the TRC 1999 depth-duration return period maps do not cover the mountain watersheds, it was decided to use the NOAA 14 data for consistency. The precipitation values used were dependent upon the general elevation and location of the different sub-basins. The precipitationvalues were assigned to general zones which include: Upper Neffs Canyon, Middle Neffs Canyon, Lower Neffs Canyon, and the Urban Area. The storm duration that will produce the highest peak runoff flow rate is dependent on rainfall- duration relationships, the characteristics of the basin, and upon the level of detention storage. Generally speaking, the longer runoff takes to flow through a drainage basin or detention basin, the longer the critical storm duration. A duration sensitivity analysis of the hydrologic study area was performed by successive model runs using 1-hour, 3-hour, 6-hour, 12-hour, and 24-hour storm durations. The 24-hour storm duration was found to produce the largest peak and was used as the basis for Neffs Canyon design flows. Critical runoff events from urban areas along the Wasatch Front are caused by cloudburst type storms, characterized by short periods of high intensity rainfall. During the 1960's and early 1970's, Dr. Eugene E. Farmer and Dr. Joel E. Fletcher completed a major study of the precipitation characteristics for storms in northern Utah based on data from two rainfall gage networks located in central and north-central Utah. These gage networks are referred to as the Great Basin Experimental Area (GBEA) and the Davis County Experimental Watershed (DCEW) respectively. This effort has become the definitive source for rainfall distributions appropriate for the Wasatch Front area. Because this study applied to short duration storms, it was not applied to durations exceeding the 6-hour event. Thirteen separate gaging stations in the Great Basin Experimental Area (ranging in elevation from 5,500 feet to over 10,000 feet) were maintained for varying periods of time from 1919 to 1965. Fifteen gaging stations were maintained in the Davis County Experimental Watershed (ranging in elevation from 4,350 to 9,000 feet) for varying periods of time between 1939 and 1968. After completing their analyses of the data, Farmer and Fletcher found that “more than 50 percent of the storm rainfall depth occurs in 25 percent of the storm periods;” and that “usually more than half of the total depth of rain is delivered as burst rainfall.” Farmer and Fletcher developed design storm distributions which have become accepted by governmental entities including Salt Lake County and Davis County as the characteristic distributions for storms in Utah of short duration (generally less than six hours). The work of Farmer and Fletcher was expanded in 1985 to develop a 24-hour rainfall distribution from the GBEA data (VHA, 1985). For the derivation of the design 24-hour rainfall event, a storm was defined “as a period of continuous or intermittent precipitation delivering at least 0.1 inches of rainfall during which time dry periods without rainfall did not exceed four hours.” Storms having durations ranging from 20 hours to 28 hours were accepted to be representative of a 24-hour storm duration. The 24-hour duration storms were then screened to include only storms ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-6 which contained rainfall meeting the burst criteria of having over 50 percent of the precipitation occurring in less than 25 percent of the time. Storms meeting the burst criteria were further categorized in accordance with which quartile of the storm the burst had occured (i.e. the first, second, third or fourth quarter of the storm period). Identified storms were used to develop a 24-hour design storm distribution for use in Utah. A sensitivity analysis for all storm distributions developed shows the 3rd quartile storm distribution to produce the higher runoff peaks. The SCS Type II distribution is an extreme distribution which includes a very intense burst of rainfall with over 35 percent of the 24-hour total rainfall occurring within a half hour. The GBEA 3rd Quartile storm distribution developed in 1985 includes a burst of rainfall with an approximate 10 percent of the 24-hour total rainfall falling within a half hour period. In a similar comparison, the SCS Type II distribution allows approximately 62 percent of the total precipitation to occur within the same period. Because the distribution was developed based on local data, the GBEA distribution is believed to be the best available storm distribution for Utah for storms lasting between 6 and 24 hours. For the same reason, the Farmer-Fletcher distribution is the best available storm distribution for durations of less than 6 hours. Comparisons of the predicted runoff peaks from the GBEA storm distribution and from the Farmer Fletcher storm distribution reveal good agreement for a 6-hour duration storm. Aerial reduction factors were applied to the model based on the Salt Lake City Hydrology Manual. These factors were developed to compensate for the aerial differences associated with different storm durations and drainage basin area. The total area for the combined sub- basins is 4.52 square miles which results in an aerial reduction factor of 0.96 or an equivalent precipitation depth reduction of 4% for the 24-hour event. The respective areal reduction amounts shown in Table II-4 were applied to each of the precipitation depths obtained from the NOAA 14 Atlas. 30-minute 0.82 1-hour 0.86 3-hour 0.91 6-hour 0.93 12-hour 0.95 24-hour 0.96 ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan II-7 Rainfall is assumed to produce the peak runoff for Neffs Canyon Creek. The NOAA Atlas 14 did not include an update to the May-October rainfall amounts included in NOAA Atlas 2. The precipitation values found in NOAA Atlas 14 are based on the complete data set (full year including snow). In order to predict the rainfall values based on the NOAA Atlas 14, a ratio was calculated using the NOAA Atlas 2 May-October rainfall versus the full year precipitation from NOAA Atlas 2. This ratio was applied to the NOAA Atlas 14 full year precipitation values to produce design storm rainfall amounts. The precipitation values from NOAA 14 with areal and rainfall adjustments are shown in Table II-5. Upper Neffs Canyon 1.20 1.58 1.98 2.32 3.10 3.97 Middle Neffs Canyon 1.20 1.56 1.95 2.26 3.01 3.77 Lower Neffs Canyon 1.16 1.51 1.86 2.12 2.74 3.32 Urban Area 1.14 1.49 1.80 2.04 2.60 3.12 Transmission losses result from infiltration along the drainage channel reaches and are calculated using methodology presented in the “National Engineering Handbook , Section 4 - Hydrology, Chapter 19 - Transmission Losses.” These losses apply to ephemeral streams in semiarid regions typical of the Neffs Canyon area. The losses are calculated using regression equations based on the effective hydraulic conductivity. A gaining stream is defined as a stream that receives groundwater discharge. The upper reaches of Neffs Canyon upstream of about 7,400 feet and tributary channels were assumed to be gaining, therefore, no losses were applied to those reaches. A storm rainfall runoff model was prepared for the Neffs Canyon watershed using the U.S. Army Corps of Engineers Hydrologic Modeling System (HEC-HMS) software. A summary of the design creek flow rates for a 10-Year and a 100-Year return period (a 100-year return period event has a 1% chance of being equaled or exceeded in any given year) are provided in Table VI-1. A duration sensitivity analysis was performed and the 24-hour storm was found to govern both the 10-year and 100-year events. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan III-8 Canyon Mouth 70 300 Wasatch Blvd 90 350 Historical snowmelt peak flows are not available for Neffs Canyon. Regression equations developed by Gingery and Associates ("Hydrology Report, Flood Insurance Studies, 20 Utah Communities, F.I.A. Contract H-4790", 1979) were used to estimate snowmelt runoff. The equations rely on the size of the basin area and the return period for the snowmelt event. Table II-7 gives a summary of expected snowmelt flows at the canyon mouth. Mouth of Canyon 50 70 75 ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan III-9 An evaluation of the debris flow hazard potential for Neffs Canyon was completed by Applied Geotehcinal Engineering Consultants (AGEC), P.C. (Project No. 1050097, August 10, 2005, see copy on CD in appendix). The debris flow hazard study included a review of geologic literature, an evaluation of aerial photographs, filed reconnaissance, and analysis. AGEC findings are summarized below. • “The mouth of Neffs Canyon is situated approximately 400 feet above the Bonneville Shoreline. The Neffs Canyon Alluvial fan extends o u t o n t o a n d coalesces with Lake Bonneville deposits.” • “Study of the aerial photographs did not identify discrete debris flow lobes on the fan. However, the distal portion of the fan is irregular in extent, which may be interpreted as a series of discrete flows with variable run-out distances.” • “Personius and Scott (1992) map the area of the Neffs Canyon alluvial fan as af2, which is assigned the age of middle H o l o c e n e t o u p p e r m o s t Pleistocene 5000 years old).” ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan III-10 • “Landslides typically do not form in limestone and quartzite, which is the bedrock underlying Neffs Canyon, indicating that this debris flow triggering mechanism would be less likely than storm-induced erosion on denuded areas.” • “The southern reaches of the Neffs Canyon drainage basin contain abundant exposed bedrock, which promotes rapid surface-water runoff that could help generate a debris flow. However, these north-facing slopes also contain large areas of dense brush and trees that act to inhibit mobilization of slope colluviaum.” • “The potential for debris flow would be increased if a significant portion of the drainage is burned.” • “Possible geomorphic evidence of past debris flow activity was observed in the lower reach of Norths Fork tributary, where boulder trains and levees were observed between roughly parallel channels on either side of the drainage.” • although the lower drainage channel is relatively broad it contains an incised channel that would act to partially confine a debris flow.” • Two methods were used to calculate the potential debris flow volume for each channel segment. The total volume of debris flow calculated is 154,700 cubic yards and 148,200 cubic yards for the different methods. • “The portion of the Neffs Canyon drainage below approximate elevation 6800 feet has a gradient suggesting deposition rather than erosion and would decrease the volume of sediment reaching the canyon mouth. The potential deposition in this reach is estimated at 13,000 cubic yards.” • “Overall, it is clear from the literature that debris flows have occurred in the past more commonly in Davis County than Salt Lake County. The drainages that produce these events are typically much smaller than Neffs Canyon.” • “The predicted debris flow volumes represent an event that occurs over the entire Neffs Canyon drainage basin. The potential for a smaller flow to occur within one of the tributary channels, or within tributary channels in a portion of the canyon, is greater than the potential for debris flows to occur simultaneously within the entire basin. Further, many of these smaller flows may be deposited before reaching the canyon mouth due to the low gradient of the main channel below approximate elevation 6800 feet.” It is difficult to assign a probability to the potential debris flow events. In discussion with the geologist and Salt Lake County, it was decided that taking the average of the predicted debris flow from the largest channel segment, upper Neffs Canyon, [(35,000 + 58,600)/2] = 46,800 cubic yards and subtracting the estimated deposition in the lower reach (13,000 cubic yards) provides an estimated debris flow volume (33,800 cubic yards) which may be an appropriate design volume for facilities with the objective of providing protection to developed ares below the canyon mouth. The design debris flow volume (33,800 cubic yards) is about 21 acre-feet. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan IV-1 The existing Neffs Canyon Creek conveyance system consists of open channels and culverts. The existing channel alignment is shown on Figure IV-1. The conveyance system flows through the Olympus Cove subdivision. The Olympus Cove subdivision was constructed in about 1958. The Forest Service boundary defines the east border of the Olympus Cove subdivision. After development of the subdivision, the area was identified as an active alluvial fan, with significant flood and debris flow risk. This condition is exacerbated because the Neffs Creek low flows currently are delivered to the subdivision from a channel which is higher than the thalweg (lowest part) of the canyon. The higher channel appears to be the result of a past diversion (possibly for irrigation purposes). In places the water elevation in the current channel is significantly higher than the lower thalweg. The alignment of the current channel and the thalweg are shown on Figure IV-2. The diversion to the current channel from the Neffs Canyon thalweg occurs about 1300 feet east of the homes. The diversion is somewhat fragile and storm runoff often spills into the lower thalweg. The capacity of the existing conveyance system through the residential area was estimated by surveying the culverts (inlet flow line, outlet flow line, and available headwater elevation at the inlet) and surveying typical channel cross sections. A HEC-RAS model was prepared of the conveyance system and culvert capacities were estimated (see Appendix). Culvert capacities are provided in the following table. Zarahemla Dr. 6375 175 2.5 50 Abinadi Rd 5476 59 3 100 Mathews Way 5192 60 4 130 Parkway Dr. 4597 29 3 50 Adonis Dr. 4232 70 3 55 Brockbank Dr. 3543 68 5 230 ---PAGE BREAK--- ---PAGE BREAK--- E N E E R S G I N ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan IV-2 Neptune Dr. 2505 166 5 160 Jupiter Dr. 2099 93 5 138 Fortuna Way 1408 95 5 140 Achillies Dr. 715 45 5 150 Existing channel capacities vary significantly through the Olympus Cove subdivision. The existing channel between Abinadi Road and Zarahemla Drive has an estimated bank full channel capacity of less than 200 cfs (assuming no backwater effects from the culvert at Abinadi Road). The smallest existing channel capacity is located adjacent to Helaman Circle below Zarahemla Drive and has an estimated bank full capacity of about 120 cfs. The safe carrying capacity is much less than the bank full carrying capacity due to high erosion potential with higher flows on the steep channel slopes. The channel adjacent to Helaman Circle has a safe carrying capacity of less than 70 cfs (due to the risk to a berm). The channel below Abinadi Road generally has sufficient capacity (in excess of the 100-year event assuming that the backwater effects are eliminated by replacing the culverts), but there is a high erosion potential and risk that the channel will move affecting existing buildings. