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REVIEWED RESUB 1+ENG2021-0183+Stormwater Report+06.09.2022Ellis Station Unit Lot Subdivision 23325, 23403, 23409, and 23415 Edmonds Way Edmonds, WA 98026 Stormwater Site Plan Report PFN: 21- 109006 PSD/SPA 21- 109011 LDA Prepared for: Pacific Ridge Homes Date: April 23, 2021 Updated: December 23, 2021 12/23/2021 Prepared by: Rob Long, PE RAM Engineering, Inc. 16531 13th Ave W, Suite A108, Lynnwood WA 98037 (425) 678-6960 RAMengineeringinc.com Job No. 20-021 Ellis Station Stormwater Site Plan Report Job # 19-034 Page i RAM Engineering, Inc. Ellis Station Project Owner/Applicant: Pacific Ridge Homes 17921 Bothell-Everett Highway Bothell, WA 98012 Site Address: 23325, 23403, 23409, and 23415 Edmonds Way, Edmonds, WA 98026 TAX ACCOUNT NUMBER(S): 00555300400600, 00555300400502, 00555300400501, 00555300400403 SW ¼ and NW ¼ of Section 31, Township 27N, Range 4E Table of Contents PROJECT OVERVIEW ...................................................................................... 1-1 Drainage Information Summary Form. .......................................................................................... 1-1 Project Overview. ........................................................................................................................... 1-2 Minimum Requirements Summary. ............................................................................................... 1-3 EXISTING CONDITIONS .................................................................................. 2-1 Existing Conditions Summary. ........................................................................................................ 2-1 2.1.1 Existing Conditions Exhibit 2-2 2.1.2 NRCS Web Soil Survey 2-3 OFF-SITE ANALYSIS REPORT ........................................................................... 3-1 Upstream Analysis. ......................................................................................................................... 3-1 Downstream Analysis. .................................................................................................................... 3-1 3.2.1 Downstream Drainage Inventory Map 3-2 3.2.2 Drainage Subbasin Map Error! Bookmark not defined. PERMANENT STORMWATER CONTROL PLAN ................................................. 4-3 Permanent Stormwater Control Plan Summary. ........................................................................... 4-3 Existing Conditions Hydrologic Model. .......................................................................................... 4-1 Developed Conditions Hydrologic Model. ..................................................................................... 4-1 4.3.1 Developed Conditions Areas 4-1 4.3.2 Developed Conditions Exhibit 4-2 4.3.3 Flow Control Analysis and Design 4-3 4.3.4 Offsite Upstream flows Error! Bookmark not defined. Water Quality Analysis. ................................................................................................................ 4-17 Stormwater System Capacity Analysis ......................................................................................... 4-19 4.5.1 Stormwater Conveyance Pipe Capacity 4-19 4.5.2 Control Structure Overflow Capacity Calculation: .............................................................. 4-19 CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN (SWPPP) ....... 5-1 SPECIAL REPORTS AND STUDIES ..................................................................... 6-1 OTHER PERMITS ............................................................................................ 7-1 OPERATION AND MAINTENANCE MANUAL .................................................... 8-1 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-1 RAM Engineering, Inc. Project Overview DRAINAGE INFORMATION SUMMARY FORM. Project Name: Ellis Station Project Area: 1.547 Ac. (Site Development Area + Edmonds Way Frontage) Number of Lots: 29 Units/Lots Summary Table: Drainage Basin Information Individual Basin Information Site Basin Area (Includes ROW Frontage) 1.547 Acres Type of Storage Proposed Detention Tank Approx. Live Storage Volume (cf) 2,098 cf 37 ft of 8.5 ft Diameter Pipe Approx. Dead Storage Volume (cf) None Water Quality: Oldcastle PerkFilter™ (Basic Treatment) Soil Type(s) (NRCS) Alderwood Urban Complex (Till Soils) Rainfall Map Data Snohomish 2012- WSDOT Data Pre-developed Runoff Rates Q (cfs.) 2 yr. 0.026 cfs 10 yr. 0.049 cfs 50 yr. 0.061 cfs 100 yr. 0.065 cfs Post-development Runoff Rates (Unmitigated, without quantity controls) Q (cfs.) 2 yr. 0.280 cfs 10 yr. 0.452 cfs 50 yr. 0.638 cfs 100 yr. 0.728 cfs Post-development Runoff Rates (Mitigated, with quantity controls) Q (cfs.) 2 yr. 0.150 cfs 10 yr. 0.246 cfs 50 yr. 0.366 cfs 100 yr. 0.430 cfs Allowable Release Rate (Edmonds Way Basin) Q (cfs.) 10 yr. (0.25 cfs per impervious acre) 100 yr. (0.45 cfs per impervious acre) 0.249 cfs (0.25 X 0.997) 0.449 cfs (0.45 X 0.997) Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-2 RAM Engineering, Inc. PROJECT OVERVIEW. The site is located at 23325, 23403, 23409, and 23415 Edmonds Way, in unincorporated Edmonds, WA; which is more generally located in SW ¼ and NW¼ of Section 31, Township 27N, Range 4E, in Snohomish County (see the vicinity map below). Vicinity Map Not to Scale The site consists of four parcels (00555300400600, 00555300400502, 00555300400501, and 00555300400403) to create a total site of approximately 1.50 acres. The site is bordered by Edmonds Way to the west, and residential properties to the north, east and south. The site contains five existing residences; each house is served by hard surface driveways which will all be removed with the proposed project. The existing structures onsite are surrounded by a mix of grass lawn, landscaping, and scattered trees throughout the site. The site slopes from east to west dropping about 20 ft at an average slope of approximately 8.5%. The Natural Resource Conservation Service (NRCS) Web Soil Survey identifies on-site soils in the area of the proposed site development as Alderwood- Urban land complex, 8% to 15% (#6) and McKenna gravelly silt loam, 0 to 8 percent slopes (#32); considered to be till soils. The project will construct 29 townhouse units, in a collection of five buildings, utilizing unit lot subdivision Code previsions. The development will install supporting infrastructure, private drive aisle, driveways, utilities, flow control (detention tank), stormwater treatment (stormwater filter system), landscaping and Edmonds Way (SR-104) frontage improvements along the site’s westerly boundary. The project’s onsite private drive aisle system will terminate onsite with no extensions provided to neighboring properties. The westerly boundary of the site is the boundary between Snohomish County and the City of Edmonds; thus, the Edmonds Way right-of-way is located within the City of Edmonds. An onsite stormwater detention tank system located in the lower northwest corner of the site will mitigate stormwater runoff from the proposed site improvements and associated frontage improvements along Edmonds Way (SR-104). Due to the fact the system will discharge to the City of Edmonds system along Edmonds Way, the stormwater flow control standards will be designed to the special basin criteria of the Edmonds Way direct discharge basin per Edmonds Community Development Code 18.30(d.)