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Modera River Trail Redmond, Washington Preliminary Stormwater Report November 2018 I Site Plan Entitlement – Final PREP ---PAGE BREAK--- ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement i Preliminary Stormwater Report November 2018 Prepared for: City of Redmond 15670 NE 85th Street Redmond, WA 98052 (425) 556-2900 Prepared by: Kelly Kobashigawa KPFF Consulting Engineers 1601 Fifth Avenue, Suite 1600 Seattle, WA 98101 (206) 622-5822 KPFF Project No. 1700901 Site Developer: Mill Creek Residential Trust 1417 116th Ave NE, Suite 208 Bellevue, WA 98004 ---PAGE BREAK--- Modera River Trail ii Preliminary Stormwater Report I Site Plan Entitlement This Page Intentionally Left Blank ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement iii Table of Contents 1. Project Overview 1 Existing Site 2 2. Minimum Requirements Summary 2 3. Off-Site Analysis 5 4. Conveyance Design 5 5. Flow Control 5 Existing Hydrology 5 Developed Hydrology 6 6. Water Quality Design 6 7. Low Impact Development (LID) Site Assessment 7 Summary of LID Implementation and feasibility 7 8. Flood Plain and Compensatory Storage 9 9. Temporary Erosion and Sedimentation Control (TESC) 9 10. Draft Operations and Maintenance Manual 9 List of Figures Figure 1-1: Vicinity Map 1 Figure 2-1: Minimum Requirements Flow Chart 3 Figure 7-1: LID MR#5 Flow Chart 8 ---PAGE BREAK--- Modera River Trail iv Preliminary Stormwater Report I Site Plan Entitlement List of Tables Table 5-1: Existing Site Area Takeoff 6 Table 5-2: Developed Site Area Takeoff 6 Appendices Appendix A – Proposed Site Conditions Map Appendix B – Existing and Proposed Surface Area Maps Appendix C – Frequently Flooded Areas Report ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement 1 1. Project Overview The project site is located at the southeast corner of Northeast 85th Street and 158th Avenue Northeast in Redmond, WA. The proposed development consists of five stories of residential units over one story of ground level retail space. The ground level retail space wraps around two levels of parking with garage access from 158th Avenue Northeast. The total project area is approximately 1.5 acres and contains two parcels. The western parcel (Parcel number: [PHONE REDACTED]) is approximately 0.81 acres and the eastern parcel (Parcel number: [PHONE REDACTED]) is approximately 0.69 acres. The project area is bound by Northeast 85th Street to the north, an existing commercial property to the east, a multi-family residential community to the south, and 158th Avenue Northeast to the west. Figure 1-1: Vicinity Map ---PAGE BREAK--- Modera River Trail 2 Preliminary Stormwater Report I Site Plan Entitlement EXISTING SITE The existing parcels consist of office and medical/dentist office buildings with asphalt paved parking lots, trees along the frontage, and perimeter landscaping features. The site is relatively flat, but both parcels have a slight slope towards the center of the project area where drainage facilities are located in the parking lots. Both sites collect stormwater through on-site drainage infrastructure and convey it to the existing 54-inch diameter storm main in Northeast 85th Street. According to the Preliminary Geotechnical Engineering Study prepared by Terra Associates, Inc. (Appendix site soils generally consist of soft silt overlying medium dense to dense sand with varying amounts of silt and gravel. The study observed groundwater approximately 10 feet below existing grades. 2. Minimum Requirements Summary The existing site has greater than 35% impervious area and therefore the project is classified as a redevelopment project. Additionally, the project will add more than 5,000 square feet of new plus replaced impervious area and the proposed improvements will exceed 50% of the assessed value of the existing site improvements. Therefore, per the 2017 Redmond Stormwater Technical Notebook Figure I-2.4.2, Minimum Requirements #1-9 apply to the new and replaced hard surfaces and converted vegetation areas. ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement 3 Figure 2-1: Minimum Requirements Flow Chart ---PAGE BREAK--- Modera River Trail 4 Preliminary Stormwater Report I Site Plan Entitlement Minimum Requirement No. 1 – Preparation of Stormwater Site Plans This stormwater report accompanies the site plans that describe the temporary and permanent drainage controls associated with this project. See the Proposed Conditions Site Map in Appendix A. Minimum Requirement No. 2 – Construction Stormwater Pollution Prevention Plan The proposed project will result in more than 2,000 square feet of new plus replaced hard surface area and will disturb more than 7,000 square feet of land. Therefore, a will be prepared for this project and approved by the City of Redmond prior to commencement of any land clearing activities. Minimum Requirement No. 3 – Source Control of Pollution The urban and residential nature of this development does not require additional structural Source Control BMP’s. Relevant operational Source Control BMP’s include Landscaping/Vegetation Management. Minimum Requirement No.4 – Preservation of Natural Drainage Systems and Outfalls The site was previously developed to collect and discharge stormwater to the City owned storm sewer system. The new development will collect and discharge stormwater to the same system in a similar location. Minimum Requirement No. 5 – On-site Stormwater Management The project drains to the Sammamish River and therefore qualifies as a Flow Control Exempt Area as the Sammamish River is classified as a flow control exempt waterbody. The project will implement Post-Construction Soil Quality and Depth to the maximum extent feasible per Section 2.5.5.1 of the City of Redmond Stormwater Technical Notebook. See Section 7 for further discussion of infiltration and dispersion feasibility. Minimum Requirement No.6 – Runoff Treatment Private runoff treatment facilities will not be required as the project area does not meet the applicable thresholds. See Section 6. Minimum Requirement No.7 – Flow Control Flow control facilities will not be required as this project discharges to a non-capacity constrained conveyance system to the Sammamish River. See Section 5. Minimum Requirement No. 8 – Wetlands Protection This project will not provide wetland protection because the parcel does not discharge stormwater to a wetland. Minimum Requirement No. 9 – Operation and Maintenance Manual An Operation and Maintenance Manual for the stormwater infrastructure will be provided through the Coordinated Civil Review (CCR). ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement 5 3. Off-Site Analysis The total proposed impervious area closely matches the existing impervious area on the site. Therefore, the storm drainage infrastructure is considered adequate to safely convey stormwater discharge that results from this development. There are no recorded drainage complaints per King County iMap GIS data. A complete off-site analysis will be prepared for this site and will be included in the Final TIR to be submitted as part of the City of Redmond Coordinated Civil Review. 4. Conveyance Design Stormwater runoff will be collected in the building and conveyed to the existing 12-inch diameter public storm drain in the frontage along 158th Avenue Northeast, thus matching existing stormwater runoff discharge from the site. Runoff from the sidewalk areas along Northeast 85th Street and 158th Avenue Northeast will sheet flow away from the site and be collected in existing public storm infrastructure in the roadways, matching existing drainage patterns. Off-site conveyance design will be further discussed in the following section. 