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APPROVED STM_RESUB1 BLD2021-0657+Storm_Drainage_Report+12.16.2021_4.48.36_PM+2578439C � ENGINEERING civil & structural engineering & planning DRAINAGE REPORT Kopak Residence 346 Sunset Ave N Edmonds, WA 98020 1211412021 250 4th Ave S Ste 200 CG Project No. 21335.20 Edmonds, WA 98020 Phone: (425) 778-8500 Fax: (425) 778-5536 Table of Contents Section I — Project Overview Section 11 — Minimum Requirements Section III — Off -Site Analysis Section IV — Permanent Stormwater Control Plan Section V — Construction Stormwater Pollution Prevention Section VI — Special Reports and/or Studies Section VI — Other Permits Section VIII — Operation and Maintenance Manual 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section I, Page 1 Section I — Project Overview Section I Summary Overview Existing Condition Developed Condition Overview This drainage report has been written for the addition of a new detached garage, covered patio area, and an associated driveway to a residential property, the Kopak Residence. The parcel is 0.25ac and located at 346 Sunset Ave N in Edmonds, WA 98020 (TPN: 27032300100400). The site has an existing single-family residence, shed, and associated driveways and walkways. The site is on a flat landscape, with little variance in elevation. In the developed condition, there will be a new detached garage and covered patio area. New and replaced impervious surfaces total 2,167 sf and the area of land disturbance is approximately 3,000 sf. The project must comply with ECDC 18.30 — Stormwater Management Code, the 2014 Stormwater Management Manual of Western Washington (DOE Manual), and the 2017 Edmonds Stormwater Addendum (Edmonds Stormwater Addendum). It is classified as a Category 1 project per ECDC 18.30 and must meet Minimum Requirements #1-5 because the amount of new plus replaced impervious surfaces are greater than 2,000 and less than 5,000 sf. This report is based on the steps recommended in Chapter 7 of the Edmonds Stormwater Addendum. Existing Condition The site has an existing single-family residence on the northwestern portion of the site, a shed in the easternmost corner, and associated driveways and walkways. Any pervious surfaces on site are landscaped with grass. The site is relatively flat, with little to no grade change. Per the geotechnical engineering report, site soils consist of loose, dry to moist undocumented fill, becoming medium dense to dense moist till at two feet below grade. The existing parcel areas are as follows: Pervious Areas Landscaping: 5,550 sf (0.127 ad Total: 5,550 sf (0.127 ac) Impervious Areas Roof: 2,125 sf (0.049 ac) Driveway (Conc.): 1,988 sf (0.046 ac) Sidewalk (Conc.): 508 sf (0.012 ac) Shed: 217 sf (0.005 ac) 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section I, Page 2 Gravel: 324 sf (0.007 ac) Total: 5,162 sf (0.119 ac) Total Lot: 10,712 sf (0.246 ac) The site borders Sunset Ave to the northwest, and existing single-family residences in all other directions. Utilities are available at the site, including stormwater. There are existing catch basins that lead to a detention facility located in the driveway. These convey stormwater to the storm main located in the Sunset Ave N ROW. Please see Section III for the off -site analysis. Developed Condition A detached garage and covered patio are proposed for the site. An associated driveway and walkway will be included in the site development. Utilities are available at the site, including stormwater pipes. Stormwater travels northwest in proposed conveyance pipes within the site until it ties into the existing stormwater pipe. The flow is then routed to a catch basin outside the property, entering the City's MS4 within Sunset Ave. The site is within the Puget Sound Piped Watershed. See Section III for the off -site analysis. On -site stormwater BMPs were evaluated for a project that discharges directly to Puget Sound through the City's MS4. All dispersion and infiltration BMPs were found to be infeasible. See Section IV for infeasibility criteria. The developed parcel areas are as follows: Pervious Areas Landscaping: 3,383 sf (0.077 ac) Total: 3,383 sf (0.077 ac) New/Replaced Impervious Areas Garage Roof: 1,532 sf (0.035 ac) Driveway Addition: 635 sf (0.015 ac) Total: 2,167 sf (0.050 ac) Total Lot: 10,712 sf (0.246 