(5.)(b.)(iv.)(B). Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-3 RAM Engineering, Inc. On-site stormwater runoff generally flows in a westerly direction collecting in the stormwater system within Edmonds Way (within the City of Edmonds) that flows north. The Edmonds Way system flows north and eventually discharges to Puget Sound about 2.7 miles from the site. The proposed project onsite stormwater infrastructure will be designed according to the vested 2016/2017 Snohomish County Drainage Manual (Drainage Manual), Snohomish County Engineering Design and Development Standards (EDDS), and Snohomish County Code (SCC) Chapter 30.63A. All of the frontage and right-of-way improvements will be constructed to the City of Edmonds standard and a right-of-way permit shall be obtained from the City of Edmonds. One exception to the onsite facilities design is that the flow control (MR-7) standard will be based on the special basin criteria of the Edmonds Way direct discharge basin per Edmonds Community Development Code 18.30.060(D.)(5.)(b.)(iv.)(B) due to the fact the stormwater system discharges into the City of Edmonds stormwater system. MINIMUM REQUIREMENTS SUMMARY. This section summarizes compliance with the Minimum Requirements SCC Chapter 30.63A SCC: Minimum Requirement #1: Preparation of Stormwater Site Plans: Civil construction plans under separate cover and this Stormwater Site Plan Report addresses Minimum Requirement #1. Minimum Requirement #2: Stormwater Pollution Prevention Plans (SWPPPs): A construction Stormwater Pollution Prevention Plan (SWPPP) has be incorporated into the site development plans. Due to the fact the project site disturbance area (1.547 ac, includes the site and frontage area) is greater than one acre, a full SWPPP report using the DOE standard template has been prepared. Prior to start of construction, a NPDES notice of intent (NOI) application will be submitted to Department of Ecology (DOE) by the applicant. Minimum Requirement #3: Water Pollution Source Control: Specific source controls are limited for residential use sites. General requirements for residential sites include preventing the discharge of pollutants to the public storm drainage system per the Snohomish County Code (Chapter 7.53 Water Pollution Control). Additional construction source control BMPs are addressed within the SWPPP report. The proposed townhouse (multifamily) development use will not have a significant or adverse impact to the local critical aquifer recharge area. No listed uses in SCC 30.62C.340 are proposed and therefore, there are no additional special protection measures or conditions applicable per Code. Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls: The site slopes in a westerly direction towards the stormwater conveyance system within Edmonds Way (City of Edmonds). The development of Ellis Station will discharge flows to the existing stormwater conveyance system in Edmonds Way, thus maintaining the natural discharge outfall of the site. The system within Edmonds Way flows northwesterly and eventually direct discharges to Puget Sound about 2.7 miles from the site. A full downstream description is provided in Section 3. Minimum Requirement #5: On-Site Stormwater Management: Due to existing dense unweathered till (hard pan) soils observed at relatively shallow depths (exhibit poor soil infiltration characteristics) and lack of minimum flow path lengths, no infiltration or dispersion BMPs are proposed. All disturbed pervious areas will be amended per Snohomish County equivalent BMP T.5.13 Post Construction Soils Quality and Depth. The following summary table provides a summary evaluation of the project’s low impact development (LID) performance standard. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-4 RAM Engineering, Inc. Ellis Station LID BMP Performance Standards Evaluation Lawn and landscaped areas: BMP Viable Limitations / Infeasibility Criteria BMP T5.13: Post-Construction Soil Quality and Depth (Volume V, Chapter 5) Yes Limitation: Considered infeasible on slopes greater than 33 percent are present., BMP T5.13 shall be applied to the site post construction. Roofs: BMP T5.30: Full Dispersion (Volume V, Chapter 5) No Infeasibility: Comprehensive Plan and Zoning minimum density requirements do not allow for large native vegetation retention and flow path areas need to meet dispersion requirements. BMP T5.10A: Downspout Full Infiltration Systems (Volume III, Chapter 3) No Infeasibility: Depth to dense unweathered glacial till (hardpan) low permeability layer from final grade and/or bottom of facility elevation will not satisfy minimum specified by SCDM Volume III, 3.1.1. See additional discussion in the Geotechnical Engineering Study by Earth Solutions Northwest. Additionally, the proposed intense site development of the multiple residential (MR) zone limits available area to place infiltration systems that meet set back requirements to property lines, buildings, retaining walls and other utility trenches (ie: sewer, water, power, gas, electrical, communications). Bioretention/Rain Garden (Volume V, Chapter 7) No Infeasibility: The minimum one foot of vertical separation from hardpan (impervious layer) per SCDM Volume V, 7.3 would not be feasible. Additionally, the proposed intense site development of the multiple residential (MR) zone limits available area to place infiltration systems that meet set back requirements to property lines, buildings, retaining walls and other utility trenches (ie: sewer, water, power, gas, electrical, communications). BMP T5.10B: Downspout Dispersion Systems (Volume III, Chapter 3) No See BMP T5.30 dispersion infeasibility discussion above. BMP T5.10C: Perforated Stub-out (Volume III, Chapter 3) No Infeasibility: Not recommended due to shallow depth to dense unweathered glacial till (hardpan). Additionally, the proposed intense site development of the multiple residential (MR) zone limits available area to place infiltration based BMP systems that meet set back requirements to property lines, buildings, retaining walls and other utility trenches (ie: sewer, water, power, gas, electrical, communications). In accordance with SCDM Volume III, 3.1.1., sections of the stub-out located under impervious or heavily compacted surface (e.g., driveways and parking areas) shall be non-perforated pipe. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-5 RAM Engineering, Inc. Other Hard Surfaces: BMP T5.30: Full Dispersion (Volume V, Chapter 5) No See BMP T5.30 dispersion infeasibility discussion above. BMP T5.15: Permeable Pavement (Volume V, Chapter 5) No Infiltration-based stormwater control measures are not recommended for the project site, see BMP T5.10A infeasibility discussion above. It is anticipated that the site development areas will have seasonal high ground water and underlying impermeable/low permeable layer that would create saturated conditions within one foot of the bottom of the lowest gravel base course of the proposed pavement areas. Bioretention (Volume V, Chapter 7) No See BMP T5.10A infiltration-based and ground water infeasibility discussion above. BMP T5.12: Sheet Flow Dispersion BMP T5.11: Concentrated Flow Dispersion (Volume V, Chapter 5) No See BMP T5.30 dispersion infeasibility discussion above. Minimum Requirement #6: Runoff Treatment: Placement of general use approved basic water quality treatment stormwater filters are included the proposed site development. The stormwater treatment filters will be placed online and downstream of the proposed detention vault. A full summary and design of the stormwater treatment filters are included in Section 5. Minimum Requirement #7: Flow Control: The project discharges to the existing stormwater system along Edmonds Way in the City of Edmonds. The Edmonds Way conveyance system directly discharges to the Puget Sound and does not discharge to a freshwater system. Therefore, the project does not meet the threshold for requiring flow control per MR 7 (Drainage Manual Volume I Section 2.5.7). Minimum Requirement 7 (Vol. 1, Section 2.5.7) states the threshold for require flow control “… apply to projects that discharge stormwater directly or indirectly through a conveyance system into a fresh water system.” The following is a summary of the MR 7 Flow Control thresholds: As described in the first statement in thresholds for requiring flow control, flow control applies to projects that discharge stormwater directly or indirectly into a fresh water Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-6 RAM Engineering, Inc. system. The Ellis Station project discharges to the stormwater conveyance system of Edmonds Way that is continuously piped all the way to the Puget Sound (salt waterbody) The Ellis Station does not directly discharge to any of these types of water bodies. See attached downstream map from City of Edmonds GIS. The Ellis Station does not directly discharge to a lake, stream, or Category IV wetland. The Ellis Station downstream conveyance is piped (manmade conveyance) all the way to Puget Sound. See attached downstream map in Section 3.2. The Ellis Station stormwater control (detention) system has been designed to the special basin criteria of the Edmonds Way direct discharge basin per Edmonds Community Development Code 18.30.060(D.)(5.)(b.)(iv.)(b.). This Edmonds Way direct discharge flow requirements are in place to limit peak flows from development and maintain the hydraulic capacity of the downstream conveyance system. The City of Edmonds has reviewed the project’s stormwater site plan report and accepted the flow control design with the condition that the stormwater modeling shall use the WSDOT “Puget East 36” rainfall data (sizing revisions will be made to the system design). ECDC 18.30.060(D.)(5.)(b.)(iv.): Ellis Station Stormwater Site Plan Report Job # 20-021 Page 1-7 RAM Engineering, Inc. City of Edmonds Review comment (June 22, 2021): The Ellis Station stormwater system does not discharge to an erodible element; the downstream conveyance is piped all the way to Puget Sound. Due to the fact the system will discharge to the City of Edmonds system along Edmonds Way, the stormwater flow control standards will be designed to the special basin criteria of the Edmonds Way direct discharge basin per Edmonds Community Development Code 18.30(d.)(5.)(b.)(iv.)(B). A detail summary of the collection and sizing calculations is included in Section 5 of this report. Analysis supporting the City of Edmonds, Edmonds Way direct discharge basin flow control facility design is provided in Section 5. The placement of the flow control facilities (detention tank) is shown on the Civil (LDA) Construction Plans. A detention tank in the lower northwest corner of the site will provide stormwater flow control for the project development areas. Minimum Requirement #8: Wetlands Protection: No wetlands or streams are located on site. See the project’s wetland and stream reconnaissance report (by Wetland Resources) for the site evaluation. Minimum Requirement #9: Operation and Maintenance: An operation and maintenance summary is provided in Section 8 of this report. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 2-1 RAM Engineering, Inc. Existing Conditions EXISTING CONDITIONS SUMMARY. The site consists of four parcels (00555300400600, 00555300400502, 00555300400501, and 00555300400403) to create a total site of approximately 1.50 acres. The subject site is bordered by residential zoned properties to the north, east and south; and Edmonds Way (SR-104) along the west. The site contains five existing residences (two residences on parcel 00555300400403 ); each residence is served by hard surface driveways from/to Edmonds Way which will all be removed with the proposed project. The existing structures onsite are surrounded by a mix of grass lawn, landscaping, underbrush with several mature trees throughout the site. On-site stormwater runoff generally flows in a westerly direction to Edmonds Way. The existing stormwater pipe system Edmonds Way flows northwesterly and eventually discharges to Puget Sound about 2.7 miles from the site. A full downstream description is provided in Section 3. There are no wetlands or streams located on or adjacent to the site; see project reconnaissance letter include in the application submittal. The site slopes from west to east dropping about 25 ft at an average slope of 7%. The