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Drainage Assessment-4198
DRAINAGE ASSESSMENT September 23, 2021 Revised February 1, 2022 PROJECT Straight Arrow Homes 910 N 31st St Renton SFR 907 N 32nd St, Renton, WA 98056 DEVELOPER/OWNER Straight Arrow Homes LLC C/O Brandon Shimizu 12015 NE 8th St, Ste 5 Bellevue, WA, 98005-3141 ENGINEER/PREPARED BY Andy Epstein Third Day Design, LLC 5918 Waller Road E, Tacoma WA 98443 (253) 256-1812 andy@thirddaydesign.com DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 2 of 16 TABLE OF CONTENTS PROJECT ENGINEERS CERTIFICATE ................................................................................................................ 4 DRAINAGE ASSESSMENT ............................................................................................................................... 4 Project Overview and Summary ............................................................................................................... 4 Figure 1.1 – Developed Basin Area Summary ................................................................................... 5 Figure 1.2 – Developed Basin Map ................................................................................................... 6 Drainage Review Type ....................................................................................................................... 7 Figure 1.3 – Renton Surface Water Design Manual Flow Chart for Determining Type of Drainge Review Required ............................................................................................................................... 9 On-Site BMP Feasibility and Design ........................................................................................................ 10 Targeted Drainge Review ........................................................................................................................ 11 Special Requirement #1: Other Adopted Areas-Specific Requirement .......................................... 11 Special Requirement #2: Flood Hazard Area Delineation ............................................................... 11 Special Requirement #3: Flood Protection Facility ......................................................................... 11 Special Requirement #4: Source Control ........................................................................................ 11 Special Requirement #5: Oil Control ............................................................................................... 11 Special Requirement #6: Aquifer Protection Area .......................................................................... 11 Erosion Control Evaluation ..................................................................................................................... 12 Conveyance Analysis and Design ............................................................................................................ 14 APPENDICIES ............................................................................................................................................... 15 APPENDIX A – Geotechnical Report ............................................................................................................ 16 DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 3 of 16 INTENTIONALLY LEFT BLANK DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 4 of 16 PROJECT ENGINEERS CERTIFICATE I hereby state that this report for the residential project at 910 N 31st St, Renton, Washington has been prepared by me or under my supervision and meets the standard of care and expertise which is usual and customary in this community for professional engineers. DRAINAGE ASSESSMENT Project Overview and Summary The project site is a single-family residential lot with an attached garage on King County Tax 3342101750 and within the City of Renton. The existing single family development will be completely removed and replaced with the redevelopment of this site. Access to the site is currently via N 31st St and will be via the alley is being completed under separate permit associated with the lot to the north. The site is bound by single family residences to the east and west, N 31st St to the south and alley right-of-way to the north followed by single family residences. The site is relatively flat and slopes away from the existing residence in all directions. This development and the associated offsite improvements are within a single threshold discharge area (TDA) that that drains to the city’s stormwater conveyance in N 31st St and Burnett Ave N. The city’s system discharges directly to Lake Washington. Total proposed project impervious area is approximately 3,467 SF, including the required offsite improvements. Refer to Figures 1.1 and 1.2 for the developed areas and basin map. Stormwater will be managed with new permeable paver driveway and walkway. New roof runoff will be managed in part with a rain garden at the front (south) end of the site and the remainder will be managed with a perforated stub out. Utility services are available in N 31st St. DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 5 of 16 Based on the City Renton code, the 2016 Renton Surface Water Design Manual (2016 RSWDM), the City of Renton COR Maps website, King County iMap, and the survey this site is as follows: Site Address 910 N 31st St, Renton, WA 98056 King County Parcel No. 3342101750 Zoning R-8 Aquifer Aquifer Protection Area Zone 2 Steep slope/ Erosion/Landslide Hazard Areas Outside of and at least 200-ft away from all hazard areas Lot Area (SF) 5,098 SF per survey Max Impervious Surface Area per Zoning 65% Onsite Slopes 1% to 10% Subsurface investigation was completed on July 20, 2021, by Pioneer Engineers, Inc., with two test holes each north of the existing residence and documented in a project specific report prepared by Pioneer Engineers, Inc., dated August 16, 2021. The observed soil conditions in