ac) 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section I, Page 3 Jay Dee Contractors v Caspers St Cospers St o Edmonds Food Bank a` ryr Ciltner Ln FUtIl.ree Libary v"" Nl Sala Ln PROJECT SITE Brackett s 346 Sunset Ave N, Holy Rosary School IV Landing Edmonds, WA 98020 Shoreline • Sanctuary... �P Sunset Beach 19,41, Brackett's Landing North Fd s Q 6, Demetris Sprague St �.j Woodstone Taverna 0rderpoorDash AQ, ghnuts b Salt and Iron ©p Bell St z 7"EdmondsE3 S © Rusty Pelican Cafe Edmonds Ferry Terminal J-1111, s The MARKET I Q a, fishmonger 8 eatery Figure 1-1. Vicinity Map (Google Maps). Holy Rosary Church 0 Aluh. St clm, St 81h Ave N Dai , 9 Sprague St Ed�,a�ds St Be11 St z Frances Anderson Center 0 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section I, Page 4 i 4 .4j P ROJ EMSITE »?T [i �7 z4a Figure 1-2. Aerial Photograph (Google Maps). 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section II, Page 1 Section II — Minimum Requirements Section II Summary Narrative The project must comply with ECDC 18.30 — Stormwater Management Code, the 2014 Stormwater Management Manual of Western Washington (DOE Manual), and the 2017 Edmonds Stormwater Addendum (Stormwater Addendum). It is classified as a Category 1 project per ECDC 18.30 and must meet Minimum Requirements #1-5 because the amount of new plus replaced impervious surfaces are greater than 2,000 sf and less than 5,000 sf. Minimum Requirement #1: Preparation of Stormwater Site Plans: The stormwater site plan consists of this report and the civil drawings and is prepared in accordance with Chapter 3 of Volume I of the DOE Manual and the requirements in the Edmonds Stormwater Addendum. Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP): The SWPPP shall include a narrative and drawings. The SWPPP narrative shall include documentation that addresses the 13 elements of Construction Stormwater Pollution Prevention. See Section V and the civil drawings. Minimum Requirement #3: Source Control of Pollution: All known, available and reasonable source control BMPs are required for all projects approved by the city. All single-family residential projects shall, at a minimum, incorporate required BMPs from SWMMWW Volume IV, S411— BMPs for Landscaping and Lawn/Vegetation Management. Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls: Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. The manner by which runoff is discharged from the project site must not cause a significant adverse impact to downstream receiving waters and down -gradient properties. All projects shall submit an off -site qualitative analysis. A qualitative analysis of the upstream and downstream system entering the site is presented in Section III. A conveyance capacity calculation for the downstream system is included in Section IV. Minimum Requirement #5: On -Site Stormwater Management: The proposed project discharges directly to Puget Sound through the City's MS4. Per ECDC 18.30, it must implement BMP T5.13 Post -Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.106 Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion. On -Site Stormwater BMPs were found to be infeasible for all hard surfaces due to low infiltration rates and lack of space. See Section IV. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section II, Page 2 Does the project result in 2,000 square feet, or greater, of new plus replaced hard surface area? OR I)ocs the land disturbing activity total 7,000 square feet or greater? Yes No %In-11:11um Requirements No. I through 5 apply Minimum Requirement No. 2 applies Ne�R Question Does the project add 5,000 square feet or more of new plus replaced hard surfaces' OR Convert 0.75 acres or more of vegetation to lawn or landscaped areas? OR Convert 2.5 acres or more of native vegetation to aasture? No Yes Is this a road related project? I:1^ Yes Does the project add 5,000 square feet or Ne more of new hard surfaces? Yes All Minimum Requirements apply to the new and replaced Yes Do new hard surfaces add 50% or hard surfaces and converted more to the existing hard surfaces vegetation areas. within the project limits? All Minimum Requirements apply to the new hard surfaces AL - No I additional and converted vegetation areas. re requirements. Figure II-1 Flow Chart for Determining Requirements for Redevelopment (Figure 