USDA Natural Resource Soil Conservation Service (NRCS) Web Soil Survey identifies onsite soils as Alderwood- Urban Complex, 2% to 15%; the NRCS Web Soil Survey information is included at the end of this section. A site specific geotechnical engineering study has been specifically prepared for the project site; under separate cover a copy of the report is included in the permit application materials. Alderwood soils are considered to be till soils for stormwater modeling considerations. The site slopes from east to west dropping about 20 ft at an average slope of approximately 8.5%. The Natural Resource Conservation Service (NRCS) Web Soil Survey identifies on-site soils in the area of the proposed site development as Alderwood- Urban land complex, 8% to 15% (#6) and McKenna gravelly silt loam, 0 to 8 percent slopes (#32); considered to be till soils. The NRCS Web Soil Survey information is included at the end of this section. A site specific geotechnical engineering study has been specifically prepared for the project site; under separate cover a copy of the report is included in the permit application materials. The Alderwood soils are considered to be till soils for stormwater modeling considerations. Refer to the Existing Conditions Exhibit on the following page. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 2-2 RAM Engineering, Inc. 2.1.1 EXISTING CONDITIONS EXHIBIT Ellis Station Stormwater Site Plan Report Job # 20-021 Page 2-3 RAM Engineering, Inc. 2.1.2 NRCS WEB SOIL SURVEY Ellis Station Stormwater Site Plan Report Job # 20-021 Page 3-1 RAM Engineering, Inc. Off-site Analysis Report UPSTREAM ANALYSIS. Approximately 1.527 acres of upstream area east of the site may contribute upstream flows to the site. See the attached site basin map provide in Section 4 below. Included in the upstream basin are two spreader dispersion trenches from the neighboring development of Natalie’s Place Condo (AFN 201911225001) and 88th Ave West Condominium (AFN 20140815001). Per the as-builts on file at Snohomish County the Natalie’s Place Condo system is an overflow for an infiltration system that provides flow control for the Natalie’s Place development and the spreader for 88th Ave West Condominium is a dispersion spreader for post detention release flows for the 88th Ave West Condominium. A site visit was made in the fall of 2021 after a period of rainfall to observe the function and operation of the two spreader trench systems. The areas downstream of the systems appeared to be stable and not saturated; there was no observed surface or concentrated flows in the area. To eliminate the potential of flooding impacts to the Ellis Station site, stormwater collection systems have been added to the easterly boundary of Ellis Station, downstream of the spread trench systems. CB 6B and a collection swale (on the Ellis Station site) in the vicinity of 88th Ave West Condominium has been added; and a wall drainpipe extend to the proposed cut rockery has been added in the vicinity of the Natalie’s Estates Condo spreader. See the revised Civil plans with the added stormwater facilities. Due to the fact the project is exempt from flow control standards (MR 7), there is no impact to a flow control detention facility on the Ellis Station site. DOWNSTREAM ANALYSIS. On-site stormwater runoff generally flows in a westerly direction to Edmonds Way along the site’s westerly boundary. The Edmonds Way right-of-way is located within the City of Edmonds boundary. The municipal separate storm sewer system (MS4) stormwater conveyance system of Edmonds Way flows northwesterly and ultimately discharges to the Puget Sound 2.7 miles downstream of the site. The Edmonds Way MS4 direct discharges to the Puget Sound without any discharge to any freshwater system. City of Edmonds GIS stormwater inventory map is included below for reference. No apparent flooding or system inadequacies within downstream of the site were discovered during site visits or research of the downstream system. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 3-2 RAM Engineering, Inc. 3.2.1 CITY OF EDMONDS GIS STORMWATER MAP Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-3 RAM Engineering, Inc. Permanent Stormwater Control Plan PERMANENT STORMWATER CONTROL PLAN SUMMARY. The proposed development includes the construction of 29 residential unit lots with typical onsite utilities, and stormwater flow control and water quality treatment facilities. Additional off-site improvements will include frontage improvements along Edmonds Way (per City of Edmonds standards). An underground detention tank (to provided flow control) will be located in the lower northwest corner of the site. An Oldcastle PerkFilter™ system providing basic water quality treatment will be installed offline and downstream of the proposed stormwater detention tank. Controlled and treated runoff will then discharge to the stormwater system (MS4) in Edmonds Way that flows south eventually discharging directly to the Puget Sound. The Ellis Station project site consists of a gross site area of 1.498 acers and 0.049 acres of redevelopment area within Edmonds Way for a total site development area of 1.547 acres. The Ellis Station Edmonds Way frontage improvements and a lower portion of the site will by-pass the stormwater collection system of the project’s stormwater detention tank. The non-collected by-pass area (0.308 ac) will be included in the WWHM flow control modeling. Below is a summary of the project site areas and a project areas map. Project Site Area Summary: Ellis Station Project Site Area + 1.498 acres Edmonds Way Frontage Redevelopment Area + 0.049 acres Development Area = 1.547 acres Ellis Station Project Site Lower By-Pass Area + 0.258 acres Edmonds Way Frontage Redevelopment Bypass Area + 0.049 acres Total By-pass Area 0.308 acres Site Development Area Routed to Detention System 1.239 acres Total Development Area = 1.547 acres Upstream Area = 1.527 acres (Note, upstream basin area is not part of site development thus, flow control for the Edmonds Way Direct Discharge Basin not required. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-4 RAM Engineering, Inc. Site Areas Map: Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-1 RAM Engineering, Inc. EXISTING CONDITIONS HYDROLOGIC MODEL. Existing project area and frontage redevelopment areas are modeled as forested condition. The project site area in Section 4.1 above delineates the site and Edmonds Way frontage redevelopment areas. Below is a summary of the existing condition site areas: Existing Conditions: Project Area: Ellis Station Project Site Area + 1.498 acres Edmonds Way Frontage Redevelopment Area + 0.049 acres Total Development Project Area = 1.547 acres DEVELOPED CONDITIONS HYDROLOGIC MODEL. 