the test holes are described below from the surfaces to the termination point: · 3 inches of topsoil. · 21 inches of fill. · 6 feet of light gray, silty, fine SAND, trace coarse gravel, occasional cobble, medium-dense, slight moist. (Vashon Till; Sandy Loam). Becomes dense around 5 feet. Test pits were terminated at 8 feet. No groundwater was encountered during exploration. The geotechnical report is included in Appendix A. Figure 1.1 – Developed Basin Area Summary Developed Basin Area Summary Surface No.Description Hard Surface Area (SF) Area (Acres) Pevious Surface Area (SF) Area (Acres) Areas Addressed with Flow Control BMPs 1 Roof 2,445 0.05613 - - 1,264 2A Driveway - PPS 359 0.00824 - - 359 2B Driveway - Impervious 47 0.00108 - 4 Steps 6 0.00014 3 Walk (flat)64 0.00147 - - 64 Onsite Impervious Total 2,921 0.06706 5 Sidwealk (flat)250 0.00574 - - - 6 TEMP Walk (flat) 106 0.00243 - - - 7 Street curb and gutter/pavement 190 0.00436 - - Onsite Pervious - - 2,177 0.04998 Planter Strip - - 397 0.00911 TOTAL New/Replaced 3,467 0.07959 2,574 0.05909 Target Impervious Surface Area 3,467 0.07959 - - Percent of Onsite Imprevious Surface Addressed with FCBMPs 48.66% DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 6 of 16 Figure 1.2 – Developed Basin Map NEW SFR LEVEL 1 FFE 115.5' 114.48114.48 CB - TYPE 1 'SADDLE' SOLID LID RIM ~113.6 EX IE @ CL ~111.13 (12" PVC E & W) IE-IN 111.63 (6" N) SURFACE P1-ROOF: 2,445 SF SURFACE P2A-DRIVEWAY: 359 SF (PERMEABLE PAVERS) SURFACE P4-WALK: 64 SF SURFACE P5-SIDEWALK: 250 SF SURFACE P6-TEMP WALK: 106 SF SURFACE P7-STREET C&G/PAVEMENT: 190 SF UTILITY RESTORATION-EXEMPT SURFACE P2B-DRIVEWAY: 47 SF20' BLDG SETBACK 15' BLDG. SETBACK PERFORATED STUBOUT CONNECTION PORTION OF ROOF TO RAIN GARDEN : 1264 SF RAIN GARDEN MIN. BOT. AREA @ SURFACE 28.75 SF (2.5-FT x 11.5-FT) FACILITY BOT. AREA ELEV. = 112.4 IE IN 112.9 4" ROOF DRAIN OVERFLOW RISER ELEV. 112.9 (MIN. 6" PONDING) MIN. TOP ELEV. 113.4 (MIN. 6" FREEBOARD) WATER SURF. AREA AT OVERFLOW ELEV. = 68.5 SF (DESIGN); 67.2 SF (MIN.) 10 LF PERFORATED STUBOUT SUMP PUMP FOR FD SOLID LID RIM ~113.8 IE IN ~111.2 4" FD IE OUT 112.6 2" 5'5.2'5.4'8.5%6.5%112.95 113.28 112.38 112.72 114.13 114.43 113.86 6" Ø SDR 35 PVC ROOF DRAIN LINE @ 1.00% MIN., UON 4" Ø SDR 35 PVC ROOF DRAIN LINE @ 2.00% MIN., UON SURFACE P3-STEPS: 6 SF 5'6.7'4" Ø SDR 35 PVC ROOF DRAIN LINE @ 2.00% PLOTTED: 2/1/2022 10:01:48 PM FILE: 31ST_E21012C-DEV BASN.DWG LAST SAVED BY: ANDYEDATE DESCRIPTION ©2021 THIRD DAY DESIGN, LLC. All rights reserved. No part of this document may bereproduced in any form without permission from Third Day Design, LLC. # THIRD DAY DESIGN 5918 WALLER RD E TACOMA, WA 98443 (253) 256-1812 CIVIL ENGINEERING 44221 DESIGNED CHECKED DRAWN CHECKED PERMITKnow what'sbelow.Callbefore you dig. 21012 2/1/2022 DEVELOPED BASIN MAP FIG 1.2 STRAIGHT ARROW HOMES N 31ST ST SFR 910 N 31ST ST RENTON, WA 98056 PERMIT PERMIT NUMBER B21005318 S32 - T24 - R5 SINGLE FAMILY RESIDENTIAL PROJECT AND ALLEY IMPROVEMENTS TO THE NORTH UNDER SEPARATE PERMIT SCALE IN FEET 10 5 0 10 20 N 31ST STBURNETT AVE NALLEY 2/1/2022 ABE ABE ABE ABE DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 7 of 16 Drainage Review Type Project is subject to Simplified Drainge Review and Targeted Drainage Review per Figure 1.3 below, since: · It is a single-family residential project; AND it is under the necessary thresholds: o It is well below the pollution generating surface threshold of 5,000 SF. o It is under the 5,000 SF threshold for new plus replaced impervious surface. · It is subject to Category 1 Targeted Drainage Review since it is mapped: o Within Aquifer Protection Area Zone 2 Therefore, per the RSWDM the project must comply with: · Simplified Drainage requirements detailed in Appendix C (which is presumed to comply with all core and special requirements in Sections 1.2 and 1.3); and · Targeted drainage review requirements as specified in Section 1.1.2.2. Definitions from RSWDM Section 1.1 Aquifer Protection Area (APA) means the portion of an aquifer within the zone of capture and recharge area for a well or well field owned or operated by the City of Renton delineated in the Aquifer Protection map found in the City of Renton Map Gallery (<http://rentonwa.gov/government/default.aspx?id=29885>) or viewed via COR Maps (<http://rp.rentonwa.gov/SilverlightPublic/Viewer.html?Viewer=COR-Maps>). Impervious surface means a non-vegetated surface area that either prevents or retards the entry of water into the soil mantle as under natural conditions before development; or that causes water to run off the surface in greater quantities or at an increased rate of flow compared to the flow present under natural conditions prior to development (see also new impervious surface). Common impervious surfaces include, but are not limited to, roof, walkways, patios, driveways, parking lots, or storage areas, areas that are paved, graveled or made of packed or oiled earthen materials or other surfaces that similarly impede the natural infiltration of surface water or stormwater. For the purposes of applying the impervious surface thresholds and exemptions contained in this manual, permeable pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater runoff are considered impervious surface while an open uncovered flow control or water quality facility is not. However, for the purposes of computing runoff, uncovered flow control or water quality facilities shall be modeled as impervious surfaces as specified in Chapter 3 New impervious surface means the addition of a man-made, modified, or compacted surface like roofs, pavement, gravel, or dirt; or the addition of a more compacted surface, such as resurfacing by upgrading from dirt to gravel, asphalt, or concrete; upgrading from gravel to asphalt, or concrete; or upgrading from a bituminous surface treatment (“chip seal”) to asphalt or concrete. Permeable pavement and vegetated roofs are considered new impervious surface for