3.1 of the Edmonds Stormwater Addendum) 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section III, Page 1 Section III — Off -Site Analysis Section III Summary Task 1 — Define and map the study area Task 2 — Review all available information of the study area Task 3 — Field inspect the area Task 4 - Describe the drainage system, and its existing and predicted problems Task 1— Define and map the study area An initial qualitative analysis shall document potential off -site impacts of stormwater discharges for each upstream drainage system entering a site, and each downstream drainage system leaving a site according to Section 6.2 of the Edmonds Stormwater Addendum. Runoff from the site enters the City's MS4 within one -quarter mile and therefore must evaluate and document downstream conditions up to and including runoff entry into the City's MS4. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com KopakResidence -[G#Z133S.Z0 Drainage Report December14, 2021 Figure U|'1Study Area Task 2 - Review all available information on the study areo Existing stormwater improvements were determined from the survey and the City GIS map. There is an existing detention facility with associated conveyance pipes and catch basins. Conveyance pipes from the property lead to the Edmonds MS4 at the western corner ofthe property. From there' a 12" PVC pipe routes stormwater -50' southwest and 12" concrete pipes route stormwater -115' northwest to an outfall in the Puget Sound. Critical areas were evaluated using the City GIS map and the western half ofthe lot is classified asaSeismic Hazard Area. Task 3 - Field inspect the study area A site visit was done on the afternoon of November 19, 2021. The weather was mostly cloudy, and it had rained the previous day. From evaluating the surrounding conditions, the site does not appear to take on any significant upstream flow. There are existing catch basins within the property and on Sunset Ave N. The following figures show the existing downstream conditions of the site. zso'mhAvenue South, Suite znn Edmonds, vvAyuozo - ph 4zs77uusoo | t4zs77u U�����U������U��� | � � � � .ssys | www.cgenginee,inQzom 1 -•• • ' ,R UPSTREAM I I CATCH BASIN V - ON SITE CLEANOUT BE REPLACED IN PROPOS ' CON D N Figure III-2. From the western corner of the site property facing east towards the site. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section III, Page 4 Figure III-3. From the existing driveway facing northwest towards the existing catch basin on Sunset Ave N. Task 4 — Describe the drainage system, and its existing and predicted problems Existing stormwater improvements were determined from the survey and the City GIS map. This is described in Task 2. No on -site stormwater BMPs for hard surfaces were found to be feasible for the project (see Section IV). Thus, the project proposes to route all roof and driveway runoff through conveyance pipes to a proposed catch basin northwest of the existing detention system, replacing the existing storm pipe cleanout. The catch basin connects the proposed conveyance pipe to an existing conveyance pipe that routes stormwater to a catch basin in Sunset Ave N. The existing catch basins appeared to be in good condition, and it is not anticipated that the project will create any issues if runoff from all disturbed areas is routed to the stormwater stub. See Section IV for a conveyance capacity calculation for the downstream system. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section IV, Page 1 Section IV — Permanent Stormwater Control Plan Section IV Summary Narrative BMP Feasibility Review Conveyance Capacity Calculation Narrative The project must meet Minimum Requirement #5: On -Site Stormwater Management. Per ECDC 18.30, projects that directly discharge to Puget Sound through the City's MS4 must implement BMP T5.13 Post - Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.10B Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion. Water quality treatment and flow control facilities are not required for this project. Feasibility Review Per ECDC 18.30, projects that discharges directly to Puget Sound through the City's MS4 must implement BMP T5.13 Post -Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.1013 Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion. 1. BMP T5.13 Post -Construction Soil Quality and Depth: Feasible. All new landscaping will meet the Post -Construction Soil Quality and Depth in accordance with BMP T5.13 of the DOE Manual. 