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan V-1 A key master plan study objective is to identify means for flood and debris flow hazard mitigation. The Federal Emergency Management Agency in “Guidelines for Determining Flood Hazards on Alluvial Fans” (FEMA, 2000) states: “Active alluvial fan flooding occurs only on alluvial fans and is characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard.” Alternative mitigation methods have been investigated for debris flow and conveyance system flooding. Mitigation measures for debris flows can be categorized into three types: debris basin, deflection, and watershed treatments. A debris basin positioned to intercept debris flows prior to reaching the residential area provides an embankment designed to stop the debris flow allowing the soilds portion of the debris flow to deposit in the debris basin and the liquid portion to flow through the basin outlet facilities. Debris basins have been used for years and have provided a reliable means of mitigating debris flow hazards. Deflection utilizes an armored embankment to deflect debris flows away from homes. A suitable location to receive the deflected debris flows does not exist at the mouth of Neffs Canyon, therefore this alternative was eliminated. Watershed treatments include several different types of measures which are implemented in the watershed. These measures include construction of temporary measures such as silt fences, organic debris rakes, and matting. More permanent type measures include earth retaining structures to stabilize potential trigger areas. Because these measures would need to be implemented within the designated Wilderness Area, equipment for construction of these treatments would be limited to hand tools. Measures which could be constructed with hand tools would be temporary and not sufficiently durable to provide sufficient debris flow mitigation to remove the homes from the hazard. These measures could be effective in providing short term protection such as during the re-vegetation period after a fire. Of the debris flow mitigation alternatives, only the debris basin was found to sufficiently reduce the debris flow hazard to the homes. Two alternative debris basin locations have been identified: Upper Debris Basin (located partially in the Wilderness Area), and Lower Debris Basin (located below the Wilderness Area). The alternative debris basin locations are shown on Figure V-1. ---PAGE BREAK--- ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan V-2 The Upper Debris Basin alternative is located partially within the wilderness area and would conceptually have a top of dam elevation of 5610 feet. For reference, the existing parking lot and the top of the old reservoir embankment are at about 5600 feet. This alternative would allow maintaining a portion of the existing trees between the homes and the embankment. A action of the U.S. Congress would be required to authorize construction and maintenance within the wilderness area. A typical cross section through the Upper Debris Basin is shown on Figure V-2. The Lower Debris Basin alternative is located on U.S. Forest Service property between the wilderness area and the homes. The conceptual top of dam elevation is 5595 feet (about five feet lower than the top of the existing old reservoir embankment). A typical cross section through the Lower Debris Basin is shown on Figure V-3. Conveyance system improvements without the debris basin discussed above are believed to be insufficient to remove the homes from the flood hazard designation. Four alternatives have been identified for improving the conveyance system through the residential area between Zarahemla Drive and Wasatch Blvd. Three of the alternatives (riprap channel, composite channel, and concrete low flow channel) assume that the existing under-capacity culverts (see Table IV-1) are replaced. The fourth alternative replaces the existing culverts and channels with a storm drain pipe. Conceptual cross sections of the alternatives are shown on Figure V-4. The alternatives are compared on Table V-1. An option for the composite channel alternative is included which does not include grade control structures. ---PAGE BREAK--- E N E E R S G I N ---PAGE BREAK--- E N E E R S G I N ---PAGE BREAK--- ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan V-3 1. RIPRAP CHANNEL 300 cfs SF=1 70 cfs SF=1.5 Likely the least maintenance costs. $400 2A. COMPOSITE CHANNEL 50 cfs riprap lowflow 300 cfs w/ SF=1 on matt So = 7.0%, GSBD 5' height The drops will affect the width of the improvements and will increase potential for conflict with existing structures. $550 2B. COMPOSITE CHANNEL 50 cfs riprap lowflow Mat side slopes, but no drops Potential for extensive erosion in higher flows. $250 3. CONCRETE LOW FLOW CHANNEL with MAT PROTECTED GRASS CHANNEL 50 cfs low flow with concrete channel depth for sequent depth matt lined channel above to total 300 cfs sequent depth Safety and aesthetics issues. Potential for extensive erosion in higher flows. $240 4. PIPE ALTERNATIVE 300 cfs; min. depth to pipe flowline = sequent depth Concerns over maintenance and integrity of pipeline without a debris basin. $340 Note: The comparative cost per foot does not include costs for elements common to all alternatives. For example the road repair costs are not included and are considered equivalent for all alternatives and therefore not needed to compare conveyance alternatives. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan VI-1 A key purpose of Salt Lake County Flood Control is to plan drainage improvements to better protect County residents from flooding and bring the system up to the requirements of the federal Flood Insurance Program. An analysis of Neffs Canyon Creek flooding hazard mitigation has been completed for the subdivision located between the mouth of Neffs Canyon and Wasatch Blvd. The analysis and potential mitigation measures are summarized below. A storm rainfall runoff model was prepared for the Neffs Canyon watershed using the U.S. Army Corps of Engineers Hydrologic Modeling System (HEC-HMS) software (please see Chapter II above). A summary of the design creek flow rates for a 10-Year and a 100-Year return period (a 100-year return period event has a 1% chance of being equaled or exceeded in any given year) are provided in Table VI-1. The snow melt flood flows were estimated using regional regression equations (see estimated snow melt flow rates in Table VI-2). Canyon Mouth 70 300 Wasatch Blvd 90 350 Mouth of Canyon 50 70 75 A debris flow flooding hazard associated with an alluvial fan has been identified for areas located of the mouth of Neffs Canyon (see Chapter III). The design debris flow volume (33,800 cubic yards) is about 21 acre-feet. ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan VI-2 Neffs Creek low flows currently are delivered to the Olympus Cove subdivision from a channel which is higher than the thalweg (lowest part) of the canyon. The alignment of the current channel and the thalweg are shown on Figure IV-2. The diversion to the current channel from the Neffs Canyon thalweg occurs about 1300 feet east of the homes. The diversion is somewhat fragile and storm runoff often spills into the lower thalweg. The existing channel and culvert system which conveys Neffs Canyon flood flows through the subdivision to Wasatch Blvd. has capacity for about the 10-year snow melt event (about 50 cfs). There is risk of flooding of homes for events exceeding the 10-year snow melt event. In addition, the existing channel is steep and there is risk of rapid bank erosion during a major event. The recommended alternative for providing protection to developed areas below the canyon mouth is the construction of a debris basin for a design debris flow volume of 21 acre-feet. Alternative debris basin locations are shown on Figure V-1. It is recommended that the conveyance system through the subdivision be improved to convey the 100-year flood event. It is recognized that without the debris basin recommended above, flooding risk to homes cannot be mitigated through conveyance system improvements alone. Proposed Neffs Creek conveyance improvements are shown on Figure VI-1. Alternative channel cross section improvements are discussed in Chapter V (see Figure V-4) with a cost comparison (see Table V-1). ---PAGE BREAK--- ---PAGE BREAK--- SALT LAKE COUNTY FLOOD CONTROL Neffs Canyon Creek Master Plan R-1 Farmer, E. E. and Joel E. Fletcher. 1972. . Geilo Symposium, Norway. National Oceanic and Atmospheric Administration (NOAA) website. 2006. http://hdsc.nws.noaa.gov/hdsc/pfds. Point Precipitation Frequency Estimates for Utah. National Oceanic and Atmospheric Administration (NOAA). 1972. . Natural Resource Conservation Service (NRCS) Website. 2005. http://soildatamart.nrcs.usda.gov/. Soil Survey Geographic (SSURGO) Database for Salt Lake County, Utah. RS Means. 2007. . RS Means Inc. Kingston, MA. TRC North American Weather Consultants. 1999. ” ” U.S. Army Corps of Engineers (USACE). 2006. . Davis, California. U.S. Soil Conservation Service (SCS). 1972. . United States Department of Agriculture, Washington, D.C. U.S. Soil Conservation Service (SCS). 1986. . United States Department of Agriculture, Washington, D.C. ---PAGE BREAK--- APPENDIX A GLOSSARY AND ABBREVIATIONS ---PAGE BREAK--- - The storm event that has a 10% (1 in 10) chance of being equaled or exceeded in any given year. - The storm event that has a 1% (1 in 100) chance of being equaled or exceeded in any given year. - Cross drainage structures convey storm drainage flows from one side of the street to the other and normally consist of storm drains or culverts. - A rainfall event, defined by storm frequency and storm duration, that is used to design drainage structures or conveyance systems. - An impoundment structure designed to reduce peak runoff flowrates by retaining a portion of the runoff during periods of peak flow and then releasing the runoff at lower flowrates. - A Hydrologic Modeling System developed by the U.S. Army Corps of Engineers. - The drainage system which provides for conveyance of the storm runoff from minor storm events. The initial drainage system usually consists of curb and gutter, storm drains, and local detention facilities. The initial drainage system should be designed to reduce street maintenance, control nuisance flooding, help create an orderly urban system, and provide convenience to urban residents. - The drainage system that provides protection from flooding of homes during a major storm event. The major storm drainage system may include streets (including overtopping the curb onto the lawn area), large conduits, open channels, and regional detention facilities. - Generally accepted as the 100-year storm. Typically homes should be protected from flooding in storm events up to a 100-year event. - Storm event which is less than or equal to a 10-year storm. - A flood event with a very low probability, usually less than 0.2%, of being exceeded in any given year. This flood event is used as a design storm when failure of the structure could cause loss of life. - An impoundment structure designed to contain all of the runoff from a design storm event. Retention basins usually contain the runoff until it evaporates or infiltrates into the ground. - The length of time that defines the rainfall depth or intensity for a given frequency. - A measure of the relative risk that the precipitation depth for a particular design storm will be equaled or exceeded in any given year. This risk is usually expressed in years. For example, a storm with a 100- year frequency will have a 1% chance of being equaled or exceeded in a given year. (täl'veg) - The line defining the lowest points along the length of a river bed or valley. A subterranean stream. “The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2005, 2000 by Houghton Mifflin Company. Updated 2005.” ---PAGE BREAK--- acre-feet cubic feet per second (ft3/s) corrugated metal pipe detention basin detention East foot or feet Geographic Information System groundwater Hansen, Allen & Luce, Inc. inches North peak storm water flow in a 10-year event peak storm water flow in a 100-year event South West with without ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- APPENDIX D Annotated FIRM Maps ---PAGE BREAK--- A AO,2,1 AO,2,2 AO,2,2 AO,2,2 AO,2,3 AO,2,3 AO,3,3 AO,3,4 SHADED X SHADED X SHADED X SHADED X SHADED X SHADED X SHADED X Legend Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X ---PAGE BREAK--- A AO,2,3 AO,2,3 AO,3,3 AO,3,3 AO,3,4 SHADED X SHADED X SHADED X Legend Revised Floodplains Zone,Depth,Velocity A AO, 2, 1 AO, 2, 2 AO, 2, 3 AO, 3, 3 AO, 3, 4 SHADED X ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX B – HYDROLOGIC ANALYSIS TM ---PAGE BREAK--- TECHNICAL MEMORANDUM – 01 (DRAFT) TO: John Miller, P.E. City Engineer - Millcreek COPIES: Dan Drumiler, P.E. – Millcreek Stormwater Engineer FROM: Cameron Jenkins, P.E. CFM Craig Bagley, P.E. CFM Bowen Collins & Associates DATE: July 1, 2021 SUBJECT: Neffs Canyon Debris Basin Study Hydrologic Analysis JOB NO.: 656-21-02 SECTION 1 - INTRODUCTION 1.1 INTRODUCTION AND PURPOSE Millcreek, Utah contracted with Bowen Collins & Associates (BC&A) to study the feasibility of constructing a debris basin or other protective measure at the mouth of Neffs Canyon with the goal of eliminating the alluvial fan flood hazard that was recently developed by FEMA in this area. This study has been completed in accordance with FEMA, Utah Dam Safety Guidelines, and Rule R655-11 requirements. The funding for this project was provided through a FEMA Pre-Disaster Mitigation (PDM) grant. The principal objective of this study is to identify a feasible debris basin or other flood control improvements that can mitigate the alluvial fan hazards on the Neffs Canyon alluvial fan and to use the results and recommendations of this study to obtain future funding for construction of the proposed alternative. This technical memorandum (TM) is the first of several anticipated TMs that will be prepared for this feasibility study. This TM documents the results of a detailed hydrologic analysis for the Neffs Canyon drainage basin. The results of this hydrologic study will be used to identify channel capacity deficiencies and to perform a debris/detention basin sizing analysis. Additional TMs will be prepared in the future to evaluate alternative spillways, outlet works, and channel improvements. The project location is shown on Figure 1 below. 