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HomeMy WebLinkAboutRS_905 32nd and Alley Drainage Assessment_20220330_v3DRAINAGE ASSESSMENT September 20, 2021 Revised February 1, 2022 Revised March 18, 2022 PROJECT 905 N 32nd St Renton SFR 905 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 N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 2 of 27 TABLE OF CONTENTS PROJECT ENGINEERS CERTIFICATE ................................................................................................................ 4 DRAINAGE ASSESSMENT ............................................................................................................................... 4 Section 1 – 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 Section 2 – On-Site BMP Feasibility and Design ..................................................................................... 10 Section 3 – Targeted Drainge Review ..................................................................................................... 11 TDR Category 1 Requirements ............................................................................................................ 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 TDR Category 2 Requirements ............................................................................................................ 12 Core Requirement #1: Discharge at the Natural Location .............................................................. 12 Core Requirement #2: Offsite Analysis ........................................................................................... 12 Core Requirement #4: Conveyance System .................................................................................... 12 Core Requirement #6: Maintenance and Operations ..................................................................... 12 Core Requirement #7: Financial Guarantees and Liability .............................................................. 12 Section 4 – Offsite Analysis ..................................................................................................................... 12 Figure 4.1 – Downstream Map........................................................................................................ 13 Figure 4.2 – City of Renton Critical Areas Map ............................................................................... 15 Section 5 – Conveyance Analysis and Design ......................................................................................... 20 Section 6 – Erosion Control Evaluation ................................................................................................... 22 APPENDICIES ............................................................................................................................................... 25 APPENDIX A – Geotechnical Report and Supplemental Letter ................................................................... 26 APPENDIX B – Operation and Maintenance Manual .................................................................................. 27 DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 3 of 27 INTENTIONALLY LEFT BLANK DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 4 of 27 PROJECT ENGINEERS CERTIFICATE I hereby state that this report for the residential project at 905 N 32nd 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 Section 1 – Project Overview and Summary The project site is a single-family residential lot with an attached garage on King County Tax 334210-1511 and within the City of Renton. This lot was segregated in 2021 from the adjacent parcel 3342101510. The address for parcel 3342101510 is 903 N 32nd St, Renton, WA 98056. The existing single family development improvements were removed in 2021 from both of lots in preparation for the development of this site. Access to the site is currently via N 32nd St and will be via the alley improvements required for this development. The site is bound by single family residences to the east, N 32nd St to the north and alley right-of-way to the south followed by residences and the formerly shared lot to the west that will be developed by others. The site slopes from southeast to northwest at approximately 3 to 15 percent. 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 32nd St and Burnett Ave N. The city’s system discharges directly to Lake Washington. Total proposed project impervious area is approximately 4,997 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 limited infiltration trench at the front (north) end of the site and the remainder will be managed with a perforated stub out. Utility services are available in N 32nd St. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 5 of 27 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 No address has been assigned; This lot was previously a part of 903 N 32nd St, Renton, WA 98056 King County Parcel No. 334210-1511 Zoning R-8 Aquifer Aquifer Protection Area/Wellhead 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) 5,099 SF per survey Max Impervious Surface Area per Zoning 65% Onsite Slopes 3% to 15% with average slope of 10% or less Subsurface investigation was completed on April 16, 2021, by Pioneer Engineers, Inc., with two test holes. This information is included in the project specific report prepared by Pioneer Engineers, Inc., dated April 23, 2021. The observed soil conditions in the test holes included the following from the surfaces down: · 9 to 11 inches of topsoil (and in the north hole less than a foot of fill). · 3.5 to 4 feet of brownish gray, silty, find SAND, trace coarse gravel… loose to medium-dense, moist (Vashon Till; Sandy Loam). · 2.5 to 3.5 feet of light gray, silty, fine SAND, trace coarse gravel, medium dense, moist (Vashon Till; Sandy Loam). In Test Hole 1 (TP-1), which was excavated at the south end of the site, the soil became dense at 6 ft. At TP-2, at the north end of the site, the soil became very dense, cemented at 6 ft. Light groundwater seepage was encountered at 8 ft deep. The geotechnical report is included in Appendix B. 