5. Flow Control Design Per Section 2.5.7 of the 2017 Redmond Stormwater Technical Notebook, flow control is not required because the project site is located in the Downtown Regional Facility Surcharge Area. In this area, the City is constructing or has constructed stormwater trunk lines that convey 50 year flows to the Sammamish River. The project will be required to pay a fee-in-lieu of constructing on-site detention facilities. The project has approximately 31 impervious units (60,965 square feet of impervious surface) to which the City Stormwater Capital Facilities Charge and the Downtown Sub-Basin Stormwater Capital Facilities Charge will apply. EXISTING HYDROLOGY The existing surface hydrology is mainly characterized by runoff within asphalt parking areas that is collected by catch basins and conveyed to the public storm drainage system. The parking area perimeter landscaping provides the site with pervious surfaces to absorb portions of the rainfall. See the existing site surfaces map in Appendix B. Subsurface conditions are typically 8 to 11 feet of soft silt below 1.5 to 6 inches of asphalt (see the Preliminary Geotechnical Engineering Study in Appendix The existing site’s pervious and impervious surface areas are shown in Table 5-1: ---PAGE BREAK--- Modera River Trail 6 Preliminary Stormwater Report I Site Plan Entitlement Table 5-1: Existing Site Area Takeoff Land Cover Total Area Pervious 0.36 acres Impervious 1.14 acres NPGIS 0.35 PGIS 0.79 Total 1.5 acres DEVELOPED HYDROLOGY The developed site has the same total area and a similar surface and subsurface hydrologic condition as the existing site, with building roof area replacing existing asphalt parking area. See the proposed site surfaces map in Appendix B. The developed site’s impervious surface areas are also shown in Table 5-2: Table 5-2: Developed Site Area Takeoff Land Cover Total Area Pervious 0.08 acres Impervious 1.42 acres NPGIS 1.31 PGIS 0.11 Total 1.5 acres 6. Water Quality Design Water quality is not required as the project is located in the Downtown Regional Facility Surcharge Area. Additionally, water quality treatment is only necessary if the total new plus replaced Pollution Generating Impervious Surface (PGIS) is greater than 5,000 square feet. This project proposes 3,248 square feet of PGIS; therefore thresholds for water quality treatment are not exceeded. ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement 7 7. Low Impact Development (LID) Site Assessment The following documents, required for the LID Site Assessment, are included in this report: Survey (included as a base on the Proposed Conditions Map in Appendix Geotechnical report (Preliminary Geotechnical Study in Appendix D) Flow chart for determining LID Requirements (Figure 7-1) Frequently Flooded Areas Report (Appendix C) The following documents are not included in this report, but have been submitted separately to the City for review: Critical Areas Report: the project site is located in the City of Redmond’s Seismic Hazard Area and Critical Aquifer Recharge Area for the supply of the City’s wells (Wellhead Protection Zone The project is not located in any other Geologically Hazardous Areas per Redmond Environmental Standard Maps. SUMMARY OF LID IMPLEMENTATION AND FEASIBILITY Per the requirements of the LID Site Assessment, infiltration and/or dispersion should be considered for all projects that trigger Minimum Requirement Full dispersion is not feasible as the project cannot accommodate the required vegetated flow paths. Per the Preliminary Geotechnical Study, full infiltration is not feasible as the upper silt soils have low permeability and the site is anticipated to be exposed to flooding during peak rain events. Per Figure 7-1, BMP T5.13: Post-Construction Soil Quality and Depth is feasible and will be implemented on-site. A complete summary of proposed BMPs will be prepared for this site and included in the Final TIR to be submitted as part of the Coordinated Civil Review. ---PAGE BREAK--- Modera River Trail 8 Preliminary Stormwater Report I Site Plan Entitlement Figure 7-1: LID MR#5 Flow Chart ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement 9 8. Frequently Flooded Areas and Compensatory Storage The project site is located in the 100-year floodplain per City of Redmond and FEMA flood maps. Therefore, the project must demonstrate that 100-year flood storage on the site will not change from the existing to the proposed condition. A comprehensive Frequently Flooded Areas Report demonstrating the project’s compliance with RZC 21.64.040, which addresses City requirements for development in flood hazard areas, can be found in Appendix C of this report. 