1.2 BACKGROUND AND PREVIOUS STUDIES Millcreek, Utah is located on the east side of Salt Lake County with Neffs Canyon being located on the west face of the Wasatch Mountains. Millcreek has experienced flooding from Neffs Creek in the past due to runoff from intense rainfall or snowmelt events as recently as May of 2019. In early May 2019, a spring runoff event resulted in shallow flooding on a large area below the mouth of Neffs Canyon. The flooding was caused by debris that accumulated on some logs that were placed across the main ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 2 Neffs Creek channel by hikers. The debris created a flow restriction that diverted water out of the active main creek channel into an historic inactive channel. The inactive channel traverses behind some homes and terminates along Zarahelmla Drive where there are no storm drains or flood control facilities to collect and manage runoff from the natural watershed. The active Neffs Creek channel runs through yards and under roads in the Mount Olympus neighborhood and discharges into a storm drain that runs under I-15 and continues west through Holladay before discharging into Big Cottonwood Creek. During the May 2019 event, there was shallow flooding in yards and streets. The flooding was caused more by the diversion than by the peak discharge in the creek. The area where water left the main channel is located on Federal Wilderness Area managed by the U.S. Forest Service, thus it is difficult to access and maintain. The area of the mouth of the canyon is mostly built out or developed and the roads have minimal amounts of curb and gutter or storm drain. Many homes also have down facing driveways. Due to the lack of curb and gutter and the down facing driveways in the area, the roadways do not have adequate storm water runoff conveyance capacity. Those homes and structures with down facing driveways and those without curb and gutter are more susceptible to flooding, even from a minor rainfall event. 2007 SALT LAKE COUNTY STUDY In 2007, Salt Lake County commissioned the “Neffs Canyon Creek Master Plan” by Hansen Allen and Luce (HAL). The purpose of that study was to develop conceptual mitigation options to help reduce the flooding hazard in the area. A geotechnical, hydrological and hydraulic analysis was performed as part of this study. The study concluded that the area sits on an alluvial fan and recommended that a debris basin be constructed at the mouth of Neffs Canyon. The study also included a hydrologic analysis for the purpose of evaluating conceptual flood control improvements. A 100-year discharge estimate of 300 cfs was developed for a point at the mouth of the Neffs Canyon. The amount of potential debris, debris basin sizing, and channel improvements were also estimated. 2016 FEMA STUDY In 2016, JE FULLER performed the Neffs Creek Flood Hazard Assessment for FEMA and developed alluvial fan floodplain mapping (see Figure 1) using a 2D model with the discharges developed in the 2007 HAL study. Those maps are set to become effective later this year and impact a large number of structures in Millcreek. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 3 Figure 1. Project Location ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 4 1.3 EXISTING DATA The hydrologic analysis of the of the Neffs Canyon Project utilized the data sources presented in Table 1. Table 1 Study Data Sources Data Source Description LiDAR Utah Automated Geographic Reference Center, (AGRC) 2013 1-meter resolution bare-earth digital terrain model (DTM) data set of Wasatch Front. Aerial Imagery Hexagon 30cm, via Utah AGRC, 2021 Aerial imagery was used for the background of the figures and drawings and to determine existing land uses for hydrologic models Soil Data NRCS Web Soil Survey Soil Survey Geographic Database (SSURGO) mapping data used to determine Hydrologic soil type for hydrologic models Hydrologic Analysis Utah Dam Safety Hydrologic analysis of spillway, and as-builts of inlet structures and spillways. Used to evaluate the capacity of the channel and structures. Land Cover Type NLCD 2016 The National Land Cover Database (NLCD) provides nationwide data on land cover and land cover change at a 30m resolution with a 16-class legend based on a modified Anderson Level II classification system. Elevation Datum NAVD 88 For this report the elevations are based on NAVD 88. Previous Studies and Reports Millcreek Reports used to gather relevant information of the Neffs Canyon Drainage area: 2007 Salt Lake County Study, 2016 FEMA Study, and the 2020 CRS Study. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 5 SECTION 2 - HYDROLOGY 2.1 INTRODUCTION/OVERVIEW In accordance with Utah Dam Safety Guidelines and Rule R655-11 of the Utah Administrative Code and FEMA guidelines, BC&A developed detailed hydrologic computer models of the Neffs Canyon drainage area for use in evaluating alternatives with various debris basin sizes, spillway discharges, and other mitigation options. The analyses include developing several critical storm hyetographs, routing computed runoff hydrographs through potential detention or flood retarding facilities, and computing the Inflow Design Flood (IDF) for the project. The model was developed using the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) computer software. The development of the various parameters and elements of the model is discussed in detail in the sections that follow. In 2007, a Salt Lake County study was completed and used as part of the 2016 FEMA study to update the floodplain mapping for the Neffs Creek alluvial fan. The results and findings of that analysis are referenced in this section to provide a basis of methodologies, and for comparison of results and assumptions. 2.2 WATERSHED CHARACTERISTICS The Neffs Canyon drainage area at the canyon mouth contains about 2,300 acres (3.7 square miles), with an average basin elevation of 7,800 ft and an average basin slope of 60.5 percent based on the USGS StreamStats website. The ground cover in Neffs Canyon Watershed is mainly composed of Juniper, Oak-Aspen, and herbaceous vegetation. The drainage area is also almost entirely comprised of public land managed by US Forest Service land (see Figure 1) and has two different flow paths. The current Neffs Creek channel appears to be manmade up on the hillside. The original channel is located at the topographic low point of the canyon which conveys runoff from any flow breakouts from the current active main channel to Zarahemla Drive where the channel terminates. SUBBASIN BOUNDARIES Utilizing the data sources listed in Table 1, along with field investigation, watershed characteristics, and reviewing the 2007 County study, it was determined a single subbasin would be appropriate for this analysis. The subbasin boundary is shown on Figure 2. 2.3 STORM EVENTS If a debris basin is constructed at the mouth of Neffs Canyon it will likely include a dam embankment to impound water and debris. Due to its proximity to homes and development, such a dam would likely be classified as a High Hazard Dam by the Utah State Engineer and would have to be designed to meet minimum State dam safety criteria. Per Utah Dam Safety and Rule R655-11 (requirements for the design, construction and abandonment of dams), two theoretical probable maximum precipitation (PMP) events must be evaluated: a 6-hr local storm and a 72-hr general storm (see Section 2.4 for more information). Additionally, the standard FEMA 10- to 500-year events based on a 6-hr storm were also simulated so that impacts to the FEMA flood hazards could be evaluated. A 6-hr storm event was chosen as the FEMA design flood event to be consistent with other recently studied creeks by the County and FEMA for FEMA floodplain mapping. It should be noted that the effective discharge for nearby creeks (Mill Creek, Big Cottonwood Creek, and Little Cottonwood Creek) are based on 3-hr storm events. In total, 11 storm events (see Table 2) with various durations and rainfall temporal distributions were evaluated to meet various FEMA and Utah Dam Safety regulations associated with the design and construction of dams and flood control facilities. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 6 Figure 2. Detailed Sub-basin Boundaries Map ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 7 Table 2 Summary of Critical Storm Precipitation Inputs 6-Hour Storms 24-Hour Storms 1 Local PMP (AMC II) 9 100-year (AMC III)) 2 Local SEP (AMC II) 72-Hour Storms 3 10-year (AMCII) 10 General PMP (AMC II) 4 25-year (AMCII) 11 General SEP (AMC II) 5 50-year (AMCII) 6 100-year (AMCII) 7 100-year (AMC III) 8 500-year (AMCII) 2.4 PRECIPITATION STANDARD STORMS Design storm depths, except for the PMP, were retrieved from the NOAA Precipitation Frequency Data Server (PFDS) which reports total storm depths for a specific geographic location based on NOAA Atlas 14 for various event durations and recurrence intervals. For the purposes of this project, a single large storm occurring over the basin at the same time was considered. The extent of the considered design storm was approximated by the elliptical area shown on Figure 3 which has an area of 7.8 square miles. Storm depths were obtained from the NOAA precipitation grid data with the mean value for the watershed. Mean storm depth estimates for recurrence intervals from 10- to 500- years for the 6-hr event and 100- to 500-years for the 24-hr event are provided in Table 3. 2.4.1.1 AREAL REDUCTION The NOAA Atlas 2 (1973) recommends a storm-centered areal reduction (ARF) of 0 to 15 percent for 6-hour storm cells ranging from 0 to 100 square miles in area. These factors, however, are based on data from thunderstorms in the Midwest, rather than those typical to the Salt Lake Valley. The results of a more locally pertinent depth-area precipitation analysis were taken from the Salt Lake City Hydrology Manual (1983). This method was used in this study to be consistent with the 2007 County Study and other recent FEMA studies in the county. The manual recommends the following precipitation depth-area relationship for a thunderstorm, with area in square miles: 6-hr Reduction Factor = 0.01*(100 – 3.5*Area^0.46) 1-day Reduction Factor = 0.01*(100 – 2.0*Area^0.46) This relationship is based on data from Project Cloudburst, a study completed by the U.S. Army Corps of Engineers in April 1979. This study involved collection of data from a network of rain gages in Salt Lake City and vicinity covering an area of roughly 350 square miles. The storm cell over the watershed is approximately 7.8 square miles which would have a ARF of 0.91 for the 6-hr storm and 0.95 for the 24-hr storm. 2.4.1.2 SEASONAL REDUCTION Rainfall Depths in NOAA Atlas 14 include all precipitation events, including snow which does not typically generate immediate runoff. In order to better estimate a rain precipitation event, an additional seasonal adjustment was made to the areal adjusted depths. NOAA Atlas 2 (which was superseded by Atlas 14) includes separate point precipitation estimates for annual data and for a subset of seasonal data occurring between the months of May and October. Eq 2.2 Eq 2.1 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 8 Figure 3. Generalized Storm Area ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 9 For each storm recurrence interval, the ratio of the NOAA Atlas 2 seasonal depth to the annual depth was computed and used to convert the Atlas 14 derived, areal adjusted depths to a final estimated rainfall depth. The seasonal adjustment ratio and final adjusted rainfall depths are provided in Table 3. Calculations to determine the seasonal adjustment factor are provided in Appendix B. Table 3 Standard Storm Depths Description Recurrence Interval (year) 6 hr 24 hr 10 25 50 100 500 100 NOAA Atlas 14 Mean Depth (in) 1.68 2.01 5.29 2.60 3.70 4.07 Areal Adjusted Depth (in) 1.53 1.83 2.08 2.37 3.37 3.86 Seasonal Adjustment 89 90 91 92 94 94 Final Adjusted Design Depth (in) 1.36 1.65 1.90 2.18 3.16 3.63 PMP AND SEP The PMP precipitation for the local 6-hr and general 72-hr storm depths along with their distributions are normally obtained by following the procedures from the Hydrometeorological Report 49 (HMR49) which covers the Colorado River and Great Basin Drainages and includes Utah and surrounding areas. The State of Utah has published two additional studies that modify those values. These two studies are titled “Probable Maximum Precipitation Estimates for Short- Duration, Small-Area Storms in Utah” (USUS) and “2002 Update for Probable Maximum Precipitation, Utah, 72-Hour Estimates, Area to 5,000 mi²” (USUL). The precipitation values developed from HMR49 and supplemented by USUS or USUL, are referred to as local and general Spillway Evaluation Precipitation (SEP) values and include an areal reduction. The developed PMP and SEP values for the 6 hour and 72 hour general and local storms are shown in Table 4. Once the critical SEP has been determined, it will be compared to the 100-year, 6-hr (for local storms) or the 100-year, 24hr (for general storm). Table 4 PMP and SEP Storm Precipitation Depths Storm Duration Precipitation Depth (inches) PMP1 SEP2 6-hr3 8.6 8.0 72-hr 16.26 16.21 Notes: 1. The precipitation depths were generated using HMR49 only. 2. The precipitation depths were generated using HMR49 along with USUL (72HR) and USUS (6 HR). DESIGN STORM DISTRIBUTION The selected design storm distribution for the standard 6-hr storms were based on the Quartile 2, 50 Percentile Convective Storm Pattern found in the NOAA Atlas 14 Semi-Arid Region documents. The 6-hr local storm and 72-hr general storm were developed using the methods identified in HMR49 and State of Utah standards. These unit rainfall distributions were used with the storm depths adjusted for area and seasonality as described above and provided in Appendix B. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 10 2.5 PARAMETER ESTIMATION CURVE NUMBER Runoff Curve Numbers (CN) were estimated for each sub-basin based on soil type and land use/vegetative cover. Hydrologic soil group (HSG) maps were obtained from the NRCS Soil Survey Geographic (SSURGO) dataset and are shown on Figure 4. Land use or vegetative cover was determined by inspection of aerial imagery, conducting field observations and using the USGS National Land Cover Dataset (NLCD) mapping data. Land use descriptions from the NLCD data were related to similar land cover descriptions in TR-55. A map of land uses and vegetative cover present within the study area are shown on Figure 5. Using GIS software, composite CNs representing the sub-basin was calculated on a weighted area basis. The CNs used for hydrologic soil-cover complexes were based on information from TR-55 and are summarized in Table 5. The initial calculated composite curve number for the sub-basin in the study area is provided in Table 6 with the calculations shown in Appendix B. Table 5 Curve Numbers for Hydrologic Soil-Cover Complexes Land Use/Vegetative Cover Hydrologic Soil Group A B C D Deciduous Forest, Oak-Aspen (Fair) 48 48 57 63 Evergreen Forest, Juniper (Fair). 58 58 73 80 Grassland/Herbaceous, Sagebrush (Fair) 51 51 63 70 Mixed Forest, Juniper (Poor) 75 75 85 89 Shrub/Scrub, Oak-Aspen (Poor) 66 66 74 79 ESTIMATE OF INITIAL ABSTRACTION Initial abstraction is the fraction of the storm depth after which runoff begins and is governed by the following equation: 𝐼𝑎= 0.2𝑆 Where 𝑰𝒂 = Initial abstraction (in.) 𝑺 = potential maximum soil moisture retention (in.) This relationship is based on empirical relationships between infiltration, surface depression storage, interception and evapotranspiration. The potential maximum soil moisture retention, S, is related to CN (NRCS 2004) and can be estimated using the following equation: 𝑆= 1000 𝐶𝑁−10 The preceding equations were used to compute the starting initial abstraction value shown in Table 6. Eq 2.4 Eq 2.3 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 11 Figure 4. Hydrologic Soil Group Map ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 12 Figure 5. Hydrologic Model Existing Land Cover Map ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 13 TRANSFORM METHOD As was done in the 2007 County study, the lag time was calculated using the regression equation from the M.J Simas and R.H. Hawskins “Lag Time Characteristics for Small Watersheds in the U.S.” study. This method requires the basin area, slope, and curve number characteristics: The flow length was estimated using GIS computer tools and the elevations were taken from the digital terrain model. The basin characteristics and resulting lag time for the single subbasin are provided in Table 6 Table 6 Summary of Initial Sub-basin Hydrologic Parameters Drainage Area Watershed Length Max Elev (ft) Min Elev (ft) Composite Curve Number Lag Time (min) Initial Abstraction (inches) Sq. ft Sq. mi. 102,310,000 3.7 17,500 9675 5634 66 110 1.0 2.6 CALIBRATION FLOOD FREQENCY ANALYSIS A flood frequency analysis is the preferred method for estimating the magnitude and return frequency of peak flows from gaged watersheds, however, no gage is present in Neffs Canyon. There are a few stream gages in other nearby watersheds that can provide insight on reasonable ranges for discharge. In 2020 CRS Engineers performed a study to analyze the discharge in Neffs Creek and some nearby gages were selected to run a Bulletin 17C statistical flood frequency analysis. Although the areas for the peak discharges of the three gages shown in Table 7 are larger than Neffs Creek (3.67 square miles), they provide insight on what range of values we can expect for Neffs Creek. Additionally in 2007, BC&A did a study for Alpine City using Bulletin 17B for the multiple stream gages throughout the Wasatch Front including the three by CRS. The results from the 2007 BC&A study are graphically shown in Figure 6 below and were updated to include the CRS study results. REGRESSION ANALYSIS The USGS has developed regional regression equations for estimating peak discharge rates in un- gaged watersheds throughout Utah. The regression equations are based on flood frequency analyses conducted on gaged watersheds throughout the state. The USGS published equations in 1994 under the National Flood Frequency program (NFF) and again in 2008 under the National Streamflow Statistics program (NSS). Both the NFF (Region 4) and NSS (Region 2) equations were applied to the study drainage basin for the 100-year event and NSS (Region 2) for the 500-year event. A drainage area of 3.67 square miles and an elevation of 7,800 was used in the equations. A summary of the regression equation estimates for the flood events are provided in Table 8 and graphically shown on Figure 6. The NSS equations were also applied to the same creeks with stream gages for comparison purposes and is shown in Table 9. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 14 Table 7 Nearby Stream Gage Statistical Analysis Peak Flows (CRS 2020) Stream Name Location Watershed Area (mi2) Years of Record 100-year Peak Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) Mill Creek Canyon Mouth 21.7 1899- Curent (107 Peaks) 150 134 171 Emigration Creek Canyon Mouth 18.4 1902- Current (92 Peaks) 160 135 202 Red Butte Creek Red Butte Reservoir, Fort Douglas 7.25 1964-2019 (56 Peaks) 114 90 157 Table 8 Regression Analysis Peak Runoff Rates Event Method Drainage Area Mean Basin Elev. 100-year Peak Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) 10-year NSS 3.67 7,800 55 29 84 25-year NSS 70 36 106 50-year NSS 91 46 137 100-year NSS 107 54 161 500-year NSS 156 81 237 100-year NFF 154 82 236 CALIBRATION TO PREVIOUS STUDIES Another option of calibration or reasonableness check in absence of stream gage data is to use results from previous studies. The most recent hydrology study completed on Neffs Creek is the 2007 study for the County as discussed previously. This study utilized a 24-hr 100-year storm event to estimate a 100-year peak discharge of 300 cfs at the mouth of the canyon. In speaking with the engineer who performed that hydrology study, the purpose of that study was to develop a conceptual design for a debris basin and the 100-year design storm was considered to be conservative. They felt that the computed peak flows were high and tried multiple methods to reduce the discharge. In addition, they did not perform any model calibration work to determine if the computed model discharges were reasonable based on flood frequency study results on nearby watersheds. After reviewing the hydrologic methodology used in the 2007 study, completing an analysis of nearby stream gages and regression discharges, BC&A came to the same conclusion as the 2020 CRS study, which is: that following FEMA guidance on hydrology, the 100-year peak flood estimate of 300 cfs developed in the 2007 County study is outside one standard deviation of the mean regression peak discharge and may be considered unreasonable as shown in Table 9. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 15 Table 9 Flood Frequency Analysis and Regression Analysis Peak Runoff Rates for the 100-year Runoff Event (Taken from CRS 2020 Study) ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 16 Figure 6. Graphical Peak Discharge Comparison (Modified from 2007 BC&A Study) ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK CALIBRATION RESULTS Due to the lack of historic stream gage records on the creek, discharge computed using the USGS regression equations were used to help calibrate the hydrologic model. The hydrologic parameters CN, Initial Abstraction, and Lag Time were adjusted until the computed peak discharge was within at least one standard deviation of the regression equation discharge estimate. The CN parameter is intended to simulate the runoff potential for a given area and is somewhat subjective. During the calibration process it was determined that each event from the 10-year to the 500-year would need to be calibrated separately to obtain the desired results. For the 100-year event and less, the initial composite CN value shown in Table 6 was reduced by 3% to 64. For the 500-year event, the CN value was reduced by 10% to 60. The Initial Abstraction was initially calculated to be 1.1 using the standard equation for each event. 1.1 was determined to be an appropriate value for the 100-year event. Storm return periods less than the 100-year used a lower initial abstraction value, while the 500-year event used a higher value. The final calibrated hydrologic parameters are provided in Table 10 along with the rainfall runoff discharges and a comparison to the USGS regression equations. The final calibrated hydrologic parameters resulted in rainfall runoff discharges for the 100-year event and less that were very close to the USGS regression equation discharges. The 500-year event discharge is at the upper end of the one error standard deviation which is within FEMA’s requirements. Further calibration to fit the results of the regression equation more closely would require the use of unreasonable CN or initial abstraction values. BC&A feels that the higher 500-year discharge would also be conservative for the design of the debris basin or other alternatives. For this study, the PMP and SEP hydrologic scenarios used the 500-year calibrated CN parameters as they represented the best calibration for larger events. Table 10 Summary of Final Calibrated Sub-basin Hydrologic Parameters Event Composite Curve Number Initial Abstraction (inches) Rainfall- Runoff Discharge (cfs) NSS Discharge (cfs) Low Confidence Limit (0.16) (cfs) High Confidence Limit (0.84) (cfs) 10-year 64 0.7 45 55 29 84 25-year 64 0.8 71 70 36 106 50-year 64 0.9 96 91 46 137 100-year 64 1.1 107 107 54 161 500-year 60 1.4 228 156 81 237 Note: The PMP and SEP scenarios will use the 500-year calibrated parameters 2.7 AMCIII As part of the Utah Dam Safety requirements for dams, a saturated soil condition (AMCIII) is required for the selected 100-year scenario (6-hr or 24-hr) to help size the spillway. To account for changes due to soil moisture, the NRCS provides a table for converting AMC II Curve Numbers to AMC III Curve Numbers. Using this conversion table, the composite 100-year AMC III CN for the entire drainage basin is 81. As no other events require the AMCIII, the conversions are not included. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK 2.8 SNOWMELT Historical snowmelt peak flows are not available for Neffs Canyon. The 2007 study for the County used regression equations developed by Gingery and Associates (see 2007 study) and provided the snowmelt flow rates shown in Table 11. It should be noted that Mill Creek and other large creeks in the area generally have peak discharges based on spring runoff from snow melt. Table 11 Estimated Snow Melt Flow Rates Location Predicted Snowmelt Flow Rates (cfs) 10-year 50-year 100-year Mouth of Canyon 50 70 75 2.9 HYDROLOGY RESULTS AND RECCOMENDATIONS RESULTS An HEC-HMS model was developed for the 3.67 square mile Neffs Canyon Watershed based on the parameters and inputs discussed above. Table 12 provides a summary of the peak discharges at the canyon mouth and Figure 7 shows the hydrographs. The results show that the 100-year peak discharge of 107 cfs is less than the 300 cfs from the 2007 County study which had a higher CN value and higher precipitation depth based on a ARF with a smaller area. The area is very sensitive to the CN value with even small changes to CN making large changes to discharge. The results also show that if a debris basin is constructed at the mouth of Neffs Canyon, it may have to pass over 2,500 cfs over the spillway to meet Dam Safety requirements. Table 12 Existing Conditions Model Peak Discharge Summary Storm Event Peak Discharge(cfs) 06hr_10yr_NOAA_AMCII 45 06hr_25yr_NOAA_AMCII 71 06hr_50yr_NOAA_AMCII 96 06hr_100yr_NOAA_AMCII 107 06hr_100yr_NOAA_AMCIII 445 06hr_500yr_NOAA_AMCII 228 24hr_100yr_NOAA_AMCII 207 24hr_100yr_NOAA_AMCIII 453 06hr_PMP_Local_Storm 2,898 06hr_SEF_Local_Storm 2,542 72hr_PMP_General_Storm 1,460 72hr_SEF_General_Storm 1,460 ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK Figure 7. Runoff hydrographs for Neffs Creek. 0 500 1000 1500 2000 2500 3000 3500 0 4 8 12 16 Discharge (cfs) Time (hr) 06hr_10yr_NOAA_AMCII 06hr_25yr_NOAA_AMCII 06hr_50yr_NOAA_AMCII 06hr_100yr_NOAA_AMCII 06hr_100yr_NOAA_AMCIII 06hr_500yr_NOAA_AMCII 06hr_PMP_Local_Storm 06hr_SEF_Local_Storm 0 200 400 [PHONE REDACTED] 1200 1400 1600 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 Discharge (cfs) Time (hr) 24hr_100yr_NOAA_AMCII 24hr_100yr_NOAA_AMCIII 72hr_PMP_General_Storm 72hr_SEF_General_Storm ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK CONCLUSIONS AND RECOMMENDATIONS The purpose of this study is to evaluate the feasibility of a debris/detention basin to mitigate the alluvial fan hazard of the canyon mouth. Based on the results from the updated detailed hydrologic analysis, the following conclusions and recommendations can be made: 1. The 100-year flood estimate of 300 cfs for Neffs Canyon is overly conservative. For the purposes of this study and for design of flood control facilities, a 100-year peak discharge of 107 cfs should be used. 2. Continue with the evaluation of a potential debris basin and channel improvements as 1) the channel does not have capacity for 107 cfs and 2) breakout flows are still possible out of the main channel and into the original flowpath. 