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 2,382 0.05468 - - 694 2 Driveway (flat)484 0.01111 - - 484 3 Steps (flat)74 0.00170 - - 4 Walk (flat)44 0.00101 - - 44 Onsite Impervious Total 2,984 0.06850 5 Sidwealk (flat)250 0.00574 6 TEMP Walk (flat) 84 0.00193 - - - 7 Street curb and gutter/pavement 192 0.00441 - - 8 Alley 1,487 0.03414 - - Onsite Pervious - - 2,040 0.04683 Planter Strip - - 344 0.00790 TOTAL New/Replaced 4,997 0.11472 2,384 0.05473 Target Impervious Surface Area 4,997 0.11472 - - Percent of Imprevious Surface Addressed with FCBMPs 24.45% DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 6 of 27 Figure 1.2 – Developed Basin Map UP UP 7 6 5 4 3 2 1 3 2 1 NEW SFR LEVEL 1 FFE 113.0' SURFACE P1-ROOF: 2,382 SF SURFACE P2-DRIVEWAY: 484 SF (PERMEABLE PAVERS) SURFACE P5-SIDEWALK: 250 SF 112.98112.98 107.12 SURFACE P6-TEMP WALK: 84 SF SURFACE P7-STREET C&G/PAVEMENT: 192 SF UTILITY RESTORATION-EXEMPT SURFACE P4-WALK: 44 SF (PERMEABLE PAVERS) SURFACE P3-STEPS: 74 SF ~107.6 CB - 24" NYLOPLAST RIM ~107.5 W/ GRATE FOR OVERFLOW IE-IN 106.0 (6" S) IE-OUT 105.8 (6" N) W/ TEE PER DETAIL IE-OUT 106.8 (6" W, OVERFLOW) MIN. 12" SUMP BELOW BOT. OF TEE TO ADDRESS BUOYANCY DRILL SIXTEEN 5/8" HOLES IN BOTTOM AND PROVIDE MIN. 12" WASHED ROCK BELOW STRUCTURE 25 LF LIMITED INFILTRATION TRENCH 6" PERF. PIPE IE = 105.7 SURFACE P8-ALLEY: 1480 SF4'4'9.05'5' 20' BLDG SETBACK CB - TYPE 1 RIM 105.5 IE-OUT 102.6 12" (W) IE-IN 103.1 6" (S)2.9%1.3%PORTION OF ROOF TO LIMITED INFILTRATION TRENCH: 694 SF PERFORATED STUBOUT CONNECTION UTILITY RESTORATION -EXEMPT SURFACE P2B-DRIVEWAY: 47 SF20' BLDG SETBACK 8.5%6.5%112.95 113.28 112.38 112.725'5.9'12 LF 6"Ø CPEP @ 0.5% (MIN) 14 LF 12"Ø DI @ 0.5% (MIN) 15' BLDG. SETBACK 6" CLEANOUT STA 31+25.1, 0.0' SURF. ELEV. 112.67' IE 108.62' 12" PVC (W) CB#2, TYPE 2 - VANED GRATE STA 30+31.5, 0.0' RIM 108.36' IE 101.32' 12" PVC (W) IE 104.27' 12" PVC (E) CB#1, TYPE 2 - SOLID OVERBUILD STA 29+92.8, 5.0'L RIM 107.24' IE 99.78' 12" PVC (E) IE 99.10' 18" EX (N) IE 99.10' 18" EX (S) ABE ABE ABE ABE SCALE IN FEET 10 5 0 10 20 PLOTTED: 3/17/2022 2:13:55 PM FILE: E21012C-DEV BASN-32ND.DWG LAST SAVED BY: 15094DATE 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 03/17/2022 DEVELOPED BASIN MAP FIG 1.2 STRAIGHT ARROW HOMES N 32ND ST SFR 905 N 32ND ST RENTON, WA 98056 PERMIT PERMIT NUMBER B21005352 S32 - T24 - R5 N 32ND ST BURNETT AVE NALLEY SINGLE FAMILY RESIDENTIAL PROJECT UNDER SEPARATE PERMIT DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 7 of 27 Drainage Review Type Project is subject to Direct 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. o It is also under the following thresholds for projects on predominately till soils:  The project results in no more than 7,947 square feet of target impervious surfaces AND (as shown in Figures 1.1 and 1.2 above)  proposed pervious area is equal to or less than 14,941 – 1.88 x (total target impervious surfaces); Per Figures 1.1 and 1.2 proposed pervious area = 2,435 SF which is less than 5,569 SF (=14,941-1.88*4,985)  This is a New Development, since the existing site is less than 35% impervious, therefore: target impervious surfaces include new plus proposed replaced impervious surfaces. · 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 DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 8 of 27 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 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 N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 9 of 27 Figure 1.3 – Renton Surface Water Design Manual Flow Chart for Determining Type of Drainge Review Required DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 10 of 27 Section 2 – 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: Per This project is subject to C1.3.1 Small Lot BMP Requirements, since the site is larger than 22,000 SF and it not a Large Rural Lot. The following BMPs were considered per the KCSWDM: 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 feasible since the underlying soils are classified as sandy loam and the soils do not become cemented until 6-ft deep per Test Pit 2 of the geotechnical engineering report. This BMP is used for a portion of the roof runoff located north of the residence where the setback requirement from the buildings is only 5-feet (since the building crawlspace is higher than the overflow elevation). Property line setbacks of 5- feet are also applicable. 694 SF of roof area is directed to the limited infiltration trench. This site is within rainfall region SeaTac 1.0, therefore 36 LF of trench is required for every 1,000 SF of roof area: 24.98 LF is required (694 ft x 36 / 1,000 SF); 25 LF is provided. 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. These two surfaces have slopes of less than 5% therefore no check dams are required. No impervious run is proposed onto these surfaces. ii. Rain Gardens/Bioretention are infeasible for the remaining surfaces since the horizontal setbacks cannot be met within the remaining site area. Horizontal requirements include: 5-ft from property lines and 5-ft from the residence to the overflow surface water elevation. Also, these facilities must have a minimum bottom of cell width of 1.5-ft. 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 20% of the total impervious area is mitigated as shown in Figure 1.1. 6. Requirement 6 – Protect soil moisture capacity of new pervious surfaces per C2.13 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. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 11 of 27 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 limited infiltration BMPs. Required perforated downspout length = 2447 SF x (10-FT / 5,000 SF) = 3.5 linear feet, or a minimum of 10 linear feet of perforated downspout. 10 linear feet is provided. Section 3 – 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, AND Category #2 since construction of a 12-inch pipe is required for the alley improvements. TDR CATEGORY 1 REQUIREMENTS 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, DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 12 of 27 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. TDR CATEGORY 2 REQUIREMENTS Per RSWDM, Table 1.1.2A, note 2, other requirements may be applied by the Community and Economic Development Department (CED) based on project or site-specific conditions. We understand that the CED is requiring compliance with the following additional Core Requirements: 1, 2, 4, 6 and 7. Below is a site-specific evaluation of these additional requirements with references to subsequent sections of the report where applicable. Core Requirement #1: Discharge at the Natural Location Runoff from the proposed improvements from the alley will continue to discharge to Bennet, which is the natural discharge location for this site. The site is not within a mapped landslide hazard area. Refer to the Level 1 Downstream Analysis under Section 