purposes of determining whether the thresholds for application of minimum requirements are exceeded, as are lawns, landscaping, sports fields, golf courses, and other areas that have modified runoff characteristics resulting from the addition of underdrains designed to collect stormwater runoff. Open, uncovered retention/detention facilities shall not be considered impervious surfaces for purposes of determining whether the thresholds for application of minimum requirements are exceeded. Open, uncovered retention/detention facilities shall be considered impervious surfaces for purposes of runoff modeling. New pervious surface means the conversion of a native vegetated surface or other native surface to a DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 8 of 16 nonnative pervious surface (e.g., conversion of forest or meadow to pasture land, grass land, cultivated land, lawn, landscaping, bare soil, etc.), or any alteration of existing nonnative pervious surface that significantly increases surface and storm water runoff (e.g., conversion of pasture land, grass land, or cultivated land to lawn, landscaping, or bare soil; or alteration of soil characteristics). Project means any proposed action to alter or develop a site. The proposed action of a permit application or an approval, which requires drainage review. Project site means that portion of a site and any offsite areas subject to proposed project activities, alterations, and improvements including those required by this manual. Replaced impervious surface means any existing impervious surface on the project site that is proposed to be removed and re-established as impervious surface, excluding impervious surface removed for the sole purpose of installing utilities or performing maintenance on underground infrastructure. For structures, removed means the removal of buildings down to the foundation. For other impervious surfaces, removed means the removal down to base course or bare soil. For purposes of this definition, base course is the layer of crushed rock that typically underlies an asphalt or concrete pavement. It does not include the removal of pavement material through grinding or other surface modification unless the entire layer of PCC or AC is removed. Replaced impervious surface also includes impervious surface that is moved from one location to another on the project site where the following two conditions are met: (A) runoff characteristics and volumes remain the same or are improved in the area where the existing impervious surface is removed , and (B) impervious surface at the new location is either designated as non-pollution generating or the pollution generating characteristics remain unchanged compared to that of the original location. Site means a single parcel; or, two or more contiguous parcels that are under common ownership or documented legal control; or a portion of a single parcel under documented legal control separate from the remaining parcel, used as a single parcel for a proposed project for purposes of applying for authority from the City to carry out a proposed project. For projects located primarily within dedicated rights-of-way, the length of the project site and the right-of-way boundaries define the site. Target surface area means a developed surface from which runoff impacts are required to be mitigated by a particular set of drainage requirements. Target impervious surface means that portion of a site’s new and/or replaced impervious surface from which runoff impacts are required to be mitigated by a particular set of drainage requirements (flow control facility, water quality facility, and/or on-site BMP). DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 9 of 16 Figure 1.3 – Renton Surface Water Design Manual Flow Chart for Determining Type of Drainge Review Required DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 10 of 16 On-Site BMP Feasibility and Design This project is subject to RSWDM Section C1.3.1 “Small Lot BMP Requirements” since the project is on a site/lot smaller than 22,000 SF. Below is a site-specific evaluation of the required BMPs for the onsite development: 1. Requirement 1 – Full dispersion per C.2.1: infeasible since the 100 feet of native vegetated flow path is not available onsite. 2. Requirement 2 – Full infiltration of roof runoff per C.2.2: infeasible per the geotechnical report and since the underlying soils are not classified as coarse sands or cobbles or medium sands. 3. Requirement 3 – Full infiltration per C.2.2 limited infiltration per C.2.3, rain gardens per C2.12, bioretention per C.2.6, OR Permeable Pavement per C.2.7: a. Full Infiltration is infeasible per the geotechnical report and since the underlying soils are not classified as coarse sands or cobbles or medium sands. b. Limited Infiltration is infeasible. The site soils are classified as sandy loam, however the required horizontal clearance from the building of 15 feet cannot be provided as required when the crawl space residence is below the overflow (which is the case for the building). c. Rain Gardens, Bioretention or Permeable Pavement were evaluated for the remaining surfaces: i. Permeable pavement is feasible for the driveway and walkway surfaces since this site does not meet any of the infeasibility criteria based on the subsurface investigation completed. The driveway surface slopes will require check dams since the slope is greater than 5%. For 6” of rock base, 5” tall checks are spaced at min. 7-ft on center (this is based on interpolating between 8-ft o.c. for 5% slopes and 4-ft o.c. for 10% slopes). No impervious run is proposed onto either of these surfaces. ii. Rain Gardens are feasible for a portion of the roof surface. The following horizontal setback requirements may be met at the south end of the site adjacent to the N 31st St right-of-way. Setback requirements are 5-ft from property lines and 5-ft from the buildings without basements to the top of rain garden. Rain gardens must also have a minimum bottom of cell width of 1.5-ft and must be sized with a horizontal projected surface below the overflow with at least 5% of the area draining to it. It is recommended that this area be increased to be an additional 2% of the lawn/landscape area drains to it. One rain garden is proposed onsite with the following contributing areas: 1264 SF of roof area and approximately 200 SF of lawn/landscape area. The required water surface below the overflow is 67.2 SF (1264 SF x 5% + ~200 SF x 2%); 68.5 SF is provided. 