3. The sediment/debris volume estimate developed in the 2007 Salt Lake County study should be updated in the next phase of the project. That updated estimate should be used in evaluating the size of any debris/detention basin. 4. Evaluate other possible flood control improvement alternatives such as a diversion berm or channel to direct Neffs Creek runoff from the historic channel to the existing active channel or from the existing active channel to the historic channel. Based on field observations and capacity estimates of the existing channel, it appears that it would also be prudent to construct an inlet and pipeline that will safely convey the 100-year flood of 107 cfs through the developed area. The new pipeline would be constructed in existing city streets. 5. Analyze the hydraulics of the canyon mouth using the existing 2D model and updated 100-year runoff hydrograph to determine if the revised 100-year flood would result in any significant changes in flood depths extent as defined on the new FEMA flood hazard map. That analysis could be funded by the Unified Fire Authority who are interested in pursuing a Conditional Letter of Map Revision to revise the alluvial fan hazard so they can remodel an existing fire station. 6. Begin coordinating with the U.S. Forest Service, as some of these potential improvements would need to be constructed on land they manage, requiring adherence to their rules and policies. 7. If the updated floodplain analysis results in significant reductions to the flood hazard defined by the current preliminary FEMA floodplain map, consider submitting a hydrology-based CLOMR to determine if FEMA will accept the lower discharge to update the flood hazard map. 8. Continue coordination with Salt Lake County, the Utah State Engineer’s office, and the State Department of Emergency Management regarding proposed changes to the design hydrology and potential changes to the alluvial fan flood hazard maps for the area. ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK APPENDIX A PHOTOS ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- NEFFS CANYON DEBRIS BASIN BOWEN COLLINS & ASSOCIATES MILLCREEK APPENDIX B HYDROLOGIC CALCULATIONS ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: 6-Hr NOAA Distribution (2nd Quartile - 50% Probability) = cell requiring user input 0.91 Days Hrs Mins 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 6 0.00 0.00 0.00 1.68 2.01 2.29 2.60 3.70 0.00 0.00 0.00 1.53 1.83 2.08 2.37 3.37 0.00 0.00 0.00 1.36 1.65 1.90 2.18 3.16 Unit Rainfall Start End (inches) 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr - 0:00 0.0000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0:00 0:30 0.0480 0.000 0.000 0.000 0.065 0.079 0.091 0.104 0.152 0:30 1:00 0.1240 0.000 0.000 0.000 0.169 0.204 0.235 0.270 0.392 1:00 1:30 0.2480 0.000 0.000 0.000 0.337 0.408 0.470 0.540 0.785 1:30 2:00 0.3960 0.000 0.000 0.000 0.539 0.652 0.751 0.862 1.253 2:00 2:30 0.5580 0.000 0.000 0.000 0.759 0.919 1.058 1.215 1.766 2:30 3:00 0.7100 0.000 0.000 0.000 0.966 1.169 1.346 1.545 2.247 3:00 3:30 0.8130 0.000 0.000 0.000 1.106 1.338 1.542 1.770 2.573 3:30 4:00 0.8800 0.000 0.000 0.000 1.197 1.449 1.669 1.915 2.785 4:00 4:30 0.9270 0.000 0.000 0.000 1.261 1.526 1.758 2.018 2.934 4:30 5:00 0.9610 0.000 0.000 0.000 1.308 1.582 1.822 2.092 3.041 5:00 5:30 0.9840 0.000 0.000 0.000 1.339 1.620 1.866 2.142 3.114 5:30 6:00 1.0000 0.000 0.000 0.000 1.361 1.646 1.896 2.177 3.165 ARF Adjusted Depths NOAA Atlas 14 Rainfall Depth (inches) Return Period Seasonal Adjusted Depths 30 min Interval Design Rainfall Distribution (Cumulative inches) Areal Reduction Factor (ARF) Basin Area ARF (mi2) Storm Duration 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0:00 1:00 2:00 3:00 4:00 5:00 6:00 Cumulative Rainfall (inches) Time (h:mm) Unit Rainfall 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: 24-Hr NOAA Distribution (2nd Quartile - 50% Probability) = cell requiring user input 0.95 Days Hrs Mins 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 24 0.00 0.00 0.00 0.00 0.00 0.00 4.07 0.00 0.00 0.00 0.00 0.00 0.00 3.86 0.00 0.00 0.00 0.00 0.00 0.00 3.63 Unit Rainfall Start End (inches) 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr - 0:00 0.0000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0:00 2:00 0.0680 0.000 0.000 0.000 0.000 0.000 0.000 0.247 2:00 4:00 0.1460 0.000 0.000 0.000 0.000 0.000 0.000 0.530 4:00 6:00 0.2570 0.000 0.000 0.000 0.000 0.000 0.000 0.933 6:00 8:00 0.4170 0.000 0.000 0.000 0.000 0.000 0.000 1.513 8:00 10:00 0.6010 0.000 0.000 0.000 0.000 0.000 0.000 2.181 10:00 12:00 0.7530 0.000 0.000 0.000 0.000 0.000 0.000 2.733 12:00 14:00 0.8520 0.000 0.000 0.000 0.000 0.000 0.000 3.092 14:00 16:00 0.9140 0.000 0.000 0.000 0.000 0.000 0.000 3.317 16:00 18:00 0.9570 0.000 0.000 0.000 0.000 0.000 0.000 3.473 18:00 20:00 0.9860 0.000 0.000 0.000 0.000 0.000 0.000 3.578 20:00 22:00 0.9980 0.000 0.000 0.000 0.000 0.000 0.000 3.622 22:00 0:00 1.0000 0.000 0.000 0.000 0.000 0.000 0.000 3.629 ARF Adjusted Depths NOAA Atlas 14 Rainfall Depth (inches) Return Period Seasonal Adjusted Depths 120 min Interval Design Rainfall Distribution (Cumulative inches) Areal Reduction Factor (ARF) Basin Area ARF (mi2) Storm Duration 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 Cumulative Rainfall (inches) Time (h:mm) Unit Rainfall 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: General and Local Storm Distributions PMP/SEF Storm Distributions Updated: December 11, 2018 Depth = 16.21 in Depth = 7.99 in Incremental Cumulative Incremental Cumulative 0 0.00 0.00 0.00 0.000 0.00 6 0.37 0.37 0.25 0.017 0.02 12 0.74 1.11 0.50 0.017 0.03 18 0.92 2.03 0.75 0.017 0.05 24 1.48 3.51 1.00 0.017 0.07 30 2.58 6.09 1.25 0.037 0.11 36 4.77 10.86 1.50 0.037 0.14 42 1.65 12.52 1.75 0.037 0.18 48 1.11 13.62 2.00 0.037 0.22 54 0.92 14.55 2.25 3.475 3.69 60 0.74 15.29 2.50 1.668 5.36 66 0.55 15.84 2.75 1.112 6.47 72 0.37 16.21 3.00 0.695 7.17 3.25 0.172 7.34 3.50 0.172 7.51 3.75 0.172 7.68 4.00 0.172 7.85 4.25 0.026 7.88 4.50 0.026 7.91 4.75 0.026 7.93 5.00 0.026 7.96 5.25 0.009 7.97 5.50 0.009 7.98 5.75 0.009 7.98 6.00 0.009 7.99 72-HR General Storm 6-HR Local Storm Time (hr) Rainfall Depth (in) Time (hr) Rainfall Depth (in) ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: Areal Reduction Factor (Salt Lake City - Project Cloudburst Derived) = cell requiring user input Area (mi2) 5-min 10-min 15-min 30-min 60-min 2-hr 3-hr 6-hr 12-hr 24-hr 2-day 3-day Equation variable 18.5 14.2 12.0 9.2 7.0 5.3 4.5 3.5 2.6 2.0 1.5 1.3 7.8 0.52 0.63 0.69 0.76 0.82 0.86 0.88 0.91 0.93 0.95 0.96 0.97 Note: These equations should not be used for Storms with durations longer than 3 hours. See excerpts from the Salt Lake City Hydrology Manual Below Area based on ellipse over watershed Basin Name Storm Duration Storm over the Entire Area To be consistent with the 2007 County Study and other recent studies for FEMA in the county, this method was used even though it is longer than a 3 hour storm. ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: Seasonal Reduction Factor Based on Atlas 2 Data See O:\GIS_Data\Utah_Statewide\Hydrology\NOAA Atlas II Figures for georeferenced Atlas 2 Raster Maps = cell requiring user input Storm Return Period (yr) Annual Rainfall (in) Seasonal Rainfall (in) Seasonal Reduction Ratio 1-Year, 06-Hr #NUM! 2-Year, 06-Hr #DIV/0! 5-Year, 06-Hr 5 88.0% 10-Year, 06-Hr 10 1.8 1.6 89.0% Taken from HAL 2007 study 25-Year, 06-Hr 25 90.0% 50-Year, 06-Hr 50 91.0% 100-Year, 06-Hr 100 2.6 2.4 92.0% Taken from HAL 2007 study 200-Year, 06-Hr 200 - - 93.0% 500-Year, 06-Hr 500 - - 94.0% 10-year, 24-Hr 10 2.5 2.3 92.0% Taken from HAL 2007 study 100-year, 24-Hr 100 4.1 3.8 94.0% Taken from HAL 2007 study Notes: 1. all ratios estimated based on logarithmic trendline except the 10-year and 100-year events. Compute a rainfall reduction ratio based on the May-October Rainfall and Annual Rainfall Maps from NOAA Atlas 2 maps. y = 0.0131ln(x) + 0.8591 R² = 0.9949 88.5% 89.0% 89.5% 90.0% 90.5% 91.0% 91.5% 92.0% 92.5% 1 10 100 Seasonal Reduction Ratio Return Period (yr) Seasonal Reduction Ratio ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: CN Calculation NLCD 2016 Code NLCD 2016 TR-55 Land Cover A B C D Soil Group Area (sqft) Weighted Area CN Weighted CN 41 Deciduous Forest Oak-Aspen (Fair) 48 48 57 63 B 11,704,379 0.114 48 5.49 41 Deciduous Forest Oak-Aspen (Fair) 48 48 57 63 D 6,977,560 0.068 63 4.30 41 Deciduous Forest Oak-Aspen (Fair) 48 48 57 63 C 6,756,651 0.066 57 3.76 42 Evergreen Forest Juniper (Fair) 58 58 73 80 B 33,363,076 0.326 58 18.92 42 Evergreen Forest Juniper (Fair) 58 58 73 80 D 28,085,275 0.275 80 21.96 42 Evergreen Forest Juniper (Fair) 58 58 73 80 C 3,007,967 0.029 73 2.15 71 Grassland/Herbaceous Sagebrush (Fair) 51 51 63 70 C 48,438 0.000 63 0.03 43 Mixed Forest Juniper (poor) 75 75 85 89 B 779,430 0.008 75 0.57 43 Mixed Forest Juniper (poor) 75 75 85 89 D 376,193 0.004 89 0.33 43 Mixed Forest Juniper (poor) 75 75 85 89 C 65,920 0.001 85 0.05 52 Shrub/Scrub Oak-Aspen (poor) 66 66 74 79 D 8,865,346 0.087 79 6.85 52 Shrub/Scrub Oak-Aspen (poor) 66 66 74 79 C 1,494,550 0.015 74 1.08 52 Shrub/Scrub Oak-Aspen (poor) 66 66 74 79 B 776,606 0.008 66 0.50 Total 66.0 ---PAGE BREAK--- JOB TITLE Neffs Canyon Debris Basin CALC. BY Cameron Jenkins DATE 06/30/21 CHECK BY DATE SUBJECT: Lagtime Initial Calibrated Watershed Are 102,310,000 Watershed Len 17500 17500 Max Elevation ( 9675 9675 Min Elevation ( 5634 5634 CN 66 64 Width 5846.3 5846.3 Slope 0.2 0.2 S 5.2 5.6 Lag Time (hr) 1.8 1.9 Lag Time (min) 110.0 113.1 As was done in the 2007 County study, the lag time was calculated using the regression equation from the M.J Simas and R.H. Hawskins “Lag Time Characteristics for Small Watersheds in the U.S.” study. This method requires the basin area, slope, and curve number characteristics: ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX C – GEOLOGIC/GEOTECHNICAL ANALYSIS TM ---PAGE BREAK--- 0LGYDOH8WDK JHUKDUWFROHFRP  7(&+1,&$/0(025$1'80 ,1752'8&7,21$1'%$&.*5281' 2ZQHU    ZDWHUIORZV  7R  %RZHQ&ROOLQV 'UDSHU8WDK &F  0U&UDLJ%DJOH\3(&)0 )URP  'DQLHO$%LOOLQJV3(  (OOLRWW/LSV3* 'DWH  -RE1XPEHU  6XEMHFW  ---PAGE BREAK---  E\8WDK'DP6DIHW\ 8'6   *LYHQWKH ZDVQRWDYDLODEOH    70 VHH)LJXUH   ---PAGE BREAK---   x 8*6 *HRORJLF6XUYH\ 86*6  x x x  WLPH /XQGHWDO   VXFKDV8WDK/DNHDQG*UHDW6DOW/DNH   LQDQDOOXYLDO  685),&,$/&21',7,2167232*5$3+< &DQ\RQDERYHWKHSDUNLQJDUHDIRUWKH1HII¶V&DQ\RQ7UDLOKHDG7KHJURXQGVORSHVWRZDUG 9LQSODFHV ---PAGE BREAK---     x x x x 'HEULV)ORZ x x /DQGVOLGH x 5RFNIDOO x  )LJXUH LQFOXGHGLQ)LJXUH   SURSHUW\ 8*6 WKHIDXOWLQWKLVDUHD  ---PAGE BREAK---  'X5RVVHWDO +\OODQGDQGRWKHUV   PHPR PHPR DQG0F.HDQ 7KH  ---PAGE BREAK---     26/    7KH,6%LVDQRUWKHUO\ DQGQRUWKHUQ$UL]RQD 6PLWKDQG$UDEDV]   PRGHUDWHWRODUJH 88668866   ---PAGE BREAK---   GHULYHGIURP$QGHUVRQHWDO VFDOHRI    /DQGVOLGH+D]DUG VORSHV  VHHEOXH RXWOLQHVLQ)LJXUH ---PAGE BREAK---    5RFNIDOO+D]DUG VRLO  )LJXUHVDQG  3UREOHP6RLO+D]DUG ZKHQIUHHZDWHU ---PAGE BREAK---   VROXEOHPLQHUDOVVXFKDVJ\SVXPDUHDOVRVXEMHFWWRFROODSVHDVDUHDHROLDQ ZLQG    +D]DUGV6XPPDU\  ]RQH  27+(5&216,'(5$7,216 5HXVHRI6LWH0DWHULDOV  ---PAGE BREAK---   x x 6KDOORZHUVORSHVPD\EHUHTXLUHGIRUVWDELOLW\LIWKLVPDWHULDOLVXVHG x 9HU\FRDUVHPDWHULDOLVOLNHO\±WKLVPDWHULDOFDQEHXVHGDVULSUDSRUZRXOGQHHGWR    &21&/86,21$1'5(&200(1'$7,216)25$'',7,21$/678'<  VLWH     ---PAGE BREAK---  ODQGVOLGH  EDVLQZLOOEHFODVVLILHG  /,0,7$7,216   RXURIILFH   ---PAGE BREAK---   &RQWUDFW5HSRUW 6FKZDUW]'3 =RQH8WDK86$-RXUQDORI*HRSK\VLFDO5HVHDUFK  (OOLRWW$+DQG+DUW\.0 /DQGVOLGH0DSVRI8WDK6DOW/DNH&LW\¶[¶ RIVFDOH   6XUIDFH)DXOW5XSWXUH+D]DUG0DSRIWKH6XJDU  9ROXPH   6DOW/DNH&LW\ 'DPHV 5HSRUW    ---PAGE BREAK---  [[YLL   0F.HDQ$3 *HRORJLF0DSRIWKH6XJDU+RXVH4XDGUDQJOH6DOW/DNH&RXQW\  6PLWK5%DQG$UDEDV]:-   $PHULFDQ*HRORJ\0DS(GV   8866  8866  )LJXUHV )LJXUH 6LWH0DSDQG*HRORJLF+D]DUGV )LJXUH *HRORJLF0DS )LJXUH )LJXUH  +LVWRULF$HULDO,PDJHU\ ---PAGE BREAK--- Site Map and Geologic Hazards Figure 1 Neffs Canyon Debris Basin (21-1398) ± 0 250 500 125 Feet Legend Dam Alternative Boundary Inline Detention Basin Boundary Landslide Deposits (McKean, 2020) Landslide Deposits (Elliott & Harty, 2010) Quaternary Faults (Hiscock and McKean, 2018) <130,000, Inferred Surface Fault Rupture Hazard Special Study Zone (UGS, 2020) Utah Wilderness Areas Inline Debris Basin Dam Alternative Basemap Source: Utah Geographic Resource Center (UGRC) Raster Data Discovery Possible continuation of fault in bedrock, mapped by McKean (2020) Norths Fork Neffs Canyon Diversion channel ---PAGE BREAK--- Geologic Map Figure 2 Neffs Canyon Debris Basin (21-1398) ± 0 250 500 125 Feet Source Map: Adam P. McKean, 2020, Geologic Map of the Sugar House Quadrangle, Salt Lake County, Utah. M- 285dm. UGS. 1:24,000 scale. See Figure 3 for Descriptions of Geologic Units Dam Alternative Qafy Inline Debris Basin Diversion channel Norths Fork Neffs Canyon ---PAGE BREAK--- '(6&5,37,212)*(2/2*,&0$381,76 4K WKHH[WHQWRIILOO  ODQG 4PV 3RRUO\VRUWHGFOD\ WRERXOGHUVL]HGPDWHULDOLQ VOLGHVDQGVOXPSV PRVWRIWKHODQGVOLGHVDUHVPDOODQG PRUSKRORJ\ $VKODQG VWDELOLW\ DUHWRRVPDOOWRVKRZDWPDSVFDOH 4DI\ í3RRUO\WRPRGHUDWHO\VRUWHG SHEEOHWRFREEOHJUDYHOZLWKERXOGHUVQHDUEHGURFNVRXUFHVZLWKDPDWUL[RIVDQGVLOWDQGFOD\ WKLQWR 3DUOH\VDQG0LOO&UHHNV 4DIDQG4DI VKRZVHSDUDWHO\DWPDSVFDOH 4PF 0L[HGODQGVOLGH  4DI 3RRUO\VRUWHG SHEEOHWRFREEOHJUDYHOZLWKERXOGHUVQHDUEHGURFNVRXUFHVZLWKDPDWUL[RIVDQGVLOWDQGFOD\ FODVWV IRUPVVPDOOIDQV &DQ\RQDQGLQ2O\PSXV &RYH F\FOHV 0F&DOSLQ RIWKH /LWWOH9DOOH\ODNHDWDERXW IHHW  6FRWWDQGRWKHUV RUIHHW  6FRWW  5DQJH ZLWKWKHDJHVIRUWKH3RNHV3RLQWODNHF\FOHYDU\LQJIURPPRUHWKDQaND WR0,6RUDERXWND OHVVWKDQIHHW  6HH)LJXUHIRU*HRORJLF0DS 0F.HDQ  1RWHV 1HIIV &DQ\RQ'HEULV%DVLQ  )LJXUHD ---PAGE BREAK--- '(6&5,37,212)*(2/2*,&0$381,76 0GR /LJKW WKLQ ORFDOO\VRPHEHGV QRGXODUGDUNJUD\WREODFNFKHUW VORSHIRUPHU QRUWKVLGHRI1HIIV &DQ\RQ  UHSRUWHGD0RUURZDQ (DUO\  0K 0G 0K VXFKWKDW   IHHW LQ&LW\&UHHN&DQ\RQ*UDQJHU  UHSRUWHGD+XPEXJ DQG*UDQJHUDQGRWKHUV  P 6HH)LJXUHIRU*HRORJLF0DS 0F.HDQ  1RWHV 1HIIV &DQ\RQ'HEULV%DVLQ  )LJXUHE ---PAGE BREAK--- 1937 Historic Aerial Imagery Figure 4 Neffs Canyon Debris Basin (21-1398) ± 0 250 500 125 Feet Note: 1) Historic aerial imagery is provided by the Utah Geologic Survey (UGS) from accessed August 18, 2021. Dam Alternative Inline Debris Basin Norths Fork Neffs Canyon Diversion channel ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX D – DEBRIS AND SEDIMENT VOLUME ASSESSMENT TM ---PAGE BREAK--- Neffs Creek Debris Basin Feasibility Study Debris and Sediment Volume Assessment February 2022 prepared for Bowen Collins & Associates 8400 S Kyrene Rd, STE 201 Tempe, AZ 85284 www.jefuller.com Date Signed: 02/28/2022 ---PAGE BREAK--- i Table of Contents 1 Geologic Context 1 1.1 Past Events 5 2 Empirical Volume Estimates 6 3 Field Investigation 9 4 Summary 10 5 Limitations and Assumptions 12 6 References 13 List of Figures Figure 1. Vicinity map of study area 2 Figure 2. Longitudinal profiles of main channels 3 Figure 3. Geologic mapping (source: Bryant, 2005) 4 Figure 4. Average annual sediment yield rate versus drainage area size. From BUREC, 1987 8 List of Tables Table 1. USACE Equation 2 debris yield estimates 6 Table 2. Channel and drainage areas used for Hungr et al. (1984) volume estimates 7 Table 3. Summary of debris flow volume estimations 11 Appendices Appendix A - Field Photographs ---PAGE BREAK--- 1 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 1 GEOLOGIC CONTEXT Neffs Canyon drains to the west into the Salt Lake Valley and is bounded on the north by the larger Mill Creek Canyon drainage basin and to the south by Mount Olympus. The Olympus Cove neighborhood is located on the Neffs Creek alluvial fan just west of the canyon mouth (Figure The canyon mouth has an elevation of approximately 5,600 feet and the canyon crest reaches approximately 9,800 feet. The watershed is characterized by steep channels, with slopes ranging from 4% to 16% in the lower reaches to greater than 50% in the upper reaches. This study focused on three main channels, denoted as Neffs Creek, Unnamed Tributary, and North Fork (Figure These channels have average slopes of 22%, 28%, and 38%, respectively. Longitudinal profiles for these channels are shown in Figure 2. The Neffs Canyon watershed is located in Salt Lake County, Utah, and is part of the Wasatch Mountains, a mountain range on the easternmost edge of the greater Basin and Range Province. The Wasatch Mountains are geologically diverse, ranging from alluvial deposits to limestone, sandstone, and shales, to quartzite, gneiss, and schist, ranging in age from the Holocene to the Archean. The geology of Neffs Canyon is dominated by north-northwest dipping layers of limestone and quartzite (Figure This geological diversity reflects the long tectonic and related depositional history of the area. The Wasatch Mountain Front is a result of repeated normal fault displacement over the last several million years along the Wasatch Fault Zone, a fault system composed of several independently moving fault segments. Neffs Canyon is located at the northern end of the normally faulted, Salt Lake City segment of this fault system. The regional climate is characterized by cold, snowy winters and hot, dry summers with modest to light seasonal rainfall. Based on these conditions, it is considered a humid continental climate to dry-summer continental climate. The vegetation in Neffs Canyon consists of scrub oak, maple brush, pine trees, and aspen trees. This vegetation is dense in some areas but very sparse in the areas of the exposed slabs of steeply dipping, quartzite bedrock. ---PAGE BREAK--- 2 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment Figure 1. Vicinity map of study area ---PAGE BREAK--- 3 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment Figure 2. Longitudinal profiles of main channels 5700 6200 6700 7200 7700 8200 8700 9200 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 ElevaƟon Distance Channel Longitudinal Profiles Neffs Creek Unnamed Tributary North Fork ---PAGE BREAK--- 4 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment Figure 3. Geologic mapping (source: Bryant, 2005) ---PAGE BREAK--- 5 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 1.1 PAST EVENTS There is no documentation for the debris flow history within Neffs Canyon, however, the history of the neighboring areas is well documented and can be used to better understand the regional recurrence of debris flow events. Along the Wasatch Front, debris flows generally initiate on steep slopes or in channels, onset by intense rainfall (or “cloudburst events”) or rapid snowmelt (Giraud, 2005). The term cloudburst is usually used to designate a torrential downpour of rain, characterized by short interval, high-intensity precipitation (Woolley et al., 1946). Historical records of sedimentation events in Utah indicate that debris flows in this region can be highly variable in terms of their size, travel distance, and depositional behavior, which are all largely functions of the size, slope, vegetation cover, geology of their source basin, and intensity and duration of contributing precipitation events (Giraud, 2005). In nearby Davis County an example of a rapid snowmelt induced event occurred in the spring of 1983, where rising temperatures triggered landslides that transformed into debris flows along the Wasatch Front from Willard to Salt Lake City (Wieczorek et al., 1983). The most significant of these flows occurred in Lower Rudd Canyon, where an estimated total of 83,700 cubic yards (52 acre-ft) of material were deposited at the mouth of Rudd Creek, 15-18% of which was contributed by the landslide, the rest coming from channel scour. The events of this storm motivated the researchers to evaluate the history and future potential of debris flows and debris floods in the surrounding canyons. The historic flows documented in this study all occurred within Davis County and range in volume from about 1,300 to 84,000 cubic yards (or 0.8 to 52 acre-feet). The estimated potential volumes range from 2,600 to 140,000 cubic yards (or 1.6 to 87 acre-ft), depending on the canyon of interest. However, Wieczorek et al. (1983) noted that most of the debris flows that mobilized during the 1983 event originated from basins underlain by the Farmington Canyon complex geologic formation common to Davis County. The gneiss and schist of this unit commonly decompose to zones of clayey material prone to landslides. In contrast, the limestone and quartzite that underlays Neffs Canyon do not have the same proclivity to slope failure, indicating that debris flow initiation may not occur in the same manner. In September 1991, a cloudburst storm induced event occurred in Slide Canyon, located at the base of the west-facing Wasatch Mountains in North Odgen, Utah (Mulvey and Lowe, 1991; Vaughn, 1997). Rainfall totaling 8.4 inches fell within a 24-hr period, setting a new state precipitation record and was estimated to be equivalent to a 1,000-year storm (Mulvey and Lowe, 1991). Slide Canyon is a small, third-order drainage basin with a surface area of approximately 0.30 square miles. The channel slopes of this canyon average 30% in the lower reaches and 48% in the higher reaches. Similar to Neffs Canyon, this area is underlain by Tintic Quartzite. Volume estimates of this debris flow indicate that the event moved approximately 26,000 to 28,000 cubic yards (or 16 to 17 acre-ft) of material (Mulvey and Lowe, 1991; Vaughn, 1997). ---PAGE BREAK--- 6 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 2 EMPIRICAL VOLUME ESTIMATES Several empirical methods and equations were used to better understand the debris flow volume that could be evacuated from Neffs Canyon in a high runoff event. The first method comes from the Army Corps of Engineers, Los Angeles District Method for Prediction of Debris Yield, Equation 2 (USACE, 1992). This debris flow volume estimation is calculated by the following equation: Log Dy = 0.65(Log P) + 0.62(Log RR) +0.18 (Log A) + 0.12 (FF) Dy = Unit Debris Yield (yd3/mi2), P = Maximum 1-hr Precipitation (in) RR = Relief Ratio (ft/mi) A = Drainage Area (acres) FF = Non-dimensional Fire Factor Q = Unit Peak Runoff (ft3/s/mi2) Adjustment factors are also made for local characteristics such as parent material, soil, channel morphology, and hillslope morphology. The values of these adjustment factors can be found within Appendix B of USACE, 1992. Unit Peak Runoff was computed using U.S. Geological Survey regression equation data (StreamStats) (regression equation Region One of the regression equation Region 2 input variables is mean annual precipitation (inches). Mean annual precipitation for the Neffs Canyon watershed was obtained from Oregan State University PRISM Climate Group data1. The PRISM mean annual precipitation value for the Neffs Canyon watershed is 35 inches. Applying the USACE Equation 2 to the Neffs Canyon Watershed results in potential 100-year and 500-year recurrence interval debris yield volume estimates listed in Table 1. Table 1. USACE Equation 2 debris yield estimates 100-Year Debris Volume Estimate [USACE Equation 2] 500-Year Debris Volume Estimate [USACE Equation 2] ac-ft ac-ft 8 11 The following empirical methods come from a study by Hungr et al. (1984). The first of which uses empirical relationships based on channel gradient, bed material, side slope conditions, and stability conditions to determine a range of channel debris yield rates (m3/m). A debris volume is estimated by multiplying this rate by the contributing channel length. Based on the field and quantitative assessment of the channels of interest, The Neffs Canyon channels were considered type indicating a channel debris yield rate of 5 (lower limit) to 10 (upper limit) m3/m. Using the of the three main channels, results of this method indicate a potential total debris flow volume of 36 to 73 ac-ft. 1 ---PAGE BREAK--- 7 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment The second method incorporates the drainage area and a channel erodibility coefficient of the contributing channels to estimate debris flow volume potential. The channel erodibility coefficient is also based on channel gradient, bed material, side slope conditions, and stability conditions. The channel type classification indicates the use of a 5 to 10 m3/m*km2 channel erodibility coefficient. The equation for this estimate is: V=A0.5L*e V = volume (m3) A = drainage area (km2) L = channel length e = channel erodibility coefficient (m3/m*km2) This method provides a debris flow volume range of 64 to 128 ac-ft. The channel and drainage areas used to calculate these two volume estimates are listed in Table 2. Table 2. Channel and drainage areas used for Hungr et al. (1984) volume estimates Length Drainage Area (km2) Neffs Creek 4027 4.3 Unnamed Tributary 2876 2.2 North Fork 2029 2.5 A previous study performed in 2005 by Applied Geotechnical Engineering Consultants (AGEC) also investigated the potential debris flow volume that could be evacuated from Neffs Canyon in a high runoff event. That analysis used the empirical equations from the Hungr et al. (1984) study, however, they included all the tributary channels within the watershed, whereas the present study only used the three main channels based on the field assessment that indicated the debris would be dominantly sourced from these channels. The total volume of debris flow calculated using their methods, denoted as Method A and Method B, were 154,700 cubic yards (96 ac-ft) and 148,200 cubic yards (92 ac-ft), respectively. The AGEC report suggests that approximately 13,000 cubic yards of material could be deposited in the lower slope reaches of the watershed, leaving approximately 141,700 cubic yards (87 ac-ft) and 135,200 cubic yards (84 ac-ft) of material to be transported to a debris basin near the canyon mouth. The 2007 Neffs Canyon Creek Master Plan (Hansen, Allen & Luce, 2007) evaluated the AGEC (2005) study and provided the following conclusions: It is difficult to assign a probability to the potential debris flow events. In discussion with the geologist and Salt Lake County, it was decided that taking the average of the predicted debris flow from the largest channel segment, upper Neffs Canyon, [(35,000 + 58,600)/2] = 46,800 cubic yards and subtracting the estimated deposition in the lower reach (13,000 cubic yards) ---PAGE BREAK--- 8 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment provides an estimated debris flow volume (33,800 cubic yards) which may be an appropriate design volume for facilities with the objective of providing protection to developed areas below the canyon mouth. The design debris flow volume (33,800 cubic yards) is about 21 acre-feet. The final method utilized in this analysis estimates the total yearly, non-debris flow sediment yield for a given canyon based on its drainage area. This value comes from empirical relationships by the Bureau of Reclamation, using the chart shown in Figure 4 to estimate sediment yield (BUREC, 1987). This method estimated a yearly sediment yield, exclusive of debris flows, of 4 ac-ft. Figure 4. Average annual sediment yield rate versus drainage area size. From BUREC, 1987 ---PAGE BREAK--- 9 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 3 FIELD INVESTIGATION JEF conducted a field investigation of the Neffs Canyon watershed in August 2021 to observe existing conditions of the three main channels. The channels rapidly increase in slope as illustrated in Figure 2. Debris loading of the channels was found to increase significantly in the upstream direction. The lower channel reaches exhibit minimal debris loading. The middle and upper reaches contain a substantial volume of debris loaded in the channel corridors, especially in the pool sequences. Field photos are included in Appendix A. The heavy debris loading indicates Neffs Canyon has not experienced a storm or rapid snowmelt event within recent time with the intensity to mobilize and transport the debris to the lower reaches or out of the canyon. The three main criteria for mobilization of a debris flow event are 1) slope, 2) channel confinement, and 3) debris availability in the channels. Neffs Canyon exhibits all three criteria and is poised for a debris flow