2. BMP T5.10A Downspout Full Infiltration Systems: Infeasible. A full infiltration system is not feasible as infiltration is infeasible on -site per the geotechnical report. 3. BMP T5.10B Downspout Dispersion Systems: Infeasible. There is insufficient space for a 50' vegetated flow path. 4. BMP T5.10C Perforated Stub -out Connections: Infeasible. Infiltration is infeasible on -site. 5. BMP T5.11 Concentrated Flow Dispersion: Infeasible. There is not a sufficient downgradient flow path for dispersion. Therefore, dispersion is infeasible. 6. T5.12 Sheet Flow Dispersions: Infeasible. There is not a sufficient downgradient flow path for dispersion. Therefore, dispersion is infeasible. Conveyance Capacity Calculation The project will add more impervious surfaces than was previously approved in the original design for the property. There are 5,162 sf of existing impervious surfaces on the subject parcel whereas this project will be adding 2,167 sf more of impervious surfaces. A capacity analysis is required per the City Engineer to determine that the downstream conveyance system can handle the runoff from the additional impervious surfaces. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section IV, Page 2 Assuming an additional 0.05 ac (2,167 sf) of impervious surfaces, the flow of stormwater will increase by 0.11 cfs during the 100-year storm (see Figure IV-1). This brings the total flow to 0.38 cfs during the 100- year storm. Figure IV-2 shows conveyance capacity for PVC pipes. Given that the minimum pipe size being tied into is 6", and assuming the pipe is sloped at a minimum of 0.5%, this is sufficient capacity for the existing pipe. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Rational Method - Developed Conditions for: Reserve at Renton Conveyance Pipe Q = Ca'g1A Tc = 6.00 (assumed, since all pipe flow) Dense Forest Light Forest Pasture Lawns Gravel Pavement & Roofs Open Water Other Area (ac) C 1 1 1 1 1 :1 11 1 1'1 11 1 11 Total Area (A) = 0.05 acres Cavg 0.90 i = (Pr)(ir) Pr-- total precipitation at the project site for 24-hr duration storm ir= unit peak rainfall intensity factor ir= (ar)(Tc)^(-br) ar, br= coefficients from Table 3.2.LB ofKCSWDM (ir)z= 0.56 iz= 0.838341 Q2= 0.04 (ir)10= 0.78 ilo= 1.937818 Q10= 0.09 (ir)25= 0.83 i25= 2.075025 Qz;= 0.09 00100= 0.84 i100= 2.532366 Q100= 0.11 Rational Method L5y SBG "ate 12/8/2021 ENGINEERING Developed Conditions Chkd Date 250 4th Ave. South Seale N.T.S. sheet No. Suite 200 Job No. Edmonds, WA 98020 Kopak Residence 21335.20 Rational Method - Developed Conditions for: Kopak Residence n= 0.013 � Rational Method By SBG Date 12/8/2021 ENGINEERING Developed Conditions Chkd Dale 250 4th Ave. South scale N.T.S. Sheet No. Suite 200 Job No. Edmonds, WA 98020 Kopak Residence 21335.20 Conveyance Capacity 4-inch 6-inch 8-inch 10-inch 12-inch Slope (fUft) 0.005 0.13 0.40 0.87 1.55 2.52 0.010 0.19 0.56 L23 2.20 3.57 0.015 0.23 0.69 1.50 2.69 4.37 0.020 0.27 0.79 1.74 3.11 5.05 0.025 0.30 0.89 1.94 3.47 5.65 0.030 0.33 0.97 2.13 3.81 6.18 0.035 0.36 1.05 2.30 4.11 6.68 0.040 0.38 1.12 2.46 4.39 7.14 0.045 0.40 1.19 2.60 4.66 7.57 0.050 0.43 1.26 2.75 4.91 7.98 0.055 0.45 1.32 2.88 5.15 8.37 0.060 0.47 1.38 3.01 5.38 8.75 0.065 0.49 1.43 3.13 5.60 9.10 0.070 0.50 1.49 3.25 5.81 9.45 0.075 0.52 1.54 3.36 6.02 9.78 0.080 0.54 1.59 3.47 6.21 10.10 0.085 0.56 1.64 3.58 6.40 10.41 0.090 0.57 1.68 3.68 6.59 10.71 0.095 0.59 1.73 3.78 6.77 11.01 0.100 0.60 1.78 3.88 6.95 11.29 Kopak Residence —CG #21335.20 December 14, 2021 Drainage Report Section V, Page 1 Section V — Construction Stormwater Pollution Prevention Section V Summary Narrative Erosion control details are provided consistent with the Edmonds Stormwater Addendum and the DOE Manual. Erosion control plan sheets are provided in full size as a part of the civil drawing set. A Construction SWPPP is not required by the Department of Ecology because the site is under one acre (the land -disturbing activity threshold which requires the completion of their SWPPP document and Construction Stormwater General Permit). The elements for construction pollution prevention are discussed as follows: Element 1: Mark Clearing Limits To protect adjacent