4. Requirement 4 – Basic Dispersion per C2.4 Infeasible since the onsite flow paths would not meet the minimum 25’ length. 5. Requirement 5 – Minimum BMP Implementation: BMPs must be implemented, at a minimum, for an impervious area equal to at least 10% of the site/lot for the site/lot sizes up to 11,000 SF. Since this project is located within critical aquifer recharge area, these impervious area amounts must be doubled (unless within 200 ft of steep slope hazard area, landslide hazard area or erosion hazard area – which this site is not within 200 ft of). This requirement is met since over 50% of the onsite impervious area is mitigated as shown in Figure 1.1. 6. Requirement 6 – Protect soil moisture capacity of new pervious surfaces per C2.13 DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 11 of 16 Maintaining topsoil and/or replacing soils is required for disturbed and new pervious areas. Areas to be protected are identified on the CSWPPP and TESC Site Map. 7. Requirement 7 – Perforated Downspouts per C2.11 Any proposed connection of roof downspouts to the local drainage system must be via a perforated pipe connection as detailed in Appendix C, Section C.2.11. The perforated downspout stub is sized for the remaining roof area that is not mitigated by other BMPs. Required perforated downspout length = 1,181 SF x (10-FT / 5,000 SF) = 2.4 linear feet. 3 linear feet of perforated downspout is provided. Targeted Drainge Review This project is subject to RSWDM Section C1.1.2.2 “Targeted Drainage Review” (TDR), Category #1 since it is located within an Aquifer Protection Area. Below is a site-specific evaluation of the special requirements for TDR Category 1: Special Requirement #1: Other Adopted Areas-Specific Requirement Special Requirement #1 is not applicable to the project per RSWDM Section 1.3.1, as there are no other adopted area-specific requirements. Special Requirement #2: Flood Hazard Area Delineation Special Requirement #2 is not applicable to the project per RSWDM Section 1.3.2, as the site is not located in or adjacent to the 100-year flood plain. Special Requirement #3: Flood Protection Facility Special Requirement #3 is not applicable to the project per RSWDM Section 1.3.3. The project site does not rely on an existing flood protection facility (such as a levee or revetment) for protection against hazards, nor does the project site modify or construct a new flood protection facility. Special Requirement #4: Source Control Special Requirement #4 is not applicable to the project per RSWDM Section 1.3.4 since this is a single- family residential project. Special Requirement #5: Oil Control Special Requirement #5 is not applicable to the project per RSWDM Section 1.3.5, as this project site does not have characteristics of a high-use site, nor is it an existing high-use site. Per the RSWDM, a high-use site is defined as a commercial or industrial site that (1) has an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet or gross building area; (2) is subject to petroleum storage or transfer in excess of 1,500 gallons per year, not including delivered heating oil; or (3) is subject to use, storage, or maintenance of a fleet of 25 or more diesel vehicles that are over 10 tons net weight (trucks, buses, trains, heavy equipment, etc.). For this definition, commercial and industrial site means that portion of a site’s developed area associated with an individual commercial or industrial business. Special Requirement #6: Aquifer Protection Area Special Requirement #6 is applicable to the project per RSWDM Section 1.3.6 since the site is located within Wellhead Protection Area – Zone 2. Per RSWDM liners are required for open flow control facilities, such as ponds, as well as conveyance ditches that will convey pollution generating surfaces. No such facilities are proposed for this development; therefore, the liner requirement is not applicable. DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 12 of 16 Erosion Control Evaluation A Construction Stormwater Control Plan following Best Management Practice’s (BMP’s) will be implemented during construction. Following is a summary of some of the BMPs that will be used. The CSWPP Supervisor is the responsible person who will oversee implementation and maintenance. Refer to the plans for proposed construction sequence as well as the CSWPPP that has been prepared for this project. ESC Measures The following ESC measures are required to minimize erosion and sediment transport offsite: 1. Limits of Disturbance will be marked by fencing or other means on the ground; this will be completed prior to land disturbing activity to protect vegetated areas which do not need to be disturbed with the proposed improvements. 2. Straw, mulch or plastic will be used to cover the site or to provide immediate temporary/short- term protection from erosion. 3. Perimeter Protection: A row of siltation fence is on the downhill sides of the project area; this BMP was selected to provide perimeter protection since site grades will not concentrate flows and site slopes are shallow (less than 10%, which allows for a maximum flow path of up to 250 feet). Flow path lengths max out at about 100 feet. 4. Traffic Area Stabilization: The existing driveway may be used a as stabilized construction exit in lieu of constructing a construction exist. A stabilized exit is required to prevent the tracking sediment offsite. 