event(s) under a triggering meteorological condition, or in a post- wildfire watershed condition. ---PAGE BREAK--- 10 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 4 SUMMARY Table 3 summarizes the ranges of calculated debris flow volumes based on the methods described in this report. The predicted volumes of these methods range from 8 to 128 ac-ft. This equates to approximately 0.4 to 7 cubic yards of sediment per foot of channel length. Although a significant volume, it is far less than the relationship given by Williams and Lowe (1990), where they estimate an average contributing debris volume of 12 cubic yards per foot of channel for the canyons of Davis County. Additionally, these volumes represent an event such that the entire Neffs Canyon drainage basin contributes debris. It is possible that a localized, high-intensity storm event could occur on only a portion of the Neffs Canyon watershed, triggering debris mobilization from only a single channel. The sub-basins of Neffs Canyon are more comparable to Slide Canyon as they are more similar in size, channel gradient, and underlying lithology. The 1991 event in Slide Canyon mobilized an estimated 26,000 to 28,000 cubic yards (16 to 17 ac-ft) of material (Mulvey and Lowe, 1991; Vaughn, 1997), giving a reference amount to the potential volume that could evacuate in one of the Neffs Canyon sub-basins during a similar storm (1,000 year recurrent interval estimate). A recent hydrologic assessment of the Neffs Canyon watershed estimated the 100-year peak runoff volume to be 33 ac-ft (Bowen Collins, 2021). A comparison of this water volume to the non-recurrence interval-based methods listed in Table 3 suggests that a recurrence interval event much higher than the 100-year would be required to mobilize the debris volume estimates in excess of 33 ac-ft. Given that the USACE (1992) methodology is recurrence interval storm-based, it is the most appropriate method to use with recurrence interval-based hydrologic analyses. The results of the USACE methodology indicate a 100-year storm could produce up to 8 ac-ft of debris volume, and a 500-year storm could produce up to 11 ac-ft of debris volume. ---PAGE BREAK--- 11 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment Table 3. Summary of debris flow volume estimations Summary of Volume Estimates Method Sed Type Volume - lower limit (ac-ft) Volume - upper limit (ac-ft) Recurrence Interval USACE, 1992 - LA District Method, Equation 2 Debris flow volume 8 11 100-Year I 500-Year Hungr et al., 1984 - Method 1 Debris flow volume 36 73 N/A Hungr et al., 1984 - Method 2 Debris flow volume 64 128 N/A AGEC, 2005 – Method A Debris flow volume 87 N/A N/A AGEC, 2005 – Method B Debris flow volume 84 N/A N/A Hansen, Allen & Luce (2007) Debris flow volume 21 N/A N/A Average 50 71 Median 50 73 BUREC, 1987 - Design of Small Dams Non debris flow sediment yield per year 4 N/A N/A ---PAGE BREAK--- 12 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 5 LIMITATIONS AND ASSUMPTIONS • This analysis considered existing watershed conditions. Neffs Canyon is susceptible to wildfire risk. In a post-wildfire condition, debris flow risk to life and property of the canyon mouth increases significantly. Recent wildfires in the Wasatch Range have resulted in debris flow and mudflows that have adversely impacted communities (ex. Alpine, 2013). It is recommended that a future debris basin design for Neffs Canyon consider a post-wildfire watershed condition. • This analysis was conducted as part of a feasibility study for a potential debris basin near the Neffs Canyon mouth. A more detailed debris volume analysis is needed to support a final basin design. ---PAGE BREAK--- 13 Neffs Creek Debris Basin Feasibility Study – Debris and Sediment Volume Assessment 6 REFERENCES Applied Geotechnical Engineering Consultants (AGEC), 2005, Debris Flow Hazard Study Report, Neffs Canyon, Salt Lake City, Utah: Bowen Collins, 2021, Neffs Creek Hydraulic Analysis Results. Draft Memorandum dated August 21, 2021. Bureau of Reclamation (BUREC), 1987, Design of Small Dams: 3rd Edition, Denver Colorado. Bryant, 2005, Geologic Map of the Salt Lake City 30’ x 60’ Quadrangle, North-Central Utah, and Uinta County, Wyoming. Map 190DM. Utah Geological Survey Miscellaneous Investigations Series Map I- 1944 (1990). Giraud, R.E., 2005, Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah: Miscellaneous Publication 05-6; Utah Geological Survey, doi:10.34191/mp-05-6. Hansen, Allen & Luce, 2007, Neffs Canyon Creek Master Plan. Salt Lake County Public Works Engineering, Flood Control Division. Hungr, Morgan, G.C., and Kellerhals, 1984, Quantitative analysis of debris torrent hazards for design of remedial measures.: Canadian Geotechnical Journal, v. 21, p. 663–677, doi:10.1139/t84- 073. Mulvey, W.E., and Lowe, 1991, 1991 Cameron Cove debris flow, North Ogden, Utah: Utah Geological Survey,. US Army Corps of Engineers (USACE), 1992, Debris Method – Los Angeles District Method for Prediction of Debris Yield: Vaughn, D.M., 1997, A major debris flow along the Wasatch Front in northern Utah, USA: Physical Geography, v. 18, p. 246–262, doi:10.1080/02723646.1997.10642619. Wieczorek, G.F., Ellen, Lips, E.W., Cannon, S.H., and Short, D.N., 1983, Potential for debris flow and debris flood along the Wasatch Front between Salt Lake City and Willard, Utah, and measures for their mitigation: Open-file Report 83-635, p. 76. Williams, S.R., and Lowe, 1990, “Process-Based Debris-Flow Prediction Method,” Hydrology & Hydraulics of Arid Lands, ASCE Conference Proceedings, August, 1990, p. 66-71. Woolley, Marsell, and Grover, 1946, Cloudburst floods in Utah, 1850-1938: U.S. Geologic Survey, http://pubs.usgs.gov/wsp/0994/report.pdf. ---PAGE BREAK--- APPENDIX A ---PAGE BREAK--- [7264] [7265] [7266] [7267] [7268] [7269] [7270] [7271] [7272] [7273] [7274] [7275] [7276] [7277] [7278] [7279] [7280] [7281] [7282] [7283] [7284] [7285] [7286] [7287] [7288] [7289] [7290] [7291] [7292] [7293] [7294] [7295] [7296] [7297] [7298] [7299] [7300] [7301] [7302][7303] [7304] [7305][7306] [7307] [7308] [7309] [7311] [7312] [7313] [7314] [7316] [7317] [7318] [7319] [7320] [7321] 0 750 1,500 375 Feet´ Legend Fieldphotos and photo [ID] Neffs Canyon Watershed Neffs Creek North Fork Unnamed Tributary Field Photographs Index ---PAGE BREAK--- IMG_7264 IMG_7265 ---PAGE BREAK--- IMG_7266 IMG_7267 ---PAGE BREAK--- IMG_7268 IMG_7269 ---PAGE BREAK--- IMG_7270 IMG_7271 ---PAGE BREAK--- IMG_7272 IMG_7273 ---PAGE BREAK--- IMG_7274 IMG_7275 ---PAGE BREAK--- IMG_7276 IMG_7277 ---PAGE BREAK--- IMG_7278 IMG_7279 ---PAGE BREAK--- IMG_7280 IMG_7281 ---PAGE BREAK--- IMG_7282 IMG_7283 ---PAGE BREAK--- IMG_7284 IMG_7285 ---PAGE BREAK--- IMG_7286 IMG_7287 ---PAGE BREAK--- IMG_7288 IMG_7289 ---PAGE BREAK--- IMG_7290 IMG_7291 ---PAGE BREAK--- IMG_7292 IMG_7293 ---PAGE BREAK--- IMG_7294 IMG_7295 ---PAGE BREAK--- IMG_7296 IMG_7297 ---PAGE BREAK--- IMG_7298 IMG_7299 ---PAGE BREAK--- IMG_7300 IMG_7301 ---PAGE BREAK--- IMG_7302 IMG_7303 ---PAGE BREAK--- IMG_7304 IMG_7305 ---PAGE BREAK--- IMG_7306 IMG_7307 ---PAGE BREAK--- IMG_7308 IMG_7309 ---PAGE BREAK--- IMG_7311 IMG_7312 ---PAGE BREAK--- IMG_7313 IMG_7314 ---PAGE BREAK--- IMG_7316 IMG_7317 ---PAGE BREAK--- IMG_7318 IMG_7319 ---PAGE BREAK--- IMG_7320 IMG_7321 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX E – CONCEPTUAL COST ESTIMATES ---PAGE BREAK--- Conceptual Construction Costs for Alternative 1 – Dam/Debris Basin with New Storm Drain Item Classification of Unit Price Work Quantity Unit Unit Price Amount 1 Mobilization of Subtotal) 1 LS $799,500 $799,500 2 Site Prep / survey / misc 1 LS $196,800 $196,800 3 Clear and Grub basin/future parking lot 18,000 SY $5.35 $96,300 4 1 LS $11,800 $11,800 5 Traffic Control 1 LS $164,500 $164,500 6 Debris Basin Excavation 31,880 CY $15 $478,200 7 Foundation Excavation 15,208 CY $18 $273,744 8 Embankment Fill 9,539 CY $24 $228,936 9 Stilling Basin at Bottom of Spillway Excavation 9,000 CY $18 $162,000 10 Auxiliary Spillway Excavation 1,250 CY $24 $30,000 11 Auxiliary Spillway 1 EA $705,000 $705,000 12 Stepped Spillway 2 EA $235,000 $470,000 13 Excess Material Haul Off 13,330 CY $26 $346,580 14 Access Road 600 LF $24 $14,400 15 Neffs Diversion Structure 1 EA $293,800 $293,800 16 Install Type Cap on Dam Embankment Crest 385 LF $59 $22,715 17 Type Drain Gravel 2,300 CY $106 $243,800 18 Type Filter Sand 1,300 CY $106 $137,800 19 Clay Core 8,556 CY $141 $1,206,396 20 Manhole Access for Drain Pipe 1 EA $18,200 $18,200 21 6-inch Toe Drain Pipe (Perforated) 770 LF $65 $50,050 22 Outlet Structure 1 LS $293,800 $293,800 23 48" RCP Dam Outlet Pipe 800 LF $790 $632,000 24 Turf Reinforced Mat 4,065 SY $120 $487,763 25 Stilling Basins, Low Flow Channel Riprap, Eastern Slope of Embankment Riprap 2,633 CY $180 $473,871 26 Restoration 1 LS $47,000 $47,000 27 Storm Drain RCP Pipe (36") 7,500 LF $450 $3,375,000 28 Catch Basins 50 EA $7,600 $380,000 29 Catch Basin Pipe (18") 1,000 LF $280 $280,000 30 Asphalt Replacement 150,000 SF $12 $1,800,000 31 Manhole 50 EA $18,200 $910,000 32 Utility Relocation 1 LS $499,400 $499,400 Subtotal $16,260,831 Contingency 20% $3,252,166 Engineering 12% $1,951,300 USFS Environmental Document $200,000 Total Project Cost (rounded to nearest $1,000) $21,665,000 ---PAGE BREAK--- Conceptual Construction Costs for Alternative 2 - Dam/Debris Basin and Neffs Creek Channel Improvements Item Classification of Unit Price Work Quantity Unit Unit Price Amount 1 Mobilization of Subtotal) 1 LS $507,600 $507,600 2 Site Prep / survey / misc 1 LS $212,600 $212,600 3 Clear and Grub basin/future parking lot 18,000 SY $5.80 $104,400 4 SWPP 1 LS $12,700 $12,700 5 Traffic Control 1 LS $63,500 $63,500 6 Debris Basin Excavation 31,880 CY $16 $510,080 7 Foundation Excavation 15,208 CY $19 $288,952 8 Embankment Fill 9,539 CY $25 $238,475 9 Stilling Basin at Bottom of Spillway Excavation 9,000 CY $19 $171,000 10 Auxiliary Spillway Excavation 1,250 CY $25 $31,250 11 Auxiliary Spillway 1 EA $761,400 $761,400 12 Stepped Spillway 2 EA $253,800 $507,600 13 Excess Material Haul Off 13,330 CY $28 $373,240 14 Access Road 600 LF $25 $15,000 15 Neffs Diversion Structure 1 EA $317,300 $317,300 16 Install Type Cap on Dam Embankment Crest 385 LF $63 $24,255 17 Type Drain Gravel 2,300 CY $114 $262,200 18 Type Filter Sand 1,300 CY $114 $148,200 19 Clay Core 8,556 CY $152 $1,300,512 20 Manhole Access for Drain Pipe 1 EA $19,700 $19,700 21 6-inch Toe Drain Pipe (Perforated) 770 LF $70 $53,900 22 Outlet Structure 1 LS $317,300 $317,300 23 48" RCP Dam Outlet Pipe 800 LF $860 $688,000 24 Turf Reinforced Mat 4,065 SY $130 $528,410 25 Stilling Basins, Low Flow Channel Riprap, Eastern Slope of Embankment Riprap 2,633 CY $190 $500,197 26 Restoration 1 LS $50,800 $50,800 27 Pipe (36") ([Pipe Only) 1,710 LF $490 $837,900 28 Catch Basins 17 EA $8,200 $139,400 29 Catch Basins Pipe (18") 340 LF $300 $102,000 30 Asphalt Replacement 15,390 SF $13 $200,070 31 Manhole 17 EA $19,700 $334,900 32 Utility Relocation 1 LS $539,400 $539,400 33 Concrete Box Culverts 1 LS $1,868,100 $1,868,100 34 Property Acquisition 10 AC $253,800 $2,592,769 35 Channel Improvements 1 LS $4,866,800 $4,866,800 Subtotal $18,665,309 Contingency 20% $3,733,062 Engineering 12% $2,239,837 USFS Environmental Document $200,000 Total Project Cost (rounded to nearest $1,000) $24,939,000 ---PAGE BREAK--- Conceptual Construction Costs for Alternative 3 – Below-Grade Debris Basin and New Storm Drain Item Classification of Unit Price Work Quantity Unit Unit Price Amount 1 Mobilization of Subtotal) 1 LS $723,400 $723,400 2 Site Prep / survey / misc 1 LS $212,600 $212,600 3 Clearing & Grubbing (basin area) 33,000 SY $5.90 $194,700 4 Reseeding (basin area minus existing/future parking lot areas, also basin area) 17,500 SY $4.10 $71,750 5 1 LS $31,700 $31,700 6 Traffic Control 1 LS $177,700 $177,700 7 Debris Basin Excavation 75,965 CY $20 $1,519,300 8 Excess Material Haul Off 62,350 CY $28 $1,745,800 9 Access Road 820 LF $26 $21,320 10 Stepped Spillway 2 EA $253,800 $507,600 11 Pond Outlet Structure 1 LS $317,300 $317,300 12 48" RCP Pond Outlet Pipe 800 LF $860 $688,000 13 Turf Reinforcing Mat 2,710 SY $100 $271,000 14 Revegetate Disturbed Areas in Basins 2,710 SY $13 $35,230 15 Pond Embankment Liner 400 CY $82 $32,800 16 Neffs Creek Diversion Structure 1 EA $317,300 $317,300 17 Stilling Basins and Low Flow Channel Riprap 485 CY $190 $92,150 18 Storm Drain RCP Pipe (36") 7,500 LF $490 $3,675,000 19 Catch Basins 50 EA $8,200 $410,000 20 Catch Basin Pipe (18") 1,000 LF $290 $290,000 21 Asphalt Replacement 150,000 SF $13 $1,950,000 22 Manhole 50 EA $19,700 $985,000 23 Utility Relocation 1 LS $539,400 $539,400 Subtotal $14,809,050 Contingency 20% $2,961,810 Engineering 12% $1,777,086 USFS Environmental Document $200,000 Total Project Cost (rounded to nearest $1,000) $19,748,000 ---PAGE BREAK--- Conceptual Construction Costs for Alternative 4 – Below-Grade Debris Basin and Neffs Creek Channel Improvements Item Classification of Unit Price Work Quantity Unit Unit Price Amount 1 Mobilization of Subtotal) 1 LS $901,000 $901,000 2 Site Prep / survey / misc 1 LS $212,600 $212,600 3 Clearing & Grubbing (basin area) 33,000 SY $5.80 $191,400 4 Reseeding (basin area minus existing/future parking lot areas, also basin area) 17,500 SY $4.00 $70,000 5 SWPP 1 LS $12,700 $12,700 6 Traffic Control 1 LS $63,500 $63,500 7 Debris Basin Excavation 75,965 CY $19 $1,443,335 8 Excess Material Haul Off 62,350 CY $28 $1,745,800 9 Access Road 820 LF $25 $20,500 10 Stepped Spillway 2 EA $253,800 $507,600 11 Pond Outlet Structure 1 LS $317,300 $317,300 12 48" RCP Pond Outlet Pipe 800 LF $860 $688,000 13 Turf Reinforcing Mat 2,710 SY $130 $352,300 14 Revegetate Disturbed Areas in Basins 2,710 SY $13 $35,230 15 Pond Embankment Liner 400 CY $82 $32,800 16 Neffs Creek Diversion Structure 1 EA $317,300 $317,300 17 Stilling Basins and Low Flow Channel Riprap 485 CY $190 $92,150 18 Pipe (36") ([Pipe Only) 1,710 LF $490 $837,900 19 Catch Basins 17 EA $8,200 $139,400 20 Catch Basins Pipe (18") 340 LF $298 $101,388 21 Asphalt Replacement 15,390 SF $13 $200,070 22 Manhole 17 EA $19,700 $334,900 23 Utility Relocation 1 LS $539,400 $539,400 24 Concrete Box Culverts 1 LS $1,868,100 $1,868,100 25 Property