4 of this report. Core Requirement #2: Offsite Analysis An offsite analysis is provided under Section 4 of this report. Core Requirement #4: Conveyance System The conveyance system requiring analysis is limited to the alley improvements. The adjacent onsite improvements do not tie into the alley system via pipe. Refer to Section 5 of this report. Core Requirement #6: Maintenance and Operations Operation and maintenance instructions are provided in Appendix B. The alley improvements are all public and will be maintained by the city. Maintenance instructions are included for the onsite improvements and are also within the Declaration of Covenant prepare for the onsite development. Core Requirement #7: Financial Guarantees and Liability The owner will provide any financial guarantees and liabilities required by the right-of-way permit. Bond quantity estimates for the offsite improvements are provided as a separate document. Section 4 – Offsite Analysis A Level 1 Analysis of the downstream conditions has been completed, per Core Requirement #2, Section 1.2.2 of 2017 CRSWDM. As required, both downstream and upstream conditions were analyzed qualitatively. TASK 1 – Define and Map the Study Area The study area includes N 32nd St/Benne from the project site approximately a quarter mile downstream where the City of the Renton stormwater conveyance system discharges into Lake Washington. A downstream map is provided in Figure 3.1 below. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 13 of 27 Figure 4.1 – Downstream Map DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 14 of 27 TASK 2 – Review All Available Information on the Study Area The City of Renton COR Maps website as well as King County iMap were reviewed for the surrounding and downstream study area. Based on King County iMap there are no drainage complaints around the project site or within the downstream area. Based on City of Renton COR Map, there are moderate to high landslide hazard areas and small areas of steep slopes within the downstream study area, as shown in Figure 4.2. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 15 of 27 Figure 4.2 – City of Renton Critical Areas Map DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 16 of 27 TASK 3 – Field Inspect the Study Area A field investigation was completed by Third Day Design on April 14, 2021 at the site and on August 3, 2021 to review the remainder of the downstream system, as part of the downstream analysis. The onsite drainage patterns within the project site were observed. Refer to Task 4 below for a detailed description of the downstream drainage system and analysis. TASK 4 – Describe the drainage system, and its existing and predicted drainage and water quality problems. Runoff from the project site, and from the adjacent properties off the alley right-of-way to drains to the conveyance system in Burnett Ave N. Presently runoff from the alley right-of-way is managed on each individual abutting parcel. All stormwater from the 905 N 32nd St currently drains via overland flow N 32nd St where it is conveyed via asphalt gutter flow (Photo A) to the catch basin at the SE corner of N 32nd St/Burnett Ave N (Photo B) which is a part of the collection and conveyance system in Burnett. This conveyance system continues downhill to the north (Photo C) to N 34th St where it turns west and is conveyed via pipe through undeveloped right-of-way (Photo D) to the piped system on the east side of Lake Washington Blvd N. The pipe system in Lake Washington Blvd continues three parcels to the south where it turns westerly at a type 2 catch basin to cross Lake Washington Blvd N followed by the Eastside Rail Corridor Trail (Photo E) before discharging to Lake Washington on private property (on King County Parcel 3124059076) (Photo F). The discharge to the Lake Washington is located behind a fence and could not be observed from the adjacent trail. No storm drainage system or flooding problems system problems were observed during the visit. Neither were any erosion or landslide hazard concerns observed. Photographs from the site visit are included below and the letters correspond to the letters indicated on Figure 3.1. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 17 of 27 Photo A – N 32nd St, Adjacent to the project site (left), looking West toward Burnett (4/15/21): Photo B – Catch Basin inlet at SE Corner of N 32nd St/Burnett Ave N (4/15/21): DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 18 of 27 Photo C –Burnett Ave N, looking North from N 31st St (8/3/21): Photo D – Evidence of conveyance system (vaned grate for CB beyond guy wire) within undeveloped N 34th St right-of-way (8/3/21) DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 19 of 27 Photo E – From trail looking uphill (east) toward Lake Washington Blvd N (8/3/21) Photo E – General area of discharge on private property per GIS (8/3/21) DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 20 of 27 Section 5 – Conveyance Analysis and Design The proposed project includes a new closed pipe conveyance system in the alley between N 32nd and N 31st Street which includes conveyance pipes and catch basins. The conveyance system is designed to adequately convey and contain the 100-year SCS Type 1A 24-hour peak flow. Autodesk’s Storm and Sanitary Analysis 2022 was used to analyze the proposed storm system. A Conveyance Basin Map is included in Figure 5.1. Complete conveyance calculations are included in Figure 5.2. Note that all stormwater was analyzed for the shallowest pipe, which is the pipe crossing Burnett Ave N, with a slope just under 4.0%. The upstream pipe which will convey a portion of the runoff contributing to the fully built out alley has a design slope of 4.65%. 107.6 BURNETT AVE NN 32ND AVE N 31ST AVE PARK AVE NALLEY STORM SYSTEM TDA TOTAL AREA: 4.7 ACRES ASSUME 75% IMPERVIOUS PER TABLE 3.2.2.C OF THE RENTON SURFACE WATER DESIGN MANUAL FOR R-8 ZONING LOOK AT MOST CONSTRAINED PIPE 39 LF 12"Ø PVC @ 3.95% EX CB #1 RIM: 107.24 IE: 99.78 CB #2 RIM: 108.36 IE: 101.32 SCALE IN FEET 1608004080 Project Description Renton SRF.SPF Project Options CFS Elevation Santa Barbara UH SCS TR-55 Hydrodynamic YES NO Analysis Options 00:00:00 0:00:00 00:00:00 0:00:00 00:00:00 0:00:00 0 days 0 01:00:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 30 seconds Number of Elements Qty 1 1 3 2 1 0 0 0 2 0 2 0 0 0 0 0 0 Rainfall Details SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall ID Source ID Type Units Period Depth Distribution (years)(inches) 1 100-YEAR Time Series 100-YEAR Cumulative inches Washington King 100.00 4.00 SCS Type IA 24-hr Antecedent Dry Days ................................ File Name ................................................. Flow Units ................................................ Elevation Type .......................................... Hydrology Method ................................... Time of Concentration (TOC) Method ....... Link Routing Method ................................ Enable Overflow Ponding at Nodes .......... Skip Steady State Analysis Time Periods ... Start Analysis On ...................................... End Analysis On ........................................ Start Reporting On ................................... Storage Nodes .................................. Runoff (Dry Weather) Time Step .............. Runoff (Wet Weather) Time Step ............. Reporting Time Step ................................. Routing Time Step .................................... Rain Gages ............................................... Subbasins.................................................. Nodes........................................................ Junctions .......................................... Outfalls ............................................ Flow Diversions ................................ Inlets ................................................ Outlets ............................................. Pollutants ................................................. Land Uses ................................................. Links.......................................................... Channels .......................................... Pipes ................................................ Pumps .............................................. Orifices ............................................. Weirs ................................................ Subbasin Summary SN Subbasin Area Impervious Impervious Pervious Total Total Total Peak Time of ID Area Area Curve Area Curve Rainfall Runoff Runoff Runoff Concentration Number Number Volume (ac)(%)(in)(in)(ac-in)(cfs)(days hh:mm:ss) 1 Ally 4.70 75.00 98.00 86.00 3.99 3.30 15.49 1.68 0 01:53:53 Node Summary SN Element Element Invert Ground/Rim Initial Surcharge Ponded Peak Max HGL Max Min Time of Total Total Time ID Type Elevation (Max)Water Elevation Area Inflow Elevation Surcharge Freeboard Peak Flooded Flooded Elevation Elevation Attained Depth Attained Flooding Volume Attained Occurrence (ft)(ft)(ft)(ft)(ft²)(cfs)(ft)(ft)(ft)(days hh:mm)(ac-in)(min) 1 CB#2 Junction 101.32 108.36 101.32 108.36 10.00 1.68 101.70 0.00 6.66 0 00:00 0.00 0.00 2 EX_CB#1 Junction 97.10 107.24 97.10 107.24 10.00 1.68 99.41 0.00 7.83 0 00:00 0.00 0.00 3 Out-01 Outfall 94.03 1.68 95.38 Link Summary SN Element Element From To (Outlet)Length Inlet Outlet Average Diameter or Manning's Peak Design Flow Peak Flow/Peak Flow Peak Flow Peak Flow Total Time Reported ID Type (Inlet)Node Invert Invert Slope Height Roughness Flow Capacity Design Flow Velocity Depth Depth/Surcharged Condition Node Elevation Elevation Ratio Total Depth Ratio (ft)(ft)(ft)(%)(in)(cfs)(cfs)(ft/sec)(ft)(min) 1 Link-01 Pipe CB#2 EX_CB#1 39.00 101.32 99.78 3.9500 12.000 0.0130 1.68 7.08 0.24 6.63 0.36 0.36 0.00 Calculated 2 Link-02 Pipe EX_CB#1 Out-01 130.00 99.10 94.03 3.9000 18.000 0.0150 1.68 17.98 0.09 1.67 0.83 0.55 0.00 Calculated Subbasin Hydrology Subbasin : Ally Input Data Area (ac) ...............................................................................4.7 Impervious Area (%) .............................................................75 Impervious Area Curve Number ...........................................98 Pervious Area Curve Number ...............................................86 Rain Gage ID .........................................................................100-YEAR Composite Curve Number 32 Area Soil Curve Soil/Surface Description (acres)Group Number Composite Area & Weighted CN 4.7 95 Time of Concentration TOC Method : SCS TR-55 Sheet Flow Equation : Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4)) Where : Tc = Time of Concentration (hr) n = Manning's roughness Lf = Flow Length (ft) P = 2 yr, 24 hr Rainfall (inches) Sf = Slope (ft/ft) Shallow Concentrated Flow Equation : V = 16.1345 * (Sf^0.5) (unpaved surface) V = 20.3282 * (Sf^0.5) (paved surface) V = 15.0 * (Sf^0.5) (grassed waterway surface) V = 10.0 * (Sf^0.5) (nearly bare & untilled surface) V = 9.0 * (Sf^0.5) (cultivated straight rows surface) V = 7.0 * (Sf^0.5) (short grass pasture surface) V = 5.0 * (Sf^0.5) (woodland surface) V = 2.5 * (Sf^0.5) (forest w/heavy litter surface) Tc = (Lf / V) / (3600 sec/hr) Where: Tc = Time of Concentration (hr) Lf = Flow Length (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft) Channel Flow Equation : V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n R = Aq / Wp Tc = (Lf / V) / (3600 sec/hr) Where : Tc = Time of Concentration (hr) Lf = Flow Length (ft) R = Hydraulic Radius (ft) Aq = Flow Area (ft²) Wp = Wetted Perimeter (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft) n = Manning's roughness User-Defined TOC override (minutes): 113.89 Subbasin Runoff Results Total Rainfall (in) ..................................................................3.99 Total Runoff (in) ...................................................................3.3 Peak Runoff (cfs) ..................................................................1.68 Weighted Curve Number ......................................................95 Time of Concentration (days hh:mm:ss) ...............................0 01:53:53 Subbasin : Ally Rainfall Intensity Graph Time (hrs) 2423222120191817161514131211109876543210Rainfall (in/hr)1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 Runoff Hydrograph Time (hrs) 23222120191817161514131211109876543210Runoff (cfs)1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Autodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary AnalysisAutodesk Storm and Sanitary Analysis DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 21 of 27 DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 22 of 27 Section 6 – 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: A stabilized construction exit is proposed to prevent 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 disturbed earthwork and measures should be taken to cover or otherwise stabilize the material, as required. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 23 of 27 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. Fencing will be used to protect these areas. · Infiltration Trench: Construction stormwater runoff shall not be directed to the infiltration trench. Fencing will be provided around the perimeter of the trench to protect the underlying soils from compaction and sediment. 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 N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 24 of 27 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. DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 25 of 27 APPENDICIES DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 26 of 27 APPENDIX A – Geotechnical Report and Supplemental Letter PIONEER ENGINEERS, INC. Geotechnical Engineering Earth Science Water Resources ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ P. O. Box 33628Seattle, WA 98133 Phone: (206) 427-9118 Fax: (206) 306-2982 josephwu.pei@gmail.com January 3, 2022 Mr. Brandon Shimizu Straight Arrow Homes, LLC 12015 NE 8th Street Bellevue, WA 98005 Subject: Geotechnical Consultation Proposed Single-Family Residence 907 North 32nd Street Renton, Washington Dear Mr. Shimizu: Per the civil engineer’s request for clarification of full and limited stormwater infiltration in the report prepared 4/23/2021 by Pioneer Engineers, Inc. (“PEI”) for the proposed residential development at the subject site, it is PEI’s clarification that soil and groundwater conditions explored are not appropriate in support of full infiltration as a best management practice (“BMP”) of on-site stormwater management. Limited infiltration, however, may be implemented to serve this purpose if it meets the requirements and design criteria in all relevant codes. Should you have any additional questions or need further consultation, please feel free to call or email. Very truly, PIONEER ENGINEERS, INC. Joseph Wu, P.E. Engineering Consultant PIONEER ENGINEERS, INC. Geotechnical Engineering Earth Science Water Resources ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ P. O. Box 33628Seattle, WA 98133 Phone: (206) 427-9118 Fax: (206) 306-2982 josephwu.pei@gmail.com April 23, 2021 Mr. Brandon Shimizu Straight Arrow Homes, LLC 12015 NE 8th Street Bellevue, WA 98005 Subject: Geotechnical Evaluation Proposed Single-Family Residence 90x North 32nd Street Renton, Washington Dear Mr. Shimizu: At your request, Pioneer Engineers, Inc. (“PEI”) has explored subsurface conditions with two test pits (“TP-1 and TP-2”) for evaluation of stormwater infiltration at the subject site in Renton, Washington. The site is the east lot of two urban single-family residential lots subdivided from a parent parcel, approximately 50 feet east of North 32nd Street and Burnett Avenue North. The general location of the site is shown in Figure 1. Based on a topographic survey plan prepared by Crones Surveying and a preliminary architectural site plan by JW Architects, the residence will be a two- or three-story wood framed structure with an attached garage, accessible from North 32nd Street. Minor grading is anticipated to establish the driveway and foundation. 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: 1. Explore subsurface conditions with two test pits to a maximum depth of 8 feet. The underlying soils encountered are visually classified; April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 2 PIONEER ENGINEERS, INC. 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, a Zone 2 aquifer protection area, and evaluation of on-site stormwater infiltration. SITE CONDITIONS Surface Conditions The site is a rectangularly-shaped urban lot bounded to the north by North 32nd Street and adjoins private properties on the other three sides. It covers an area of 0.115 acre, measuring 50 feet along the street and 100 feet in depth. A vertical relief of 2 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 westerly to Lake Washington. Locally within the site, the ground surface slopes down mildly toward northwest. All existing structures have been removed. Groundcover consists mostly of lawn grass. 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 surficial soil unit is mapped as deposits of Vashon recessional outwash deposits (“Qvr”) at the site. Qvr was deposited mostly from the meltwater flowing from the retreating glaciers. It consists of fine to medium sand with minor amount of gravel, generally overlying Vashon till (“Qvt”). April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 3 PIONEER ENGINEERS, INC. 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 April 16, 2021, subsurface conditions were explored with two test pits to a maximum depth of 8 feet. These pits were excavated by using a track-mounted excavator (Caterpillar 303SE2 CR) provided by the owner. Locations of these pits were determined by tape measuring with reference to existing surface features shown on the design plan. Their approximate locations are shown on Figure 2. Subsurface exploration was continuously monitored by an engineer from our firm who documented the underlying soil and water 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: Nine to eleven inches of topsoil, underlain by an upper, more weathering portion of Vashon till consisting mostly of brownish gray, silty, fine sand with a trace of coarse gravel. Underlying this soil deposit was relatively unweathered Vashon till of light gray, silty, fine sand with a trace of coarse gravel, occasional cobble. A thin layer of fill was found between the topsoil and the upper till in TP-1. Denseness appears to increase with depth and degree of cementation in this till. More detailed soil information is presented in Figure 4. April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 4 PIONEER ENGINEERS, INC. Groundwater Conditions Light groundwater seepage was encountered at depths varying from 5.5 to 6 feet in the test pits. Seepage water levels and amount generally fluctuate with seasons, depending on the amount of precipitation and surface runoff, denseness of groundcover, purpose of land use, and other factors. 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- inch-thick layer of quarry spalls (generally 2 to 4 inches in size), 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. April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 5 PIONEER ENGINEERS, INC.  