Document Millcreek_doc_4e3a63bcfb

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