5. Catch Basin Inserts, which are manufactured device made of filter fabric and nests inside a catch basin, will be placed on catch basin until the site is fully stabilized. This BMP was selected to prevent coarse sediment from entering the storm drainage system and because the contributing basin area to the catch basin are small (significantly less than one acre). 6. Surface Water Collection: Not anticipated during construction. If surface water is collected it should be dispersed across established vegetation. 7. Dewatering: Accumulated water in foundation areas, excavations, and utility trenches shall be removed and disposed of in a manner that does not pollute surface waters or cause downstream erosion or flooding. See "Dewatering Control," Section D.2.1.7 (p. D-66), for detailed specifications. 8. Dust Control: Not anticipated. 9. Flow Control: Not applicable since this is a small project site. 10. Control Pollutants: Refer to SWPPS Measures below. 11. Maintain BMPs: BMPs will be maintained by the CSWPP Supervisor or designee per the Renton Surface Water Design manual or DOE BMPs. All BMPs implemented on site must be maintained and repaired until after the site is completely stabilized in order to protect the function of the BMP in erosion control practice. 12. Manage Project: o Site demolition and grading shall be performed after the erosion and sediment control measures have been constructed. o Trenches shall be opened only immediately prior to installation of the stormwater conveyance pipes and the trenches should be backfilled immediately after any required testing or inspections of the installed pipes. Trenching spoils must be treated as other DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 13 of 16 disturbed earthwork and measures should be taken to cover or otherwise stabilize the material, as required. o The Construction SWPPP plan shall be retained on-site or within reasonable access to the site at all times. The CSWPPP plan shall be modified to include any additional or modified BMPs that are deemed necessary to manage erosion and sediment on the site. Revisions to the CSWPPP must be completed within seven days. o Report spillage or discharge of pollutants within 24-hours to the City of Renton. 13. Protect Low Impact Development BMPs: This project includes the following low impact development features and the following protection requirements apply: · Permeable Pavers: the installation of permeable pavers must be completed at the end of the project to protect the system from sediment. Furthermore, underlying soils in these areas must be protected from over compaction. Flags or fencing will be used to protect these areas. · Rain Garden: prior to construction the rain garden area must be identified and protected using flagging or high visibility fencing to prevent compaction of underlying soils by vehicle traffic. SWPPS Measures The following SWPPS measures are required to prevent, reduce or eliminated the discharge of pollutants to onsite or adjacent stormwater systems from construction related activities. 1. Follow effective pollutant handling and disposal procedures. All pollutants, including waste materials and demolition debris that occur on-site during construction shall be handled and disposed of in a manner that does not cause contamination of stormwater. This is necessary to protect site soils and Lake Washington (downstream) and to comply with regulations. 2. Provide cover and containment for materials, fuel and other pollutants. This measure is necessary to protect onsite soils and Lake Washington from petroleum products, asphalt and concrete compounds, hazardous chemicals, etc. A centralized area for storage and maintenance and refueling of construction equipment must be provided onsite and away from Lake Washington. This is anticipated to be provided onsite near the site entrance (away from Lake Washington) at the beginning of construction and may be within the house after the roof is installed. Refer to RSWDM D.2.2.4 for more specific requirements. 3. Manage the project site to maximize pollutant control and minimize pollutant sources. Control of pollutants other than sediments is the responsibility of the CSWPP Supervisor. This oversight of subcontractors and laborer is important to keep the project on track with SWPPS and ESC measures. A CSWPP Supervisor is identified on the plan and will be the primary contact responsible for SWPPS and ESC issues and reporting, coordination with subcontractors and implementation of the CSWPP Plan. Refer to RCSWDM D.2.2.11 for more specific requirements. 4. Protect from spills and drips of petroleum products and other pollutants. This is important since the project drains to a system that discharges to Lake Washington. 5. Avoid overapplication or untimely application of chemicals and fertilizers. This is important since the project drains to a system that discharges to Lake Washington. 6. Prevent or treat contamination of stormwater runoff by pH modifying sources. o Concrete Handling will be employed since this site includes concrete foundation, driveway and walks. Below is a list of pertinent the Design and Installation Specifications from RSWDM: 1. Assure that washout of concrete trucks, chutes, pumps, and internals is performed at an approved off-site location or in designated concrete washout areas. Do not wash DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 14 of 16 out concrete trucks onto the ground, or into storm drains, open ditches, streets, or streams. Refer to BMP D.2.2.2 for information on concrete washout areas. 2. Return unused concrete remaining in the truck and pump to the originating batch plant for recycling. Do not dump excess concrete on site, except in designated concrete washout areas. 3. Wash off hand tools including, but not limited to, screeds, shovels, rakes, floats, and trowels into formed areas only. 4. Wash equipment difficult to move, such as concrete pavers in areas that do not directly drain to natural or constructed stormwater conveyances. 