9. Temporary Erosion and Sedimentation Control (TESC) TESC will be installed to prevent transport of sediment-laden runoff from entering adjacent properties and sewer systems. TESC plans, descriptions of how runoff will be treated, and details of the proposed TESC facilities will be provided at the CCR. 10. Draft Operations and Maintenance Manual An Operations and Maintenance Manual will be prepared for this site and will be included in the Final TIR to be submitted as part of the CCR. ---PAGE BREAK--- ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement Appendix A Appendix A Proposed Site Conditions Map ---PAGE BREAK--- ---PAGE BREAK--- PROPOSED SITE CONDITIONS MAP A A Portion of the SE 1/4 of the SW 1/4 of Section 23, Township 25 N, Range 5 E, W.M. MODERA RIVER TRAIL 15801 & 15945 NE 85TH ST Redmond, WA 04/06/2018 KAK SJD JSF Checked By: Drawing No. Job No: Date: Drawing Title Drawn By: Approved By: Revisions Scale: Horiz: Vert: 1601 5th Avenue, Suite 1600 Seattle, WA 98101 [PHONE REDACTED] www.kpff.com 1301 First Avenue, Suite 301 Seattle, WA 98101 http://www.gglo.com ---PAGE BREAK--- ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement Appendix B Appendix B Existing and Proposed Surface Area Maps ---PAGE BREAK--- ---PAGE BREAK--- EXISTING SITE SURFACES MAP B1 A Portion of the SE 1/4 of the SW 1/4 of Section 23, Township 25 N, Range 5 E, W.M. MODERA RIVER TRAIL 15801 & 15945 NE 85TH ST Redmond, WA 04/06/2018 KAK SJD JSF Checked By: Drawing No. Job No: Date: Drawing Title Drawn By: Approved By: Revisions Scale: Horiz: Vert: 1601 5th Avenue, Suite 1600 Seattle, WA 98101 [PHONE REDACTED] www.kpff.com 1301 First Avenue, Suite 301 Seattle, WA 98101 http://www.gglo.com ---PAGE BREAK--- ---PAGE BREAK--- PROPOSED SITE SURFACES MAP B2 A Portion of the SE 1/4 of the SW 1/4 of Section 23, Township 25 N, Range 5 E, W.M. MODERA RIVER TRAIL 15801 & 15945 NE 85TH ST Redmond, WA 04/06/2018 KAK SJD JSF Checked By: Drawing No. Job No: Date: Drawing Title Drawn By: Approved By: Revisions Scale: Horiz: Vert: 1601 5th Avenue, Suite 1600 Seattle, WA 98101 [PHONE REDACTED] www.kpff.com 1301 First Avenue, Suite 301 Seattle, WA 98101 http://www.gglo.com ---PAGE BREAK--- ---PAGE BREAK--- Modera River Trail Preliminary Stormwater Report I Site Plan Entitlement Appendix C Appendix C Frequently Flooded Areas Report ---PAGE BREAK--- Modera River Trail Redmond, Washington Frequently Flooded Areas Report November 2018 ---PAGE BREAK--- Modera River Trail Frequently Flooded Areas Report i Frequently Flooded Areas Report November 2018 Prepared for: City of Redmond 15670 NE 85th Street Redmond, WA 98052 (425) 556-2900 Prepared by: Sam DeBell, PE KPFF Consulting Engineers 1601 Fifth Avenue, Suite 1600 Seattle, WA 98101 (206) 622-5822 KPFF Project No. 1700901 Site Developer: Mill Creek Residential Trust 1417 116th Ave NE, Suite 208 Bellevue, WA 98004 ---PAGE BREAK--- ---PAGE BREAK--- Modera River Trail Frequently Flooded Areas Report 1 Flood Hazard Summary PROJECT OVERVIEW The project site is located at the southeast corner of Northeast 85th Street and 158th Avenue Northeast in Redmond, WA. The proposed development consists of five stories of residential units over one story of ground level retail space and parking. The total project area is approximately 1.5 acres and contains two parcels. The western parcel (Parcel number: [PHONE REDACTED]) is approximately 0.81 acres and the eastern parcel (Parcel number: [PHONE REDACTED]) is approximately 0.69 acres. The project area is bound by Northeast 85th Street to the north, an existing commercial property to the east, a multi-family