4.3.1 DEVELOPED CONDITIONS AREAS Below is a summary of the developed conditions areas for the collected and upstream areas: Developed Conditions: Collected Project Area: Impervious (hard surface): Drive Aisle = 0.165 ac. (7,190 sf) Roofs = 0.501 ac. (21,840 sf) Driveways = 0.076 ac. (3,290 sf) Sidewalks/Patios/Open Space Misc. = 0.154 ac. (6,690 sf) Total Collected Impervious = 0.896 acres (39,010 sf) Pervious: Onsite lawn and landscape = 0.343 acres (14,983 sf) Total Collected Area = 1.239 acres Frontage By-Pass Area: Impervious (hard surface): Access / Drive Aisle = 0.052 ac. (2,260 sf) Frontage Sidewalks = 0.049 ac. (2,150 sf) Total By-pass Impervious = 0.101 acres (4,415 sf) Pervious: Onsite lawn and landscape = 0.207 acres (9,000 sf) Total By-pass Area = 0.308 acres Total Impervious = 0.977 acres Total Pervious = 0.550 acres Total Area = 1.547 acres Allocated Ellis Station Lot Impervious (hard surface): Roofs = 21,840 sf (0.501 ac) Driveways = 3,290 sf (0.076 ac) Patio/Walk = 2,640 sf (0.061 ac) Total Lot Impervious = 27,770 sf (0.638 ac) Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-2 RAM Engineering, Inc. 4.3.2 DEVELOPED CONDITIONS EXHIBIT Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-3 RAM Engineering, Inc. 4.3.3 FLOW CONTROL ANALYSIS AND DESIGN An underground detention tank will be installed along in the northwesterly corner of the project site to control stormwater discharges from the project site. The proposed detention tank contains provides about 2,098 cf (37 feet of 8.5 ft diameter pipe) of storage volume. The vault outflow will discharge to an Oldcastle Perk Filter™ located downstream of the control structure (see section 4.4 for water quality calculations). Due to the fact the stormwater basin of the site does not discharge to a fresh water system (direct discharge to the Puget Sound) the threshold for requiring full flow control is not met/required (Drainage Manual Volume I, 2.5.7). However, flow control for the site will be provided in accordance with the City of Edmonds direct discharge basin requirements (Edmonds Community Development Code 18.30.060(D.)(5.)(b.)(iv.)(B)). Direct discharge basin flow control is provided to maintain capacity of the downstream stormwater system; peak release flows from the site are reduced/controlled so the downstream conveyance system is not overburdened/exceeded. City of Edmonds “Edmonds Way Specific Requirements” requires project to apply on-site stormwater management/ LID measures (MR #5); after applying the MR #5 requirements, all remaining project site runoff (e.g., from unmanaged surfaces, or from overflow from on-site BMPs), the post-development 10- and 100-year recurrence interval peak flows shall not exceed 0.25 and 0.45 cubic feet per second per acre of impervious surface area, respectively. The following table summarizes the direct discharge basin flow control criteria: Flow Control Summary Table: Project Hard (Impervious) Surface Areas 0.997 Acres (43,420 sf) Allowed Post-development Runoff Rates (Edmonds Way Direct Discharge Basin) Q (cfs.) 10 yr. (0.25 cfs per impervious acre) 0.249 cfs (0.25 X 0.997) 100 yr. (0.45 cfs per impervious acre) 0.449 cfs (0.45 X 0.997) Post-development Runoff Rates (with quantity controls + bypass) Total (Tank Release + Bypass) Q (cfs.) 10 yr. 0.246 cfs 100 yr. 0.430 cfs Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-4 RAM Engineering, Inc. The hydrologic analysis below uses WWHM2012 software to model historical storm events in the existing and proposed developed conditions to estimate the 10- and 100-year recurrence interval peak flows. The design WWHM2012 summary input and output printout is provided below. WWHM2012 PROJECT REPORT __________________________________________________________________ WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: 20-021 Ellis- Edmonds- WSDOT Site Name: Ellis Station Site Address: 20-021 City : Report Date: 10/20/2021 MGS Regoin : Puget East Data Start : 1901/10/1 Data End : 2058/09/30 DOT Data Number: 03 Version Date: 2019/09/13 Version : 4.2.17 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Mod 1.547 Pervious Total 1.547 Impervious Land Use acre Impervious Total 0 Basin Total 1.547 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-5 RAM Engineering, Inc. MITIGATED LAND USE Name : By-pass Frontage Bypass: Yes GroundWater: No Pervious Land Use acre C, Pasture, Mod .207 Pervious Total 0.207 Impervious Land Use acre DRIVEWAYS MOD 0.052 SIDEWALKS FLAT 0.049 Impervious Total 0.101 Basin Total 0.308 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Pasture, Mod .343 Pervious Total 0.343 Impervious Land Use acre ROADS MOD 0.165 ROOF TOPS FLAT 0.501 DRIVEWAYS MOD 0.076 SIDEWALKS MOD 0.154 Impervious Total 0.896 Basin Total 1.239 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Tank 1 Tank 1 ___________________________________________________________________ Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-6 RAM Engineering, Inc. Name : Tank 1 Tank Name: Tank 1 Dimensions Depth: 8.5 ft. Tank Type : Circular Diameter : 8.5 ft. Length : 37 ft. Discharge Structure Riser Height: 8 ft. Riser Diameter: 12 in. Orifice 1 Diameter: 1.5 in. Elevation: 0.5 ft. Orifice 2 Diameter: 1.5 in. Elevation: 6.5 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Tank Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 100.00 0.000 0.000 0.000 0.000 100.09 0.001 0.000 0.000 0.000 100.19 0.002 0.000 0.000 0.000 100.28 0.002 0.000 0.000 0.000 100.38 0.003 0.000 0.000 0.000 100.47 0.003 0.001 0.000 0.000 100.57 0.003 0.001 0.015 0.000 100.66 0.003 0.001 0.024 0.000 100.76 0.004 0.002 0.030 0.000 100.85 0.004 0.002 0.036 0.000 100.94 0.004 0.002 0.040 0.000 101.04 0.004 0.003 0.044 0.000 101.13 0.004 0.003 0.048 0.000 101.23 0.005 0.004 0.052 0.000 101.32 0.005 0.004 0.055 0.000 101.42 0.005 0.005 0.058 0.000 101.51 0.005 0.005 0.061 0.000 101.61 0.005 0.006 0.064 0.000 101.70 0.005 0.006 0.066 0.000 101.79 0.005 0.007 0.069 0.000 101.89 0.006 0.008 0.072 0.000 101.98 0.006 0.008 0.074 0.000 102.08 0.006 0.009 0.076 0.000 102.17 0.006 0.009 0.079 0.000 102.27 0.006 0.010 0.081 0.000 102.36 0.006 0.010 0.083 0.000 102.46 0.006 0.011 0.085 0.000 102.55 0.006 0.012 0.087 0.000 102.64 0.006 0.012 0.089 0.000 102.74 0.006 0.013 0.091 0.000 102.83 0.006 0.014 0.093 0.000 102.93 0.006 0.014 0.095 0.000 103.02 0.006 0.015 0.097 0.000 103.12 0.007 0.016 0.098 0.000 103.21 0.007 0.016 0.100 0.000 103.31 0.007 