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. Structural fill should be free of organic, construction debris and other deleterious substances. It should consist 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. Compaction Criteria 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 Utility Trench Backfill 95% for upper 4 feet, 90% below April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 6 PIONEER ENGINEERS, INC. Utility Trench Backfill Preventing settlements along utility (sewer and storm) alignments particularly in paved areas is a major concern for trench backfilling. It is important that each section of the utility line be adequately supported by the bedding material. The material should be hand tamped to ensure support is provided around the pipe haunches. Structural fill should be carefully placed and hand tamped to at least 12 inches above the crown of the pipe before any mechanical or vibratory compaction equipment is brought into use. The remainder of the trench backfill should follow the recommendations in Compaction Criteria. Controlled Density Fill 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. WELLHEAD PROTECTION AREA The site is located in a Zone 2 wellhead protection area. Zone 2 is defined as “the land area located between the 365 day groundwater travel time contour and the boundary of the zone of potential capture for a well or well field owned or operated by the City” in accordance with Renton Municipal Code (“RMC”) 4-3-050G.8. In RMC, Zone 2 occasionally refers to the property subject to water supply from a well, well field or spring which originates from a confined aquifer naturally protected by relatively impervious stratified soil layers. In a wellhead protection area, the storage, handling, use, treatment, and production of hazardous materials are restrictively regulated. Secondary containment devices, emergency collection devices and monitoring devices should be present on site in addition to the primary April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 7 PIONEER ENGINEERS, INC. containers of hazardous materials. The project site is located in a zone of residential land use where it has never been used for industrial purposes since the record time of King County iMap. Both test pits did not encounter any hazardous materials during the exploration. PEI anticipates that the development will meet the requirements for a Zone 2 wellhead protection area. STORMWATER INFILTRATION The exploration indicates that considerably impermeable Vashon till deposits underlie the locations for stormwater infiltration. These till deposits consolidated by historical glaciation contain a significant amount of fines (silt and clay particles), which are considered as a stratum of depository soils restrictive for stormwater infiltration since they will impede moisture or water from moving downwards to recharge groundwater. This soil stratum is unable to bolster stormwater infiltration, confirmed by groundwater seepage above relatively unweathered till in the test pits. With the existence of such till at shallow depths, a pilot infiltration test is not required and the best management practice (“BMP”) of stormwater infiltration inappropriate for application on site. Space constraints and/or topographic conditions in an urban area usually limit the application of full dispersion for stormwater management. In this perspective, the collective storm runoff from impervious areas may drain in an infiltration facility such as a trench or a drywell, depending on the existence of a relatively permeable soil stratum in the infiltration zone. The BMP of basic dispersion requiring a shorter vegetated flowpath appears an option when subsurface soil and groundwater conditions are not supportive for infiltration. 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. April 23, 2021 Proposed Single-Family Residence PEI Project No. G21A17 Page 8 PIONEER ENGINEERS, INC. 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. 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 90X NORTH 32ND STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G21A17 DATE 4/23/21 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 DRAINAGE ASSESMENT FOR N 32ND ST STARIGHT ARROW HOMES (21012) THIRD DAY DESIGN Page 27 of 27 APPENDIX B – Operation and Maintenance Manual APPENDIX A MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND ON-SITE BMPS 12/12/2016 2017 City of Renton Surface Water Design Manual A-10 NO. 5 – CATCH BASINS AND MANHOLES MAINTENANCE COMPONENT DEFECT OR PROBLEM CONDITION WHEN MAINTENANCE IS NEEDED RESULTS EXPECTED WHEN MAINTENANCE IS PERFORMED Structure Sediment accumulation Sediment exceeds 60% of the depth from the bottom of the catch basin to the invert of the lowest pipe into or out of the catch basin or is within 6 inches of the invert of the lowest pipe into or out of the catch basin. Sump of catch basin contains no sediment. Trash and debris Trash or debris of more than ½ cubic foot which is located immediately in front of the catch basin opening or is blocking capacity of the catch basin by more than 10%. No Trash or debris blocking or potentially blocking entrance to catch basin. Trash or debris in the catch basin that exceeds 1/3 the depth from the bottom of basin to invert the lowest pipe into or out of the basin. No trash or debris in the catch basin. Dead animals or vegetation that could generate odors that could cause complaints or dangerous gases (e.g., methane). No dead animals or vegetation present within catch basin. Deposits of garbage exceeding 1 cubic foot in volume. No condition present which would attract or support the breeding of insects or rodents. Damage to frame and/or top slab Corner of frame extends more than ¾ inch past curb face into the street (If applicable). Frame is even with curb. Top slab has holes larger than 2 square inches or cracks wider than ¼ inch. Top slab is free of holes and cracks. Frame not sitting flush on top slab, i.e., separation of more than ¾ inch of the frame from the top slab. Frame is sitting flush on top slab. Cracks in walls or bottom Cracks wider than ½ inch and longer than 3 feet, any evidence of soil particles entering catch basin through cracks, or maintenance person judges that catch basin is unsound. Catch basin is sealed and is structurally sound. Cracks wider than ½ inch and longer than 1 foot at the joint of any inlet/outlet pipe or any evidence of soil particles entering catch basin through cracks. No cracks more than 1/4 inch wide at the joint of inlet/outlet pipe. Settlement/ misalignment Catch basin has settled more than 1 inch or has rotated more than 2 inches out of alignment. Basin