properties and to reduce the area of soil exposed to construction, the limits of construction will be clearly marked before land -disturbing activities begin. Clearing limits will be to the extents of necessary land disturbance for the new building and associated parking area and walkways. The BMPs relevant to marking the clearing limits that will be applied for this project include: High Visibility Plastic or Metal Fence (BMP C103) Element 2: Establish Construction Access Construction access or activities occurring on unpaved areas shall be minimized, yet where necessary, access points shall be stabilized to minimize the tracking of sediment onto public roads. The existing entrance to the site may be used as a stabilized construction entrance and expanded to the 15' minimum as necessary: Stabilized Construction Entrance (BMP C105) Element 3: Control Flow Rates The site is small enough that a silt fence used for Element 4 may be used to control flow rates. Element 4: Install Sediment Controls All stormwater runoff from disturbed areas shall pass through an appropriate sediment removal BMP before leaving the construction site or prior to being discharged. Silt fence will be installed along the west, north, and south edges of the site and can be seen on the Temporary Erosion Control Plan, C2.1. Pollution prevention facilities on the erosion control plan must be constructed prior to or in conjunction with all clearing and grading to ensure that the transport of sediment to surface waters and adjacent properties is minimized. The specific BMPs to be used for controlling sediment on this project include: 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section V, Page 2 Silt Fence (BMP C233) Element 5: Stabilize Soils Leave existing ground cover undisturbed to the maximum extent feasible. Exposed and unworked soils shall be stabilized with the application of effective BMPs to prevent erosion throughout the life of the project. Soils must not remain exposed and unworked for more than the time periods set forth below to prevent erosion: • During the dry season (May 1— September 30): 7 days • During the wet season (October 1— April 30): 2 days The specific BMPs for soil stabilization that shall be used on this project include: Temporary and Permanent Seeding (BMP C120) Mulching (BMP C121) Nets and Blankets (BMP C122) Plastic Covering (BMP C123) Sodding (BMP C124) Topsoil ing/Composting (BMP C125) Surface Roughening (BMP C130) Dust Control (BMP C140) Element 6: Protect Slopes Exposed slopes shall be stabilized with BMPs found in Element 5. Element 7: Protect Drain Inlets Drain inlets adjacent to the site and those made operable on -site will be protected from sedimentation. Stormwater shall not enter the conveyance system without first being filtered or treated to remove sediment. Inlet protection devices shall be cleaned or removed and replaced when sediment has filled one-third of the available storage (or as specified by the manufacturer). The specific BMPs to be used for protecting drain inlets are: Storm Drain Inlet Protection (BMP C220) Element 8: Stabilize Channels and Outlets Conveyance channels are not located on or in the immediate vicinity of the site, nor are any proposed. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section V, Page 3 Element 9: Control Pollutants Design, install, implement, and maintain effective pollution prevention measures to minimize the discharge of pollutants. The suggested BMPs are: Concrete Handling (BMP C151) Sawcutting and Surfacing Pollution Prevention (BMP C152) Material Delivery, Storage and Containment (BMP C153) Element 10: Control Dewatering De -watering is not anticipated. Element 11: Maintain BMPs All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to ensure continued performance of their intended function. Element 12: Manage the Project • Phase development projects to the maximum degree practicable and consider seasonal work limits. • Inspection and monitoring— Inspect, maintain, and repair all BMPs as needed to assure continued performance of their intended function. Conduct site inspections and monitoring in accordance with the Construction Stormwater General Permit or local plan approval authority. • Maintain an Updated Construction SWPPP - This SWPPP shall be retained on -site or within reasonable access to the