residential community to the south, and 158th Avenue Northeast to the west. The existing parcels consist of office and medical/dentist office buildings with asphalt paved parking lots, trees along the frontage, and perimeter landscaping features. The site is relatively flat, but both parcels have a slight slope towards the center of the project area where drainage facilities are located in the parking lots. Both sites collect stormwater through on-site drainage infrastructure and ultimately convey it to the existing 54-inch diameter storm main in Northeast 85th Street. According to the Preliminary Geotechnical Engineering Study prepared by Terra Associates, Inc. (Appendix site soils generally consist of soft silt overlying medium dense to dense sand with varying amounts of silt and gravel. The study observed groundwater approximately 10 feet below existing grades. FLOOD HAZARD SUMMARY According to FEMA Floodplain maps, City of Redmond GIS maps and topographic survey provided BRH, Inc. on April 4, 2018, the majority of the existing project site is located within the 100-year floodplain (Zone AE). According to FEMA Floodplain maps and City of Redmond GIS maps, the project site is located approximately 500 feet east of the Sammamish River zero-rise floodway. The project therefore proposes no fill or structures within the floodway. Because the project site is located in the 100-year floodplain, the project must demonstrate that the total 100-year flood storage on the site will remain constant (or increase) from the existing to proposed condition. To meet this requirement, and because the proposed building will displace the majority of floodwater onsite, the project proposes hydraulically connected compensatory storage depressions along the southern boundary of the project site. Details of the proposed compensatory storage depressions demonstrating the capacity and hydraulic connectivity of these depressions can be found on the Floodplain Map included in this report as Attachment 1. ---PAGE BREAK--- Modera River Trail 2 Frequently Flooded Areas Report COMPENSATORY STORAGE Compensatory storage calculations are based on a Base Flood Elevation (BFE) of 33.90. Documentation of this BFE has been provided by the City of Redmond and is included in this report as Attachment 2. As described in the previous section, the project must demonstrate that the total 100-year flood storage on the site will remain constant (or increase) from the existing to proposed condition. The existing 100-year flood storage volume for the site is calculated as the total volume of floodwater between the existing ground surface on the site and the BFE of 33.9 (Figure 1-1). To be conservative, the calculated existing 100-year flood storage on the site assumes that the entirety of the project site that falls below the BFE is within the floodplain. 𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝐹𝑙𝑜𝑜𝑑 𝑆𝑡𝑜𝑟𝑎𝑔𝑒 𝑉𝑜𝑙𝑢𝑚𝑒 (𝐸𝑉) = 𝐵𝐹𝐸 33.9 – 𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝐺𝑟𝑜𝑢𝑛𝑑 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 The proposed 100-year flood storage volume is calculated as the total volume of floodwater between the proposed ground surface on the site and the BFE of 33.9 (Figure 1-2). The majority of the proposed flood storage volume will be provided via compensatory storage depressions along the southern boundary of the site. See Floodplain Map for more detailed cross sections of the proposed compensatory storage areas. 