replaced or repaired to design standards. Damaged pipe joints Cracks wider than ½-inch at the joint of the inlet/outlet pipes or any evidence of soil entering the catch basin at the joint of the inlet/outlet pipes. No cracks more than ¼-inch wide at the joint of inlet/outlet pipes. Contaminants and pollution Any evidence of contaminants or pollution such as oil, gasoline, concrete slurries or paint. Materials removed and disposed of according to applicable regulations. Source control BMPs implemented if appropriate. No contaminants present other than a surface oil film. Inlet/Outlet Pipe Sediment accumulation Sediment filling 20% or more of the pipe. Inlet/outlet pipes clear of sediment. Trash and debris Trash and debris accumulated in inlet/outlet pipes (includes floatables and non-floatables). No trash or debris in pipes. APPENDIX A MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND ON-SITE BMPS 2017 City of Renton Surface Water Design Manual 12/12/2016 A-11 NO. 5 – CATCH BASINS AND MANHOLES MAINTENANCE COMPONENT DEFECT OR PROBLEM CONDITION WHEN MAINTENANCE IS NEEDED RESULTS EXPECTED WHEN MAINTENANCE IS PERFORMED Inlet/Outlet Pipe (cont.) Damaged inlet/outlet pipe Cracks wider than ½-inch at the joint of the inlet/outlet pipes or any evidence of soil entering at the joints of the inlet/outlet pipes. No cracks more than ¼-inch wide at the joint of the inlet/outlet pipe. Metal Grates (Catch Basins) Unsafe grate opening Grate with opening wider than 7/8 inch. Grate opening meets design standards. Trash and debris Trash and debris that is blocking more than 20% of grate surface. Grate free of trash and debris. footnote to guidelines for disposal Damaged or missing grate Grate missing or broken member(s) of the grate. Any open structure requires urgent maintenance. Grate is in place and meets design standards. Manhole Cover/Lid Cover/lid not in place Cover/lid is missing or only partially in place. Any open structure requires urgent maintenance. Cover/lid protects opening to structure. Locking mechanism not working Mechanism cannot be opened by one maintenance person with proper tools. Bolts cannot be seated. Self-locking cover/lid does not work. Mechanism opens with proper tools. Cover/lid difficult to remove One maintenance person cannot remove cover/lid after applying 80 lbs. of lift. Cover/lid can be removed and reinstalled by one maintenance person. APPENDIX A MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND ON-SITE BMPS 12/12/2016 2017 City of Renton Surface Water Design Manual A-12 NO. 6 – CONVEYANCE PIPES AND DITCHES MAINTENANCE COMPONENT DEFECT OR PROBLEM CONDITIONS WHEN MAINTENANCE IS NEEDED RESULTS EXPECTED WHEN MAINTENANCE IS PERFORMED Pipes Sediment & debris accumulation Accumulated sediment or debris that exceeds 20% of the diameter of the pipe. Water flows freely through pipes. Vegetation/root growth in pipe Vegetation/roots that reduce free movement of water through pipes. Water flows freely through pipes. Contaminants and pollution Any evidence of contaminants or pollution such as oil, gasoline, concrete slurries or paint. Materials removed and disposed of according to applicable regulations. Source control BMPs implemented if appropriate. No contaminants present other than a surface oil film. Damage to protective coating or corrosion Protective coating is damaged; rust or corrosion is weakening the structural integrity of any part of pipe. Pipe repaired or replaced. Damaged pipes Any dent that decreases the cross section area of pipe by more than 20% or is determined to have weakened structural integrity of the pipe. Pipe repaired or replaced. Ditches Trash and debris Trash and debris exceeds 1 cubic foot per 1,000 square feet of ditch and slopes. Trash and debris cleared from ditches. Sediment accumulation Accumulated sediment that exceeds 20% of the design depth. Ditch cleaned/flushed of all sediment and debris so that it matches design. Noxious weeds Any noxious or nuisance vegetation which may constitute a hazard to City personnel or the public. Noxious and nuisance vegetation removed according to applicable regulations. No danger of noxious vegetation where City personnel or the public might normally be. Contaminants and pollution Any evidence of contaminants or pollution such as oil, gasoline, concrete slurries or paint. Materials removed and disposed of according to applicable regulations. Source control BMPs implemented if appropriate. No contaminants present other than a surface oil film. Excessive vegetation growth Vegetation that reduces free movement of water through ditches. Water flows freely through ditches. Erosion damage to slopes Any erosion observed on a ditch slope. Slopes are not eroding. Rock lining out of place or missing (If applicable) One layer or less of rock exists above native soil area 5 square feet or more, any exposed native soil. Replace rocks to design standards. MAINTENANCE INSTRUCTIONS FOR LIMITED INFILTRATION Your property contains an on-site BMP (best management practice) called “limited infiltration,” which was installed to mitigate the stormwater quantity and quality impacts of some or all of the impervious surfaces on your property. Limited infiltration is a method of soaking runoff from impervious area (such as paved areas and roofs) into the ground. Infiltration devices, such as gravel filled trenches, drywells, and ground surface depressions, facilitate this process by putting runoff in direct contact with the soil and holding the runoff long enough to soak most of it into the ground. To be successful, the soil condition around the infiltration device must be able to soak water into the ground for a reasonable number of years. This on-site BMP shall be maintained per Appendix A of the City of Renton’s Surface Water Design Manual. Infiltration Devices The infiltration devices used on your property include the following as indicated on the site plan (CHECK THE BOX(ES) THAT APPLY):  gravel filled trenches,  drywells MAINTENANCE RESTRICTIONS The size, placement, and composition of these devices as depicted by the site plan and design details must be maintained and may not be changed without written approval from the City of Renton or through a future development permit from the City of Renton. INSPECTION FREQUENCY AND MAINTENANCE GUIDELINES To be successful, the soil condition around the infiltration device must be able to soak water into the ground for a reasonable number of years. • Infiltration devices must be inspected