5. Do not allow washdown from areas, such as concrete aggregate driveways, to drain directly to natural or constructed stormwater conveyances. 6. Contain wash water and leftover product in a lined container when no formed areas are available. Dispose of contained concrete in a manner that does not violate ground water or surface water quality standards. 7. Concrete Washout Area will be employed offsite. The developer is planning for washout of concrete trucks to be completed at an approved off-site location, since this is a small lot. Refer to RSDWM D.2.2.2 for specifications, details and maintenance standards. Refer to RSDWM D.2.2. for additional specifications, details and maintenance standards. Conveyance Analysis and Design The proposed drainage conveyance systems will be adequate to provide stormwater conveyance from the proposed development. Detailed analysis may will be provided if required by the City of Renton. DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 15 of 16 APPENDICIES DRAINAGE ASSESMENT FOR 910 N 31ST ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 16 of 16 APPENDIX A – Geotechnical Report PIONEER ENGINEERS, INC. Geotechnical Engineering Earth Science Water Resources ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ P. O. Box 33628Seattle, WA 98133 Phone: (206) 427-9118 Fax: (206) 306-2982 josephwu.pei@gmail.com August 16, 2021 Mr. Brandon Shimizu Straight Arrow Homes, LLC 12015 NE 8th Street Bellevue, WA 98005 Subject: Geotechnical Evaluation Proposed Single-Family Residence 910 North 31st Street Renton, Washington Dear Mr. Shimizu: Per your request, Pioneer Engineers, Inc. (“PEI”) has explored subsurface conditions with two test pits (“TP-1 and TP-2”) for evaluation of on-site stormwater infiltration at the subject site in Renton, Washington. The site is located approximately 80 feet east of the intersection of North 31st Street and Burnett Avenue North, where its general location shown in Figure 1. Based on the property information and site plan excerpted from King County iMap and communication with you, PEI understands that a single-family residence will be constructed at the approximately central portion of the site, consisting of a two- or three-story wood framed structure with an attached garage, accessible from North 31st Street or from Burnett Avenue North by an alley. Minor grading is anticipated to establish the driveway and foundation. In accordance with Bellevue’s database of geographic information system (“GIS”), the majority of the site is located in an area requiring evaluation of on-site stormwater infiltration. SCOPE OF WORK The purpose of this geotechnical evaluation report is to evaluate on-site stormwater infiltration by characterizing subsurface conditions with test pit explorations. To achieve the purpose, the scope of work specifically comprises the following items: August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 2 PIONEER ENGINEERS, INC. 1. Explore subsurface conditions with two test pits to a maximum depth of 8 feet. The underlying soils encountered are visually classified; 2. Collect soil samples at selected depths and seal them in sampling bags for further examination; 3. Perform a site reconnaissance to observe and document existing surface features; 4. Review geologic and surficial soil conditions by referencing public geologic or soil information; 5. Prepare a written report to present the findings from subsurface exploration and recommendations for grading and site preparation, structural fill and compaction, and evaluation of on-site stormwater infiltration. SITE CONDITIONS Surface Conditions The site is a developed property rectangular in shape, bounded to the south by North 31st Street and adjoins private properties on the other three sides. It is measured approximately 50 feet along the street and about 100 feet in depth, encompassing an area of 0.11 acre. A vertical relief less than 5 feet stretches horizontally across the site. In a broad view of topography, the site is situated in the mid-hill of a regional slope descending northwesterly to Lake Washington. Locally within the site, the ground surface slopes down mildly toward west. A one-story residence is currently located in the central portion of the site, attached with a patio. To its northwest is a shed. The remaining open space is covered mostly with grass lawn and scattered trees. Geologic Mapping A geologic map, Geologic Map of Surficial Deposits in the Seattle 30’ x 60’ Quadrangle, Washington, prepared by James C. Yount, James P. Minard and Glenn R. Dembroff in 1993 was referenced for the geologic and soil conditions at the site. According to this map, the site is mapped in a soil unit of Vashon recessional outwash (“Qvr”) deposits, closely underlain by Vashon till (“Qvt”) deposits. August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 3 PIONEER ENGINEERS, INC. Qvr was deposited mostly from the meltwater flowing from the retreating glaciers during the last glaciation period, consisting of poorly to moderately sorted and stratified gravel and sand with local inclusions of silt and clay. Qvt was deposited directly by glacial ice during the most recent glaciation as it advanced over an eroded, irregular surface of older formations and sediments. It is a very dense mixture of unsorted clay, silt, sand, gravel, and scattered cobbles and boulders, locally referred to as "hard pan.” Subsurface Exploration On July 20, 2021, subsurface conditions were explored with two test pits to a maximum depth of 8 feet. Both pits were excavated using a track-mounted excavator (Caterpillar 303.5E2 CR) provided by the owner. Locations of pits were determined by tape measuring with reference to existing surface features shown in the site plan and their approximate locations shown in Figure 2. Subsurface exploration was continuously monitored by an engineer from