0.017 0.102 0.000 103.40 0.007 0.018 0.104 0.000 103.49 0.007 0.018 0.105 0.000 103.59 0.007 0.019 0.107 0.000 103.68 0.007 0.020 0.108 0.000 103.78 0.007 0.020 0.110 0.000 103.87 0.007 0.021 0.112 0.000 103.97 0.007 0.022 0.113 0.000 104.06 0.007 0.022 0.115 0.000 104.16 0.007 0.023 0.116 0.000 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-7 RAM Engineering, Inc. 104.25 0.007 0.024 0.118 0.000 104.34 0.007 0.024 0.119 0.000 104.44 0.007 0.025 0.121 0.000 104.53 0.007 0.026 0.122 0.000 104.63 0.007 0.026 0.124 0.000 104.72 0.007 0.027 0.125 0.000 104.82 0.007 0.028 0.126 0.000 104.91 0.007 0.028 0.128 0.000 105.01 0.007 0.029 0.129 0.000 105.10 0.007 0.030 0.131 0.000 105.19 0.007 0.030 0.132 0.000 105.29 0.007 0.031 0.133 0.000 105.38 0.007 0.032 0.134 0.000 105.48 0.006 0.032 0.136 0.000 105.57 0.006 0.033 0.137 0.000 105.67 0.006 0.034 0.138 0.000 105.76 0.006 0.034 0.140 0.000 105.86 0.006 0.035 0.141 0.000 105.95 0.006 0.036 0.142 0.000 106.04 0.006 0.036 0.143 0.000 106.14 0.006 0.037 0.145 0.000 106.23 0.006 0.037 0.146 0.000 106.33 0.006 0.038 0.147 0.000 106.42 0.006 0.039 0.148 0.000 106.52 0.006 0.039 0.157 0.000 106.61 0.006 0.040 0.171 0.000 106.71 0.005 0.040 0.179 0.000 106.80 0.005 0.041 0.186 0.000 106.89 0.005 0.041 0.192 0.000 106.99 0.005 0.042 0.198 0.000 107.08 0.005 0.042 0.203 0.000 107.18 0.005 0.043 0.208 0.000 107.27 0.005 0.043 0.212 0.000 107.37 0.004 0.044 0.216 0.000 107.46 0.004 0.044 0.221 0.000 107.56 0.004 0.045 0.224 0.000 107.65 0.004 0.045 0.228 0.000 107.74 0.004 0.046 0.232 0.000 107.84 0.003 0.046 0.236 0.000 107.93 0.003 0.046 0.239 0.000 108.03 0.003 0.047 0.292 0.000 108.12 0.003 0.047 0.695 0.000 108.22 0.002 0.047 1.261 0.000 108.31 0.002 0.047 1.822 0.000 108.41 0.001 0.048 2.234 0.000 108.50 0.000 0.048 2.462 0.000 ___________________________________________________________________ Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-8 RAM Engineering, Inc. ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:1.547 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.550 Total Impervious Area:0.997 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.02625 5 year 0.04074 10 year 0.0486 25 year 0.056594 50 year 0.061342 100 year 0.065245 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.149625 5 year 0.202988 10 year 0.246184 25 year 0.310611 50 year 0.366439 100 year 0.429611 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-9 RAM Engineering, Inc. Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1902 0.037 0.145 1903 0.013 0.155 1904 0.023 0.171 1905 0.015 0.113 1906 0.006 0.122 1907 0.041 0.177 1908 0.026 0.148 1909 0.028 0.144 1910 0.045 0.168 1911 0.023 0.143 1912 0.079 0.449 1913 0.039 0.125 1914 0.009 0.292 1915 0.013 0.120 1916 0.021 0.146 1917 0.010 0.121 1918 0.025 0.135 1919 0.019 0.117 1920 0.024 0.132 1921 0.025 0.118 1922 0.029 0.147 1923 0.019 0.134 1924 0.013 0.144 1925 0.013 0.109 1926 0.023 0.150 1927 0.029 0.135 1928 0.019 0.128 1929 0.041 0.165 1930 0.024 0.169 1931 0.025 0.132 1932 0.017 0.137 1933 0.022 0.129 1934 0.058 0.214 1935 0.021 0.123 1936 0.034 0.126 1937 0.028 0.170 1938 0.028 0.132 1939 0.002 0.138 1940 0.023 0.183 1941 0.023 0.153 1942 0.034 0.157 1943 0.012 0.160 1944 0.029 0.269 1945 0.025 0.169 1946 0.023 0.127 1947 0.016 0.139 1948 0.054 0.228 1949 0.044 0.211 1950 0.024 0.117 1951 0.028 0.137 1952 0.077 0.446 1953 0.068 0.461 1954 0.021 0.144 1955 0.018 0.127 1956 0.012 0.104 1957 0.030 0.126 1958 0.072 0.226 1959 0.044 0.149 1960 0.016 0.126 1961 0.045 0.399 1962 0.023 0.146 1963 0.011 0.105 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-10 RAM Engineering, Inc. 1964 0.021 0.194 1965 0.053 0.196 1966 0.009 0.115 1967 0.021 0.123 1968 0.028 0.124 1969 0.019 0.139 1970 0.030 0.149 1971 0.056 0.240 1972 0.037 0.644 1973 0.042 0.181 1974 0.024 0.159 1975 0.060 0.329 1976 0.027 0.186 1977 0.017 0.116 1978 0.052 0.214 1979 0.014 0.135 1980 0.026 0.135 1981 0.026 0.151 1982 0.018 0.118 1983 0.042 0.153 1984 0.011 0.159 1985 0.025 0.137 1986 0.020 0.122 1987 0.043 0.154 1988 0.032 0.142 1989 0.025 0.126 1990 0.032 0.131 1991 0.024 0.192 1992 0.037 0.221 1993 0.030 0.169 1994 0.054 0.164 1995 0.013 0.108 1996 0.060 0.145 1997 0.028 0.126 1998 0.027 0.166 1999 0.001 0.139 2000 0.020 0.159 2001 0.014 0.131 2002 0.037 0.179 2003 0.029 0.146 2004 0.029 0.160 2005 0.036 0.207 2006 0.019 0.116 2007 0.021 0.166 2008 0.027 0.127 2009 0.017 0.135 2010 0.013 0.170 2011 0.017 0.110 2012 0.024 0.259 2013 0.020 0.129 2014 0.014 0.130 2015 0.052 0.191 2016 0.007 0.104 2017 0.042 0.259 2018 0.073 0.172 2019 0.079 0.390 2020 0.023 0.151 2021 0.035 0.177 2022 0.013 0.165 2023 0.029 0.168 2024 0.062 0.568 2025 0.023 0.120 2026 0.040 0.143 2027 0.018 0.148 2028 0.009 0.097 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-11 RAM Engineering, Inc. 2029 0.029 0.136 2030 0.057 0.212 2031 0.016 0.106 2032 0.013 0.113 2033 0.014 0.107 2034 0.016 0.129 2035 0.064 0.248 2036 0.033 0.134 2037 0.006 0.134 2038 0.033 0.161 2039 0.003 0.175 2040 0.012 0.148 2041 0.018 0.146 2042 0.066 0.262 2043 0.029 0.165 2044 0.040 0.140 2045 0.025 0.125 2046 0.030 0.141 2047 0.019 0.152 2048 0.025 0.164 2049 0.024 0.160 2050 0.015 0.134 2051 0.026 0.263 2052 0.015 0.123 2053 0.027 0.131 2054 0.035 0.261 2055 0.011 0.131 2056 0.010 0.148 2057 0.018 0.124 2058 0.021 0.153 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.0794 0.6445 2 0.0792 0.5677 3 0.0771 0.4610 4 0.0728 0.4493 5 0.0719 0.4463 6 0.0683 0.3989 7 0.0664 0.3898 8 0.0638 0.3285 9 0.0617 0.2917 10 0.0602 0.2685 11 0.0595 0.2625 12 0.0578 0.2624 13 0.0568 0.2610 14 0.0562 0.2592 15 0.0537 0.2586 16 0.0536 0.2484 17 0.0529 0.2396 18 0.0520 0.2279 19 0.0519 0.2258 20 0.0448 0.2215 21 0.0447 0.2139 22 0.0441 0.2135 23 0.0438 0.2119 24 0.0426 0.2106 25 0.0424 0.2073 26 0.0422 0.1962 27 0.0419 0.1942 28 0.0415 0.1921 29 0.0409 0.1907 30 0.0405 0.1857 31 0.0403 0.1832 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-12 RAM Engineering, Inc. 32 0.0392 0.1808 33 0.0370 0.1792 34 0.0367 0.1772 35 0.0366 0.1766 36 0.0366 0.1752 37 0.0361 0.1721 38 0.0354 0.1706 39 0.0353 0.1703 40 0.0343 0.1700 41 0.0342 0.1694 42 0.0331 0.1690 43 0.0329 0.1688 44 0.0322 0.1683 45 0.0316 0.1675 46 0.0304 0.1661 47 0.0301 0.1659 48 0.0299 0.1654 49 0.0298 0.1654 50 0.0295 0.1653 51 0.0294 0.1642 52 0.0291 0.1637 53 0.0290 0.1612 54 0.0289 0.1602 55 0.0288 0.1600 56 0.0286 0.1598 57 0.0285 0.1593 58 0.0284 0.1593 59 0.0282 0.1590 60 0.0282 0.1569 61 0.0281 0.1546 62 0.0278 0.1541 63 0.0277 0.1533 64 0.0270 0.1528 65 0.0267 0.1527 66 0.0267 0.1523 67 0.0265 0.1510 68 0.0262 0.1508 69 0.0260 0.1502 70 