𝑃𝑟𝑜𝑝𝑜𝑠𝑒𝑑 𝐹𝑙𝑜𝑜𝑑 𝑆𝑡𝑜𝑟𝑎𝑔𝑒 𝑉𝑜𝑙𝑢𝑚𝑒 (𝑃𝑉) = 𝐵𝐹𝐸 33.9 −𝑃𝑟𝑜𝑝𝑜𝑠𝑒𝑑 𝐺𝑟𝑜𝑢𝑛𝑑 𝑆𝑢𝑟𝑓𝑎𝑐𝑒∗ *For purposes of this calculation, the proposed ground surface includes the bottom of the proposed depression and open-bottom box culvert along the southern boundary of the site. The calculation excludes the lid of culvert and the pavement section. Figure 1-1: Existing Site Cross Section (Looking East) Figure 1-2: Proposed Site Cross Section (Looking East) ---PAGE BREAK--- Modera River Trail Frequently Flooded Areas Report 3 As shown on the Floodplain Map, the depression and open-bottom box culvert are hydraulically connected to each other via a large grated opening. Both storage areas are also hydraulically connected to the existing public storm system, which will surcharge the compensatory storage areas during flood events. A trench drain along the entirety of the southern edge of the open-bottom box culvert will provide additional hydraulic connectivity of floodwaters to the storage area during major flood events. Calculations were performed using AutoCAD Civil 3D Software and are summarized in Table 1-1 below: Table 1-1: Flood Storage Volumes Calculated Quantity Volume (cubic feet) Required 100-year Flood Storage 20,970 Proposed 100-year Flood Storage 21,150 CONCLUSION Because the project proposes a total 100-year flood storage volume onsite that exceeds the existing 100-year flood storage volume onsite, and because the proposed storage areas are hydraulically connected and will not impede floodwaters, compensatory storage requirements as required by COR and FEMA are met. Furthermore, because the proposed building will be elevated to a minimum of 1’ above the BFE, the project meets the criteria set forth in the City of Redmond Zoning Code (RZC) section 21.64.040.C.e “Development Standards for new residential construction and substantial improvements.” ---PAGE BREAK--- Modera River Trail Frequently Flooded Areas Report Attachment 1 Attachment 1 Floodplain Map ---PAGE BREAK--- A Portion of the SE 1/4 of the SW 1/4 of Section 23, Township 25 N, Range 5 E, W.M. MODERA RIVER TRAIL 15801 & 15945 NE 85TH ST Redmond, WA 08/23/2018 KAK SJD JSF Checked By: Drawing No. Job No: Date: Drawing Title Drawn By: Approved By: Revisions Scale: Horiz: Vert: 1601 5th Avenue, Suite 1600 Seattle, WA 98101 [PHONE REDACTED] www.kpff.com FLOODPLAIN MAP n/a ---PAGE BREAK--- Modera River Trail Frequently Flooded Areas Report Attachment 2 Attachment 2 Base Flood Elevation Documentation ---PAGE BREAK--- 1 Kelly Kobashigawa From: Jeff Dendy [[EMAIL REDACTED]] Sent: Monday, January 08, 2018 4:32 PM To: Sam DeBell Subject: RE: NE 85th/158th Pre- App Info. (MCR - Redmond) Sam: The City had looked into creating a regional compensatory storage bank for the Sammamish River, but it didn’t get set up. Storage would have to be done on-site, or the developer make arrangements that are acceptable to FEMA and the City, (an alternative location would have to be hydraulically connected / equivalent to be considered. Storage has always been provided on-site). The BFE for the project site is 30.3 NGVD 29. So the development will need to flood proof to 31.3. An estimate to the Redmond Datum would be to add 3.6 feet to get 33.9 Jeff Dendy From: Sam DeBell [mailto:[EMAIL REDACTED]] Sent: Monday, January 08, 2018 2:50 PM To: Jeff Dendy <[EMAIL REDACTED]> Cc: Mark Steepy <[EMAIL REDACTED]>; Jeremy Febus <[EMAIL REDACTED]> Subject: RE: NE 85th/158th Pre- App Info. (MCR - Redmond) Hi Jeff, Glad to learn that we’ll be working with you on yet another project, looking forward to it. Per our discussion at last week’s pre-app meeting, the owner is eager to obtain the following information: 1. The City-determined BFE for our site. 2. Options for compensatory storage besides onsite, and associated unit cost. Let me know if you want to discuss or if you require anything further from us to provide the info above. Thanks, Sam Samuel J. DeBell, PE Civil Engineer O [PHONE REDACTED] D [PHONE REDACTED] [EMAIL REDACTED] From: Gary Lee [mailto:[EMAIL REDACTED]] Sent: Friday, January 5, 2018 12:54 PM To: Hans Fagerlund <[EMAIL REDACTED]> Subject: NE 85th/158th Pre- App Info. ---PAGE BREAK--- 2 Hans, Attached is the issues matrix and the sign in sheet for yesterday’s meeting. The portal should be open for you to view the redlines/comments on the plans documents. Gary Lee, Senior Planner (425) 556-2418 City of Redmond, MS: 2SPL PO Box 97010 Redmond WA 98073-9710 This message has been scanned for malware by Websense. www.websense.com Click here to report this email as spam.