our firm who documented the underlying soil and groundwater conditions encountered, maintained a log of each pit, obtained representative soil samples, and observed pertinent site features. The final test pit logs represent our interpretations of subsurface conditions explored. The stratification lines in the logs indicate approximate boundaries between soil types. Actual transitions may be more gradual in the natural geologic setting. The soils encountered are visually classified in general accordance with Unified Soil Classification System (“USCS”) of which the chart is shown in Figure 3, with reference to USDA Textural Triangle Classification System. Subsurface Soils Both test pits encountered considerably consistent soil conditions: three inches of topsoil, underlain by a thin layer of loose fill of brown, silty, fine sand with a trace of coarse gravel and clearing debris. Underlying this fill was relatively weathered Vashon till of light gray, silty, fine sand with a trace of coarse gravel, occasional cobble. Denseness appeared to increase with depth in these till deposits. More detailed soil information is presented in Figure 4. August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 4 PIONEER ENGINEERS, INC. Groundwater Conditions No groundwater was encountered in the test pits. In general, groundwater levels and amount fluctuate with seasons, depending on the amount of precipitation and surface runoff, denseness of groundcover, purpose of land use, and other factors. CONCLUSIONS AND RECOMMENDATIONS Based on the soil and groundwater conditions encountered in subsurface exploration, it is PEI’s opinion that, from a geotechnical engineering viewpoint, the site may be able to bolster on-site stormwater infiltration in support of the development if recommendations in this report are closely followed. It is advisable that grading work be started and completed in the official dry season in order to minimize the impacts from precipitation, as stated in Grading and Site Preparation. The findings from subsurface exploration and the results of our prior small pilot infiltration tests (“PITs”) in the region provide consistent design information for on-site stormwater infiltration. The stormwater facility such as an infiltration trench should be able to manage the collective storm runoff if space constraints are minimal or none. GRADING AND SITE PREPARATION Site preparation includes clearing and removal of ground features, and implementations of temporary erosion and sediment control (“TESC”) measures. Clearing of ground includes removal of existing structures and stripping and grubbing of surface vegetation within the clearing limits. Occasional overexcavation may be required when local weak soil pockets encountered. Overexcavation should be backfilled with structural fill and compacted to a stable condition. On-site topsoil is unsuitable for use in any structural areas. This soil should be disposed of at approved locations or used solely for landscaping purposes. Prior to starting construction activities, a silt fence should be installed along the lower boundary of the site. This fence may be implemented with a highly visible grid fence to delineate the clearing limits. The entrance, parking, and loading areas should be paved with a minimum 12- August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 5 PIONEER ENGINEERS, INC. inch-thick layer of quarry spalls, underlain by non-woven geotextile to prevent sediments from being tracked onto the street. The silt fence and spall pad serve as TESC measures during construction. They should remain in place until full establishment of permanent ESC measures. The following strategies and methods should be implemented in the grading phase to enhance TESC measures and site stability during construction: The bare and disturbed ground outside the clearing limits should be protected with a layer of straw mulch (a minimum thickness of 2 inches; about 2 bales per 1,000 square feet of land) during any period of precipitation, in order to minimize soil erosion by storm runoff. Straw needs to be air-dried and free of any undesirable weed or coarse material. Cut/fill slopes and stockpiles of soils should be covered with durable plastic sheeting weighed down by securely-anchored sand bags if they are to remain unworked for more than 12 hours; other disturbed areas should be covered with straw mulch as addressed above if they are to remain unworked for more than 2 days. TESC measures in place should have regular inspection weekly and more frequent inspection immediately before, during and after significant precipitation events. STRUCTURAL FILL AND COMPACTION Structural fill is the material placed under footings, on-grade slabs and pavements where it withstands loading or for utility trench backfilling. Structural fill should be free of organic, construction debris and other deleterious substances, consisting of clean soils with individual particles not greater than 3 inches in size. On-site till deposits containing a significant quantity of fines (silt and clay particles) are difficult to be compacted to meet the criteria in wet weather. Free-draining granular materials such as 2-inch-minus washed rock containing no more than 5 percent of fines can be used in structural areas. Recycled concrete may be used as structural fill provided that the requirements of 9-03.12(2) of the latest Washington State Department of Transportation (“WSDOT”) standard specifications are met. August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 6 PIONEER ENGINEERS, INC. With the exception of washed rock (drain fill) in the infiltration trench, structural fill should be placed per loose lift not more than 12 inches in thickness, and compacted to meet the required percentage of maximum dry density determined by ASTM D1557 (Modified Proctor Method) as summarized in the following table: Applicable Area Maximum Dry Soil Density Storm Trench Backfill 95% for upper 