0.0258 0.1492 71 0.0255 0.1488 72 0.0255 0.1482 73 0.0255 0.1480 74 0.0254 0.1479 75 0.0252 0.1478 76 0.0251 0.1475 77 0.0251 0.1465 78 0.0248 0.1464 79 0.0247 0.1462 80 0.0245 0.1455 81 0.0240 0.1445 82 0.0239 0.1445 83 0.0238 0.1445 84 0.0238 0.1443 85 0.0237 0.1436 86 0.0236 0.1432 87 0.0235 0.1428 88 0.0234 0.1416 89 0.0232 0.1408 90 0.0231 0.1403 91 0.0231 0.1392 92 0.0229 0.1389 93 0.0228 0.1387 94 0.0228 0.1377 95 0.0225 0.1375 96 0.0216 0.1374 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-13 RAM Engineering, Inc. 97 0.0213 0.1373 98 0.0213 0.1355 99 0.0212 0.1353 100 0.0208 0.1350 101 0.0208 0.1350 102 0.0207 0.1347 103 0.0206 0.1347 104 0.0199 0.1343 105 0.0198 0.1342 106 0.0197 0.1341 107 0.0193 0.1338 108 0.0189 0.1324 109 0.0188 0.1322 110 0.0187 0.1318 111 0.0186 0.1312 112 0.0186 0.1308 113 0.0185 0.1306 114 0.0183 0.1306 115 0.0178 0.1295 116 0.0176 0.1295 117 0.0175 0.1291 118 0.0175 0.1288 119 0.0171 0.1278 120 0.0171 0.1272 121 0.0167 0.1270 122 0.0161 0.1266 123 0.0159 0.1264 124 0.0158 0.1263 125 0.0158 0.1262 126 0.0154 0.1259 127 0.0152 0.1258 128 0.0149 0.1248 129 0.0144 0.1246 130 0.0141 0.1243 131 0.0139 0.1242 132 0.0137 0.1234 133 0.0134 0.1233 134 0.0132 0.1230 135 0.0128 0.1224 136 0.0127 0.1221 137 0.0127 0.1210 138 0.0126 0.1204 139 0.0126 0.1198 140 0.0126 0.1179 141 0.0122 0.1177 142 0.0120 0.1175 143 0.0116 0.1168 144 0.0111 0.1164 145 0.0110 0.1158 146 0.0107 0.1147 147 0.0103 0.1127 148 0.0101 0.1126 149 0.0095 0.1097 150 0.0092 0.1091 151 0.0088 0.1082 152 0.0072 0.1073 153 0.0064 0.1058 154 0.0060 0.1048 155 0.0029 0.1040 156 0.0018 0.1036 157 0.0013 0.0965 ___________________________________________________________________ Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-14 RAM Engineering, Inc. Stream Protection Duration POC #1 The Facility FAILED Facility FAILED duration standard for 1+ flows. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0131 53311 414418 777 Fail 0.0136 49017 407207 830 Fail 0.0141 45224 400435 885 Fail 0.0146 41794 393829 942 Fail 0.0151 38651 387389 1002 Fail 0.0156 35810 381113 1064 Fail 0.0160 33256 375222 1128 Fail 0.0165 30922 369112 1193 Fail 0.0170 28868 363221 1258 Fail 0.0175 26969 357551 1325 Fail 0.0180 25185 351826 1396 Fail 0.0185 23429 345991 1476 Fail 0.0190 21882 340321 1555 Fail 0.0195 20347 334595 1644 Fail 0.0199 18921 328760 1737 Fail 0.0204 17666 323035 1828 Fail 0.0209 16515 317640 1923 Fail 0.0214 15469 312355 2019 Fail 0.0219 14451 307180 2125 Fail 0.0224 13515 302336 2237 Fail 0.0229 12607 297381 2358 Fail 0.0234 11792 292647 2481 Fail 0.0238 11071 288188 2603 Fail 0.0243 10427 283619 2720 Fail 0.0248 9837 279325 2839 Fail 0.0253 9309 275086 2955 Fail 0.0258 8792 270902 3081 Fail 0.0263 8296 266829 3216 Fail 0.0268 7850 262590 3345 Fail 0.0272 7404 258681 3493 Fail 0.0277 7035 254497 3617 Fail 0.0282 6667 250424 3756 Fail 0.0287 6342 246295 3883 Fail 0.0292 6045 242276 4007 Fail 0.0297 5742 238257 4149 Fail 0.0302 5474 234349 4281 Fail 0.0307 5222 230550 4414 Fail 0.0311 4990 226807 4545 Fail 0.0316 4756 223119 4691 Fail 0.0321 4540 219595 4836 Fail 0.0326 4335 215962 4981 Fail 0.0331 4124 212494 5152 Fail 0.0336 3938 209081 5309 Fail 0.0341 3775 205778 5451 Fail 0.0346 3631 202420 5574 Fail 0.0350 3491 198952 5698 Fail 0.0355 3368 195594 5807 Fail 0.0360 3257 192290 5903 Fail 0.0365 3145 189043 6010 Fail 0.0370 3055 185905 6085 Fail 0.0375 2966 182767 6162 Fail 0.0380 2876 179739 6249 Fail 0.0385 2779 176766 6360 Fail 0.0389 2681 173794 6482 Fail 0.0394 2586 170821 6605 Fail 0.0399 2491 167958 6742 Fail 0.0404 2391 165151 6907 Fail 0.0409 2278 162343 7126 Fail Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-15 RAM Engineering, Inc. 0.0414 2181 159536 7314 Fail 0.0419 2080 156673 7532 Fail 0.0423 1997 153976 7710 Fail 0.0428 1929 151278 7842 Fail 0.0433 1864 148526 7968 Fail 0.0438 1790 145828 8146 Fail 0.0443 1718 143186 8334 Fail 0.0448 1664 140598 8449 Fail 0.0453 1596 138176 8657 Fail 0.0458 1537 135644 8825 Fail 0.0462 1480 133167 8997 Fail 0.0467 1417 130744 9226 Fail 0.0472 1361 128377 9432 Fail 0.0477 1298 126120 9716 Fail 0.0482 1242 123808 9968 Fail 0.0487 1188 121551 10231 Fail 0.0492 1146 119294 10409 Fail 0.0497 1100 117092 10644 Fail 0.0501 1045 114945 10999 Fail 0.0506 1000 112798 11279 Fail 0.0511 953 110596 11605 Fail 0.0516 916 108559 11851 Fail 0.0521 877 106522 12146 Fail 0.0526 830 104540 12595 Fail 0.0531 788 102504 13008 Fail 0.0535 754 100632 13346 Fail 0.0540 710 98815 13917 Fail 0.0545 666 96888 14547 Fail 0.0550 627 95017 15154 Fail 0.0555 595 93090 15645 Fail 0.0560 555 91328 16455 Fail 0.0565 515 89512 17380 Fail 0.0570 461 87805 19046 Fail 0.0574 421 86044 20438 Fail 0.0579 392 84337 21514 Fail 0.0584 360 82741 22983 Fail 0.0589 334 81034 24261 Fail 0.0594 306 79382 25941 Fail 0.0599 286 77841 27217 Fail 0.0604 264 76245 28880 Fail 0.0609 247 74703 30244 Fail 0.0613 233 73217 31423 Fail _____________________________________________________ The development has an increase in flow durations from 1/2 Predeveloped 2 year flow to the 2 year flow or more than a 10% increase from the 2 year to the 50 year flow. The development has an increase in flow durations for more than 50% of the flows for the range of the duration analysis. ___________________________________________________________________ Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0.0961 acre-feet On-line facility target flow: 0.0963 cfs. Adjusted for 15 min: 0.0963 cfs. Off-line facility target flow: 0.0587 cfs. Adjusted for 15 min: 0.0587 cfs. ___________________________________________________________________ Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-16 RAM Engineering, Inc. LID Report ___________________________________________________________________ Perlnd and Implnd Changes No changes have been made. ___________________________________________________________________ This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by: Clear Creek Solutions, Inc. 2005-2021; All Rights Reserved. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-17 RAM Engineering, Inc. WATER QUALITY ANALYSIS. Perk Filter™ media filter catch basin by Oldcastle Stormwater Solutions will be used to treat the collected surface water from the road and driveway areas. The Perk Filter™ system has Washington State DOE approval as a general use level designation (GULD) for basic water quality (attached). Using the DOE WWHM2012 modeling the off-line water quality treatment rate of 0.0587 cfs is required for development area. Below is the WWHM2012 output analysis of the water quality treatment areas: Per the manufacture’s design criteria, three 18” stacked cartridges provide a treatment flow rate of 0.068 cfs and thus will adequately treat the project required treatment rate of 0.0587 cfs. Below is a manufacture detail of the 60 inch Perk Filter Manhole with an 18” stack system that will meet the water quality treatment requirements of the project. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-18 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 4-19 RAM Engineering, Inc. STORMWATER SYSTEM CAPACITY ANALYSIS 4.5.1 STORMWATER CONVEYANCE PIPE CAPACITY The peak 100-year un-detained flows from the site are estimated at 0.728 cfs according with the WWHM2012 stormwater modeling. All pipes onsite shall be installed at a minimum slope of 0.5%; at this slope, the 12” stormwater pipes will have capacity (2.94 cfs) to adequately convey the 100-year unmitigated peak flow of 0.728 cfs. 4.5.2 CONTROL STRUCTURE OVERFLOW CAPACITY CALCULATION: An emergency overflow standpipe has been provided within the proposed control structure of the detention system. The riser shall be 12” in diameter with the main overflow elevation set at 380.20 (see Civil Construction Plans). The emergency overflow standpipe will adequately convey the peak 100-year unmitigated flow of 0.728 cfs with the 0.5 ft of freeboard above the standpipe. Below is a stage- discharge table demonstrating the capacity (2.674 cfs) of a 12” riser with 0.5 ft of head. RISER FLOW EQUATION INPUT DATA: RISER DIAMETER = 12.0 inches RISER CREST EL. = 380.20 Elev. ELEV INCREMENT = 0.1 feet OUTPUT DATA: WEIR EQUATION: Qw = 9.739 H1.5 (BELOW OR @ EL.) 380.6 ORIFICE EQUATION: Qo = 3.782 H0.5 (ABOVE OR @ EL.) 380.6 RISER EQUATIONS: Weir Flow: Orifice Flow: Q = 9.739 * D * (H)^1.5 Q = 3.782 * D * (H)^.5 WEIR ORIFICE CONTROL. ELEV HEAD FLOW FLOW FLOW [NGVD] [feet] [cfs] [cfs] [cfs] 380.20 0.00 0.000 0.000 0.000 380.30 0.10 0.308 1.196 0.308 380.40 0.20 0.871 1.691 0.871 380.50 0.30 1.600 2.071 1.600 380.60 0.40 2.464 2.392 2.392 380.70 0.50 3.443 2.674 2.674 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 5-1 RAM Engineering, Inc. Construction Stormwater Pollution Prevention Plan (SWPPP) A separate construction SWPPP report (WADOE template) has been prepared in conjunction with this report. The project site is over one acre in size, thus a Washington State Department of Ecology (DOE) Notice of Intent (NOI) and Construction SWPPP will be processed with DOE prior to construction activities onsite Ellis Station Stormwater Site Plan Report Job # 20-021 Page 6-1 RAM Engineering, Inc. Special Reports and Studies 1. Geotechnical Engineering Study Prepared by: Earth Solutions NW, LLC Dated: April 4, 2021 Ellis Station Stormwater Site Plan Report Job # 20-021 Page 7-1 RAM Engineering, Inc. Other Permits This report may supplement the follow additional/pending permits:  Washington State Department of Ecology NPDES Construction NOI Permit with  Snohomish County Building Permit for the stormwater detention vault  Snohomish County Building Permit for on-site retaining walls.  Snohomish County Demolition Permit for removal of site structures. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-1 RAM Engineering, Inc. Operation and Maintenance Manual Maintenance of the stormwater management system shall meet Snohomish County requirements per SCC 30.36A and SCC 7.53.140. The detention and water quality facility are private systems and shall be maintained by the residents of the Ellis Station subdivision. Drainage systems (pipes and catch basins) within the adjacent public right-of-way are to be maintained by the City of Edmonds/WSDOT. The maintenance standard tables for detention vault, control structure, catch basins, storm conveyance pipes, facility discharge locations and access roads from the 2016/2017 Snohomish County Drainage Manual Volume V are attached on the following pages. Also attached are the maintenance guide from Oldcastle for the PerkFilter™ for the project’s water quality filter system. Semi-annual inspections shall be conducted to ensure proper operations of the drainage system. The inspections should occur prior to the winter rain season (Oct/Nov), leaving sufficient time to correct any detected maintenance problems, and at the end of the season (April/May) to determine the effect of the season’s runoff. Once a historical basis is developed the frequency of inspection may be modified as necessary. Sediment can build up inside control structures and catch basins, blocking or restricting flow to the inlet. To prevent this problem, these structures should be routinely cleaned. Regular inspections of control structures should be conducted to detect the need for non-routine cleanout. The detention tank, control structure, stormwater filter, junction catch basins, and conveyance pipes within the private property of the project shall be privately maintained by the residents of the two projects. Maintenance shall include the removal of sediment from the vault bottom, using a vactor truck or equally effective method. Regular inspections of the control structure within the catch basin shall be conducted to detect if non-routine maintenance is required due to sediment or debris blocking or clogging the fixed orifice or standpipe inlet. Regular sediment removal from the catch basins with a vactor truck or equally effective method shall be conducted as determined by regular inspections. Regular inspection and maintenance of the stormwater systems should occur according to the attached maintenance tables from the 2016/2017 Snohomish County Drainage Manual. All drainage components within Edmonds Way (public right-of-way) are to be maintained by City of Edmonds/WSDOT. Regular inspections of the drainage system are recommended to alert the City if system needs immediate attention. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-2 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-3 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-4 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-5 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-6 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-7 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-8 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-9 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-10 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-11 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-12 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-13 RAM Engineering, Inc. Ellis Station Stormwater Site Plan Report Job # 20-021 Page 8-14 RAM Engineering, Inc. 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