site. - The SWPPP shall be modified whenever there is a change in the design, construction, operation, or maintenance at the construction site that has, or could have, a significant effect on the discharge of pollutants to waters of the state. - The SWPPP shall be modified if, during inspections or investigations conducted by the owner/operator, or the applicable local or state regulatory authority, it is determined that the SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater discharges from the site. The SWPPP shall be modified as necessary to include additional or modified BMPs designed to correct problems identified. Revisions to the SWPPP shall be completed within seven (7) days following the inspection. Element 13: Protect Low Impact Development BMPs N/A — All infiltration and dispersion BMPs were found to be infeasible. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section VI, Page 1 Section VI — Special Reports and/or Studies Section VI Summary: Narrative The following report is included in this section: 1. Stormwater Infiltration Evaluation by Nelson Geotechnical Associated, Inc., dated September 3, 2021. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com NELSON GEOTECHNICAL ASSOCIATES. INC. September 3, 2021 Mr. Kevin Kopak 346 Sunset Avenue North Edmonds, Washington VIA Email: jkopak@eastsidestaple.com Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration 346 Sunset Avenue North Edmonds, Washington NGA File No. 1287921 17311-135" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com Dear Mr. Kopak: This letter documents our explorations and provides our opinions and recommendations for the feasibility of stormwater infiltration for the proposed detached garage addition located at 346 Sunset Avenue North in Edmonds, Washington as shown in the Vicinity Map in Figure 1. The parcel number for the subject property is 27032300100400. INTRODUCTION The site is currently occupied by a single-family residence and a detached shed within the western and northeastern portions of the property, respectively. The ground surface within the site is generally level. We understand the proposed development will include constructing a new detached garage structure within the north -central portion of the property. The existing site layout is shown on the Schematic Site Plan in Figure 2. For use in preparing this letter, we have been provided with a plan set titled "Kopak Residence," dated May 10, 2021 and prepared by Strobl Design, LLC, showing the existing and proposed site layout. We understand that stormwater generated from the proposed development may be directed to onsite infiltration systems, if feasible. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 2 We have been requested to evaluate the infiltration capacity of the site soils within the property. The City of Edmonds utilizes the 2014 Department of Ecology (DOE) Stormwater Management in Western Washington Manual (2014 SWMMWW) to determine the design of infiltration or detention facilities. According to this manual, long-term design infiltration rates for this site are to be determined by performing on -site infiltration testing consisting of the Small Pilot Infiltration Test (PIT). SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions and provide general recommendations for site development. Specifically, our scope of services included: 1. Reviewing available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site with trackhoe excavated test pits. The trackhoe was provided by NGA. 3. Performing grain -size sieve analysis on selected soil samples, as needed. 4. Providing long-term design infiltration rates based on one on -site small-scale Pilot Infiltration Test (PIT) per the 2014 SWMMWW. Location and depth of the test was determined by civil engineer. Water for the test was secured by client. 5. Providing recommendations for infiltration system installation. 6. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The site consists of a rectangular -shaped parcel covering approximately 0.24 acres. The site is occupied by a single-family residence with a detached shed within the western and northeastern portions of the property, respectively. Topography within the site is generally level. Vegetation within the site consists of grass -covered yard areas bordered by shrubs. The site is bordered by Sunset Avenue North to the northwest and by residential properties to all other side. We did not observe surface water within the site during our site reconnaissance on August 12, 2021. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation NGA File No. 1287921 Kopak Detached Garage Addition Infiltration September 3, 2021 Edmonds, Washington Page 3 Subsurface Conditions Geology: The geologic units for this area are shown on Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by Minard, J.P., (USGS, 1983). The site is mapped as Whidbey Formation (Qw) with Vashon glacial till (Qvt) mapped nearby. Whidbey formation is generally described as bedded, compact, commonly oxidized, medium- to coarse -grained sand. Glacial till is described as a non -sorted, compact diamict of silt, sand, and sub -rounded to well-rounded clasts. Our explorations generally encountered undocumented fill soils underlain by gray to gray -brown, silty fine to medium sand with varying amounts of gravel and iron -oxide staining, consistent with the description of glacial till deposits at depth. Explorations: The subsurface conditions within the site were explored on August 12, 2021 with one test pit to an approximate depth of 4.0 feet below the existing ground surface using a mini-trackhoe. We also performed one hand auger to a depth of approximately 4.0 feet below the existing ground surface. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the test pits. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are attached to this letter and are presented in Figure 4. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions, the logs of the explorations should be reviewed. At the surface of Infiltration Pit One and Hand Auger One, we generally encountered approximately 2.0 - feet of dark brown, silty sand with varying amounts of roots, gravel, and organics which we interpreted as topsoil and/or undocumented fill soils. Underlying the topsoil and/or undocumented fill soils in each of the exploration, we generally encountered gray -brown, fine to medium silty sand with varying amounts of iron -oxide staining, gravel, and trace boulders which we interpreted as native glacial till soils. Infiltration Pit One and Hand Auger One were both terminated within the native glacial till soils at a depth of 4.0-feet below the existing ground surface. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 4 Hydrogeologic Conditions We did not encounter ground water seepage in any of our explorations. If groundwater is encountered within the site, we would interpret this to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered soils and undocumented fill. The low permeability soil consists of relatively silty native deposits. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. INFILTRATION TESTING We conducted a Small PIT in accordance with the 2014 Stormwater Management Manual for Western Washington (2014 SWMMWW) within Infiltration Test Pit One located as shown on the attached Site Plan in Figure 2. Infiltration Pit 1 measured 4.0 feet long by 3.0 feet wide by 4.0 feet deep. The subsurface soils generally consisted of silty fine to medium sand with gravel, which we interpreted as native glacial till deposits. The pit was filled with 12-inches of water and this level was maintained for six hours for the pre-soak period. At the end of the pre-soak the water level within the pit had remained at 12-inches with no additional water being added to the pit. As a result, the infiltration testing was concluded. CONCLUSIONS Based on the results of the small -PIT performed within the site, and the relatively silty compact nature of the native glacial till deposits that underlie the site, it is our opinion that the on -site native glacial soils encountered at depth are not conducive for traditional stormwater infiltration systems. Stormwater generated from proposed structures and other hard surfaces within the site should be directed to on - site detention systems and ultimately into an approved point of discharge likely found within the adjacent roadways. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 5 USE OF THIS LETTER NGA has prepared this letter for Mr. Kevin Kopak and their agents, for use in the planning and design of the above referenced project only. Our services included an evaluation of the infiltration capability of the site soils at specific locations and should not be considered as an in-depth geotechnical study of the site or an evaluation of the overall site stability. This letter may be used for bidding and estimating purposes, but our letter, conclusions, and interpretations should not be construed as a warranty of the subsurface conditions. The subsurface conditions between explorations may vary. A contingency for varying conditions should be incorporated into the project plans. We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre -construction meetings if requested. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 6 It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Sarah L. Dunn Staff Geologist Lee S. Bellah, LG Project Geologist 09.03.2021 Khaled M. Shawish, PE Principal SLD:LSB:KMS:dy Four Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale �Q Portofino o�� \ Restaurant & Bar \ Maplewood P,90 Dr _Puget Ot & Gerr MdodrLn Edmonds E T z Elementary School < z e N Mlndev Ln I V4w1a06 Way � n.. Project Z J Edmonds Food Bank Site 0 - theta PI THE BOWL Brackett s OF EDMONDS a Landing Shoreline z r Sanctuary... Pf' 104 Brackett s -• "y SI I A Daley St Landing North F Sprague St >' Hummingbird z Hill Park !nEdmonds Ferry Terminal sell St D Bill St > dsa z z Edmonds ant Main St ds W oayton s Dayton St gMapla Way Port of Edmonds Maple St ' > Maple St w J Yost Pool Q Alder St Alder St a All Beck Ln y Walnut Street CoffeeQ Walnut St Yost Park Edmonds Shelleberger „�„yn, o° Marsh Creek �`��`�' �dO�n waY N SDlucest-_ < r Spruce St Edmonds D CityPark = q hemlock Way N w _ 214th PI SW Edwards Q a n miniums Edmonds, WA Project Number IIELson GEOTECHIIICAL No. Date Revision By cK Z Kopak Residence Detachedlimb1 1287921 Garage Addition Infiltration ASSOCIATES, incR 8/25/21 Original DPN ABR Figure 1 Vicinity Map W135th -NE,e Wenatohee° t. 17311-1351h Ave. NE, A-500 105 Palouse St. Wood-ille, WA 98072 Wenatchee, WA 98801 0 OU i vrvuw.nelsongeolech.com (425)486-1669/Fax. 481-2510 (509)665-7696/Fax : 665-7692 5 K, Schematic Site Plan w _ 4 z fD CONJ C CD Cl)v a Cj- a � (0 3 CD m 3 a� CD ; ? m Existing Residence CD CD v n 0 3 � CD Q 3 M - 6Na a Cn cn O D , ° cn = a m r m —1 cn m - � :0 i►- All ,Z LEGEND v — — Property line INF-1 —�— Number and approximate ° I location of infiltration test pit 3 HA-1 W _ I Number and approximate Z ` I location of hand auger D 0 M X Reference: Site plan based on field measurements, observations, and aerial Mill\company\2021 NGA Project Folders\12879-21 Kopak Residence Detached Garage Addition Infiltration Edmonds\Drafting\SP.dwg map review. Existing Shed 0 30 60 Approximate Scale: 1 inch = 30 feet _40 UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests is based on ASTM D 2488-93. Moist - Damp, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number nELSOn GEOTEnninl_ No. Date Revision By cK Kopak Residence Detached � 1287921 Garage Addition Infiltration ASSOCIATES, inc1 8/25/21 Original DPN ABR Figure 3 Soil Classification Chart ""135thA°e°""e Wa"a`°hee.S"e 17311-1351h Ave. NE, A-500 105 Palouse St. Woodinville, WA 98072 Wenatchee, WA 98801 www.nelsongeoiech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION INFILTRATION TEST PIT ONE 0.0-2.0 SOD UNDERLAIN BY DARK BROWN, ORGANIC -RICH, SILTY SAND WITH ROOTS AND GRAVEL (LOOSE, DRY TO MOIST) (UNDOCUMENTED FILL) 2.0-4.0 SM ORANGE -BROWN TO GRAY -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON -OXIDE STAINING, AND TRACE BOULDERS (MEDIUM DENSE TO DENSE, MOIST) (TILL) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 4.0 FEET ON 08/12/21 HAND AUGER ONE 0.0 - 2.0 SOD UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ORGANICS, AND ROOTS (LOOSE, DRY) (UNDOCUMENTED FILL) 2.0-4.0 SM LIGHT BROWN TO GRAY -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND IRON -OXIDE WEATHERING (MEDIUM DENSE, MOIST) (TILL) SAMPLE WAS COLLECTED AT 4.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER WAS COMPLETED AT 4.0 FEET ON 08/12/21 SLD:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1287921 FIGURE 4 Kopak Residence — CG #21335.20 Drainage Report December 14, 2021 Section VI I, Page 1 Section VII — Other Permits Section VII Summary: Narrative This project will not require permits beyond those required from the City of Edmonds. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com