2 feet, 90% below Controlled Density Fill (“CDF”) may be used as an alternative for structural fill. CDF (a flowable, self-compacting, rigid-setting and low-density material) is generally used in overexcavation in a foundation or utility trench. Wherever applicable, neither compaction efforts are required to densify this fill, nor density tests needed to ensure the adequacy of compaction efforts. This material’s flowability enables it to access difficult spots and displace standing water in the trench. CDF has a typical slump of 10 inches and a 30-day compressive strength of 200 pounds per square inch (“psi”) or less. Low compressive strength allows CDF for convenient excavation in case of any design alteration during construction. STORMWATER INFILTRATION In accordance with City of Renton Surface Water Design Manual (“CORSWDM”) issued December 12, 2016, LID is “a stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation and use of on-site natural features, site planning and distributed stormwater management practices that are integrated into a project design.” In recent years, this strategy has been applied primarily to prevent potential pollutants from entering natural water bodies and to reduce the amount of stormwater into a public storm drainage system. To achieve the objectives, LID measures consisting mainly of drywells, infiltration trenches, green roofs, bioretention cells, stormwater harvesting devices and pervious pavements have been widely used to enhance safety of water environment, street landscape and urban amenities. August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 7 PIONEER ENGINEERS, INC. These measures have higher application priority than conventional (structural) stormwater management practices such as detention vaults or pipes. They are taken into consideration prior to application of conventional practices. In the CORSWDM, the application priority of such flow control BMPs and LID measures is full dispersion, full infiltration, limited infiltration, and basic dispersion in sequence. PIT and Design Infiltration Rate In general, the PIT was conducted at a specific depth with a bottom area of 12 square feet in a test pit. A plastic splash plate or washed rock was placed at a corner of the bottom to disperse water and/or minimize scour. Tap water was carefully dispersed to soak soils in the infiltration zone. A hydraulic head of 12 inches was maintained during this period of time. After soaking, the pit was filled up to have a hydraulic head of 12 inches and the drop of water levels measured over elapsed time to obtain measured infiltration rates. Drops of water levels were taken from the readings at a scaled wood stake accurate to half an inch placed in the middle of the pit. This process was repeated to obtain the final field infiltration rate after a steady-state rate was reached. Measured water levels versus elapsed time were logged for the measured infiltration rate. The measured infiltration rate is thus attained from the test, generally in a range of 0.30 to 0.50 in/hr. This rate is believed to reasonably represent the infiltration rate of Vashon till deposits measured on site. In accordance with the aforementioned design manual, correction should be made to account for factors (types of field infiltration tests, Ftesting; the depth from the infiltration facility bottom to the groundwater table, Fgeometry; flow channels plugged by fine particles in soil mass, Fplugging) that may affect long-term performance of on-site stormwater infiltration and then attain the design infiltration rate. Similarly, a design infiltration rate varying from 0.11 to 0.20 in/hr was attained from the regional development projects. An average value of 0.16 in/hr, therefore, is appropriate for use as the design infiltration rate. Wherever applicable, an infiltration trench or multiple can be used for on-site stormwater management. August 16, 2021 Proposed Single-Family Residence PEI Project No. G21A23 Page 8 PIONEER ENGINEERS, INC. Infiltration Trench The facility consists of a level perforated PVC pipe with a (cleanout) stand pipe of same size and material at its end. The level pipe should be surrounded by 0.75- to 1.5-inch washed rock wrapped with non-woven geotextile, where the bottom area left open to infiltration subgrade soils. Such subgrade should be scarified or grooved prior to placement of washed rock. The setback distance may be reduced to 5 feet from structures provided that no basement or crawl space will be constructed. Trenches should have a minimum clearance of 10 feet when multiple trenches are applied. LIMITATIONS This report has been prepared for the specific application to this project for the exclusive use of Mr. Brandon Shimizu of Straight Arrow Homes, LLC and his authorized personnel. The conclusions and recommendations in this report, however, should not be construed as a warranty of the subsurface conditions. PEI’s recommendations are based on the subsurface conditions encountered in the test pits, in-situ tests and our experience and engineering judgment. The recommendations are professional opinions derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in local areas. Respectfully submitted, PIONEER ENGINEERS, INC. Joseph Wu, P.E. Engineering Consultant Four figures included. SCALE:1"=300' PROPERTY FROM STANDARD PENETRATION TEST SILTY OR CLAYEY SOILS Relative Density Friction Angle %φ 0 - 15 15 - 35 26 - 30 35 - 65 28 - 35 65 - 85 35 - 42 USCS CHART PROPOSED SINGLE-FAMILY RESIDENCE 910 NORTH 31ST STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G21A23 DATE 8/16/21 FIGURE 3 Hard > 50 85 - 100 38 - 46 Very Dense > 4.00 15 - 30 > 30 0.50 - 1.00 1.00 - 2.00 2.00 - 4.00 8 - 15 4 - 8 Very Stiff Medium Stiff Stiff Very Soft Description Soft2 - 4 < 0.25 0.25 - 0.50 Unconfined Strength qu, tsf Loose Medium Dense Dense < 2 Blow Count N PIONEER ENGINEERS, INC. SANDY SOILS N Blow Count 0 - 4 4 - 10 10 - 30 Description 30 - 50 Very Loose