Acquisition 10 AC $253,800 $2,538,000 26 Channel Improvements 1 LS $4,866,800 $4,866,800 Subtotal $18,416,773 Contingency 20% $3,683,355 Engineering 12% $2,210,013 USFS Environmental Document $200,000 Total Project Cost (rounded to nearest $1,000) $24,511,000 ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX F – CONCEPTUAL RENDERINGS ---PAGE BREAK--- VIEW FROM THE SOUTHEAST ---PAGE BREAK--- VIEW FROM THE SOUTH ---PAGE BREAK--- VIEW FROM THE NORTHWEST ---PAGE BREAK--- VIEW FROM THE NORTHWEST ---PAGE BREAK--- VIEW FROM THE NORTHEAST ---PAGE BREAK--- VIEW FROM THE NORTH ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX G – CONCEPTUAL DESIGN DRAWINGS ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- NEFFS CREEK DEBRIS BASIN STUDY BOWEN COLLINS & ASSOCIATES MILLCREEK CITY APPENDIX H – PUBLIC MEETING ---PAGE BREAK--- Neffs Canyon Debris Basin Feasibility Study Millcreek, Utah Public Meeting Public Meeting The public is invited to attend, discuss, and submit comments during the public meeting. December 15, 2021 Time: 6:00 to 7:30 pm Millcreek City Hall Promise Room 3330 South 1300 East Millcreek, Utah Written comments must be received by February 28, 2022 Project Information Neffs Creek, located in the southeast portion of Millcreek, is part of an active alluvial fan that was recently studied by the Federal Emergency Management Agency (FEMA) to better define the alluvial fan flood hazards in the area. The updated Flood Insurance Rate Map (FIRM) went into effect in November 2021. An active alluvial fan on a FIRM has several ramifications that include the requirement to purchase flood insurance for properties with mortgages and restrictive building requirements (i.e. no walk out basements, remodels must be less than 50% of fair market value of the structure or the entire structure must be brought up to current FEMA zone requirements, and new construction must meet FEMA building restrictions). The only way to remove the alluvial fan flood hazards from the FEMA FIRM is to design and construct a structural flood mitigation facility at the apex of the fan which will collect debris and remove the flood flow path uncertainties associated with alluvial fans. Purpose of Study The purpose of the Neffs Canyon Debris Basin Feasibility Study is to evaluate alternative methods that could potentially be implemented to: • Mitigate the recurring flood problems and possible debris flow hazards associated with the Neffs Creek alluvial fan, and • Eliminate the alluvial fan floodplain recently defined on the FEMA FIRM. Constraints for Flood Mitigation Alternatives Each of the potential flood mitigation alternatives considered in this study met the following constraints: 1. Project improvements could not impact USFS Wilderness area 2. Project improvements could not impact existing homes and structures on private property 3. The project improvements on USFS land must meet USFS requirements 4. Flows up to 15 cfs would be maintained in the existing channel during periods of runoff on Neffs Creek 5. Improvements would need to address site geologic and geotechnical issues (Wasatch Fault, landslides, limited debris flows, etc.) 6. Improvements would eliminate the alluvial fan flood hazards associated with a 100-year runoff event defined on the new FEMA FIRM. Mitigation Alternatives Five mitigation alternatives were evaluated that could potentially mitigate the recurring flooding and existing debris flow hazards associated with the Neffs Creek alluvial fan which would allow the Neffs Creek alluvial fan flood hazards on the new FEMA FIRM to be eliminated. The alternatives considered include the construction of dry detention/debris basins on USFS land that would convey runoff from a typical 100-year flood event and convey runoff from a typical 500-year flood event to a point where it could be partially dispersed into city streets. The mitigation alternatives analyzed include: 1. Debris Basin Dam and Storm Drain in Street 2. Debris Basin Dam and Improve Existing Channel 3. Below-Grade Debris Basin and Storm Drain in Street 4. Below-Grade Debris Basin and Improve Existing Channel 5. Debris Basin in Wilderness Area (Currently Not Feasible) A Do Nothing (No Mitigation) was also considered. Contact Information All comments regarding this Project should be direct to: Dan Drumiler, P.E. Millcreek City Address: 3330 South 1300 East Millcreek, UT 84106 Phone (801) 214-2714 Email:[EMAIL REDACTED] Comments may be mailed or emailed to the above address on or prior to February 28, 2022. ---PAGE BREAK--- Neffs Canyon Debris Basin Feasibility Study Millcreek, Utah Public Meeting Recommended Alternative The Below-Grade Debris Basin and Storm Drain in Street is the recommended flood mitigation alternative. This alternative includes the construction of a debris/detention basin at the apex of the alluvial fan. The outlet pipeline from that facility would connect to a new storm drain that would be constructed in public streets. The proposed new storm drain would convey runoff from at least the 100-year flood event to a point where it would discharge into Neffs Creek near Wasatch Blvd. A schematic of the recommended alternative is shown below. This is the preferred alternative since it has minimal visual impacts, has fewer property impacts to residents adjacent to the creek the alternative could include an improved canyon trailhead parking lot with firefighting facilities for public lands, and it has the lowest estimated construction cost. What Happens Next? The following steps must be completed to proceed with the implementing the recommended flood mitigation alternative: • Obtain conditional approval from FEMA that proposed improvements will mitigate the 100-year flood hazard • Obtain funds for final design and construction (complete an environmental assessment for the USFS, work out permits/issues with USFS, obtain needed pipeline easements on private property, complete final design) • Perform a more detailed geologic/geotechnical analyses • Construct the debris basin and storm drain • Obtain a Letter of Map Revision from FEMA to eliminate alluvial fan 100-year flood hazard ---PAGE BREAK--- Neffs Canyon Debris Basin Feasibility Study Performed for Millcreek Presented by: Bowen Collins & Associates and Millcreek December 15, 2021 1 ---PAGE BREAK--- Overview • Purpose of Study • History of Neffs Creek Flood Problems • Alluvial Fan Flood Hazards • Constraints for Mitigation Methods • Flood Mitigation Alternatives • Next Steps • Answer Questions 2 ---PAGE BREAK--- Purpose of Study • Evaluate the feasibility of alternative methods to mitigate the reoccurring flood problems and possible debris flow hazards associated with the Neffs Creek alluvial fan. 3 ---PAGE BREAK--- History of Neffs Creek Flowpath Historic (Natural) Flowpath Existing (Diverted) Channel Historic Flowpath Large Discharges Flow to Historic Channel 4 ---PAGE BREAK--- • “Fan”- shaped area where silt, sand, gravel, boulders and woody debris are deposited by rivers and streams over a long period of time. • Associated Flood hazards – Braided, unpredictable flow paths – High-velocity flow – Erosion and scour – Sediment transport and deposition – Debris flow – Mud flow – Flash flooding – Sheet flow • New Flood Insurance Rate Map (FIRM) reflects updated regulatory flood zones What is an Alluvial Fan Source: FEMA 5 ---PAGE BREAK--- FEMA-Based Flood Hazard Designation Description of Mapped Flood Hazard Zone X (shaded) Average 100-year flow depth between 0.5’ and 1.0’. Zone A Ultrahazardous zone near the alluvial fan topographic apex. Area subject to the highest degree of flowpath uncertainty. In other areas where the average flow depths are greater than 3 feet. Approximate 100-year floodplain. (No flood depths defined) Zone AO2,1 100-year flow depth between 1.5 foot and 2.5 feet. Average flow velocities of 1 foot/second. Zone AO2,2 100-year flow depth between 1.5 feet and 2.5 feet. Average flow velocities of 2 feet/second. Zone AO2,3 100-year flow depth between 1.5 feet and 2.5 feet. Average flow velocities of 3 feet/second. Zone AO3,3 100-year flow depth between 2.5 feet and 3.0 feet. Average flow velocities of 3 feet/second. Zone AO3,4 100-year flow depth between 2.5 feet and 3.0 feet. Average flow velocities of 4 feet/second. Visit the Neffs Creek Floodplain Webpage for More Information 6 ---PAGE BREAK--- Constraints for Flood Mitigation Methods • Safely convey runoff from 100-yr flood • Avoid Improvements in USFS Wilderness Area • Do Not Impact Existing Homes and Structures • Meet USFS Requirements (Permitting, etc.) • Maintain Existing Channel (up to 15 cfs) • Address Geologic/Geotechnical Issues – Wasatch Fault – Landslide Potential – Regulatory Issues (Dam Safety) Existing Active Creek Channel Existing Homes Project Location (Limited Area) Historic Flowpath Forest Service Land Private Property Wilderness Area 7 ---PAGE BREAK--- Mitigation Alternatives (Based on Existing Watershed Conditions– No Post-Fire Analysis) • Design to solve two problems – reoccurring flooding and existing debris flow hazards Mitigation Alternatives include: 1. Debris Basin Dam and Storm Drain in Street 2. Debris Basin Dam and Improve Existing Channel 3. Below-Grade Debris Basin and Storm Drain in Street 4. Below-Grade Debris Basin and Improve Existing Channel 5. Debris Basin in Wilderness Area (Currently Not Feasible) Or … 6. Do Nothing (No Mitigation) 8 ---PAGE BREAK--- • The flood hazards on the new FEMA map are based on a 100-year flood discharge of 300 cubic ft per second (cfs) • A new scientific analysis shows that the peak 100-year flood discharge should be reduced to about 107 cfs • Lower number better matches discharges for nearby gaged streams • Although the new 100-yr discharge is lower, updating the FEMA floodplain map with the lower discharge would not likely result in a smaller active fan floodplain footprint • Alternative design is based on the 100-year design discharge of 107 cfs and 228 cfs for the 500-year flood event. Updated Hydrology (107 cfs vs. 300 cfs) 9 ---PAGE BREAK--- Mitigation Alternative #1 – Dam/Storm Drain • Debris Basin Dam and Storm Drain – Dam at apex of alluvial fan – Debris basin outlet connects to new storm drain pipe – Storm drain follows road and discharges into Neffs Creek storm drain at Wasatch Blvd – Maintain discharges up to about 15 cfs in existing creek channel 10 ---PAGE BREAK--- Mitigation Alternative #2 – Dam/Channel • Debris Basin Dam and Channel Improvements – Dam at apex of alluvial fan – Debris basin outlet connects to storm drain pipe – Storm drain follows road and discharges into Neffs Creek at Parkview Drive – Channel Improvements below Parkview Drive – Maintain discharges up to about 15 cfs in creek channel 11 ---PAGE BREAK--- Mitigation Alternative #3 – Below-Grade/Storm Drain • Below-Grade Debris Basin and Storm Drain – Basin at apex of alluvial fan – Outlet of debris basin connects to storm drain – Storm drain follows road and discharges into Neffs Creek storm drain at Wasatch Blvd – Maintain discharges up to about 15 cfs in existing creek channel 12 ---PAGE BREAK--- Mitigation Alternative #4 – Below-Grade/Channel • Below-Grade Debris Basin and Channel –Basin at apex of alluvial fan –Outlet of basin connects to storm drain –Storm drain follows road and discharges into Neffs Creek at Parkview Drive –Channel Improvements below Parkview Drive –Maintain discharges up to about 15 cfs in creek channel 13 ---PAGE BREAK--- Mitigation Alternative #5 – Basin in Wilderness Area • Debris Basin in Wilderness Area –Not feasible without act of Congress –Still need improvements below the debris basin to convey flood flow safely past existing homes and to Neffs Creek storm drain. Historic (Natural) Flowpath Existing (Diverted) Channel Wilderness Area Private Property Forest Service Land 14 ---PAGE BREAK--- Mitigation Alternative #6 – Do Nothing • Do Nothing –Hazards stay the same –Cannot revise floodplain maps –Flood Insurance Requirements –Restrictive Building requirements –Rebuilds must meet new building restrictions –Current Fire Station serving the Mount Olympus area cannot be replaced with a new building under current restrictions 15 ---PAGE BREAK--- Recommended Alternative (Alternative • Below Grade Debris Basin and Storm Drain –Utilize available USFS land –Least impactful to properties along existing creek channel –Minimal visual impacts –Storm drain can pick up localized street runoff –Improve trailhead parking lot with fire fighting facilities –Least Alternative Estimated Cost: $12,000,000 16 ---PAGE BREAK--- Existing vs. Proposed 17 ---PAGE BREAK--- Recommended Alternative •View Renderings 18 ---PAGE BREAK--- Existing vs. Proposed 19 ---PAGE BREAK--- Existing vs. Proposed 20 ---PAGE BREAK--- 21 ---PAGE BREAK--- 22 ---PAGE BREAK--- 23 ---PAGE BREAK--- 24 ---PAGE BREAK--- 25 ---PAGE BREAK--- Recommended Alternative •Video of Below-Grade Debris Basin 3D Rendering 26 ---PAGE BREAK--- Study Limitations • Fire – if a large fire occurs, the recommended basin may be undersized to protect homes from a debris flow • While the debris basin may be undersized for a large post-fire runoff event, the available area is being utilized to the maximum extent that is practicable. 27 ---PAGE BREAK--- Next Steps • Obtain conditional approval from FEMA that proposed improvements will mitigate the 100-year flood hazard • Obtain funds for final design and construction –Complete an environmental assessment for the USFS –Work out permits/issues with USFS –Obtain needed pipeline easements on private property –Complete final design • Perform a more detailed geologic/geotechnical analysis • Construct the debris basin and storm drain • Obtain a Letter of Map Revision from FEMA to eliminate alluvial fan 100-year flood hazard 28 ---PAGE BREAK--- Questions? Or Email Dan Drumiler at: [EMAIL REDACTED] by end of February 2022 29 ---PAGE BREAK--- WWW.BOWENCOLLINS.COM DRAPER, UTAH OFFICE 154 E 14075 S DRAPER, UTAH 84020 PHONE: [PHONE REDACTED] BOISE, IDAHO OFFICE 776 E RIVERSIDE DRIVE SUITE 250 EAGLE, IDAHO 83616 PHONE: [PHONE REDACTED] ST. GEORGE, UTAH OFFICE 20 NORTH MAIN SUITE 107 ST.GEORGE, UTAH 84770 PHONE: [PHONE REDACTED] OGDEN, UTAH OFFICE 2036 LINCOLN AVENUE SUITE 104 OGDEN, UTAH 84401 PHONE: [PHONE REDACTED]