annually and after major storm events to identify and repair any physical defects. • Maintenance and operation of the system should focus on ensuring the system's viability by preventing sediment-laden flows from entering the device. Excessive sedimentation will result in a plugged or non-functioning facility. • If the infiltration device has a catch basin, sediment accumulation must be removed on a yearly basis or more frequently if necessary. • Prolonged ponding around or atop a device may indicate a plugged facility. If the device becomes plugged, it must be replaced. • Keeping the areas that drain to infiltration devices well swept and clean will enhance the longevity of these devices. • For roofs, frequent cleaning of gutters will reduce sediment loads to these devices. X EXHIBIT B - OPERATIONS AND MAINTENANCE INFILTRATION TRENCH AND DRYWELL SYSTEMS 6" 24" 12" 24" WASHED ROCK 1 12"- 34" LEVEL INFILTRATION TRENCH SECTION A NTS PLAN VIEWNTS SECTION VIEWNTS VARIES A A 6" 6" 4" RIGID OR 6"FLEXIBLEPERFORATEDPIPE CB SUMP w/SOLID LID ROOF DRAIN 4" RIGID OR 6" FLEXIBLEPERFORATED PIPE OVERFLOW SPLASH BLOCK ROOF DRAIN CB SUMP w/SOLID LID 12" FINE MESHSCREEN SETBACKFROMBUILDING5' MIN. 1' MIN.1' MIN. FILTER FABRIC COMPACTED BACKFILL 4" RIGID OR 6" FLEXIBLEPERFORATED PIPE 112" - 34" WASHED ROCK MIN. 1' ABOVE SEASONAL HIGH GROUNDWATER TABLE, SEE SECTION C.2.2.2 TRENCH SETBACK FROMBUILDING AS REQUIRED,15' MIN. DRIVEWAY X-SECTION NTS 2' MIN. FILTER STRIP SEE SECTIONS6.3.4 & 6.3.5 18" MIN. 6" MIN. 3/4" TO 1-1/2"WASHED DRAINROCK GRASS OVERFLOW NOTE:SEE C.2.2.3 FORTRENCH LENGTHS, TRENCH SPACING ANDSITE LIMITATIONS MIN. 1' ABOVESEASONAL HIGHGROUNDWATERTABLE, SEESECTION C.2.2.2 X-SECTIONNTS GRASS 2' MIN. 18" MIN. 6" MIN. 3/4" TO 1-1/2"WASHED DRAINROCK OVERFLOW 15' MIN. ASREQUIRED,SEE C.2.2.3 FROM ROOF NS T T S MIN. 1' ABOVESEASONAL HIGHGROUNDWATERTABLE, SEESECTION C.2.2.2 HOUSE FLOW 48 INCH DIAMETERHOLE FILLED WITH 1 12" - 3" WASHEDDRAIN ROCK SECTIONNTS MARK CENTER OF HOLEWITH 1" CAPPED PVCOR OTHER MEANSFLUSH WITH SURFACE ROOF DOWNSPOUT OVERFLOW SPLASH BLOCK TOPSOIL FINE MESH SCREEN MIN. 4" DIA.PVC PIPE SIDES OF HOLELINED WITHFILTER FABRIC CATCH BASIN(YARD DRAIN) 15' MIN. AS REQUIRED, SEE SECTION C.2.2.3 VARIES 1' MIN. MIN. 1' ABOVE SEASONALHIGH GROUNDWATER TABLE,SEE SECTION C.2.2.2 5' MIN. SETBACKFROM BUILDING FLOW PLAN VIEWNTS HOUSE ROOFDOWNSPOUT ROOFDOWNSPOUT CATCH BASIN(YARD DRAIN) 48 INCH DIAMETERHOLE FILLED WITH1 12" - 3" WASHEDDRAIN ROCK MAINTENANCE INSTRUCTIONS FOR PERMEABLE PAVEMENT (NON-VEGETATED) Your property contains an on-site BMP (best management practice) called “permeable pavement,” which was installed to minimize the stormwater quantity and quality impacts of some or all of the paved surfaces on your property. Permeable pavements reduce the amount of rainfall that becomes runoff by allowing water to seep through the pavement into a free-draining gravel or sand bed, where it can be infiltrated into the ground. This on-site BMP shall be maintained per Appendix A of the City of Renton’s Surface Water Design Manual. Permeable Pavements The type(s) of permeable pavement used on your property is (CHECK THE BOX(ES) THAT APPLY):  Porous concrete  Porous asphalt  Permeable pavers  Modular grid pavement MAINTENANCE RESTRICTIONS The area covered by permeable pavement as depicted by the site plan and design details must be maintained as permeable pavement and may not be changed without written approval from the City of Renton or through a future development permit from the City of Renton. INSPECTION FREQUENCY AND MAINTENANCE GUIDELINES • Permeable pavements must be inspected after one major storm each year to make sure it is working properly. More frequent inspection is recommended. • Prolonged ponding or standing water on the pavement surface is a sign that the system is defective and may need to be replaced. If this occurs, contact the pavement installer or the City of Renton for further instructions. • A typical permeable pavement system has a life expectancy of approximately 25 years. To help extend the useful life of the system, the surface of the permeable pavement should be kept clean, stable and free of leaves, debris, and sediment through regular sweeping or vacuum sweeping. Aggregate fill in modular grid pavement may need periodic surface replenishment. • The owner is responsible for the repair of all ruts, deformation, and/or broken paving grids or pavers. • Modular grid pavement and permeable pavers filled with gravel or with gravel in the joints may need to be refilled periodically. MAINTENANCE INSTRUCTIONS FOR A PERFORATED PIPE CONNECTION Your property contains an on-site BMP (best management practice) called a “perforated pipe connection,” which was installed to reduce the stormwater runoff impacts of some or all of the impervious surface on your property. A perforated pipe connection is a length of drainage conveyance pipe with holes in the bottom, designed to “leak” runoff, conveyed by the pipe, into a gravel filled trench where it can be soaked into the surrounding soil. The connection is intended to provide opportunity for infiltration of any runoff that is being conveyed from an impervious surface (usually a roof) to a local drainage system such as a ditch or roadway pipe system. This on-site BMP shall be maintained per Appendix A of the City of Renton’s Surface Water Design Manual. MAINTENANCE RESTRICTIONS • The size and composition of the perforated pipe connection as depicted by the site plan and design details must be maintained and may not be changed without written approval from the City of Renton or through a future development permit from the City of Renton. • The soil overtop of the perforated portion of the system must not be compacted or covered with impervious materials. RECORDING REQUIREMENT These perforated pipe connection on-site BMP maintenance and operation instructions must be recorded as an attachment to the required declaration of covenant and grant of easement per Requirement 3 of Section C.1.3.4 of the City of Renton Surface Water Design Manual. The intent of these instructions is to explain to future property owners, the purpose of the BMP and how it must be maintained and operated. These instructions are intended to be a minimum; the City of Renton may require additional instructions based on site-specific conditions. See the City of Renton’s Surface Water Design Manual website for additional information and updates. TYPICAL PERFORATED PIPE CONNECTION FOR A SINGLE FAMILY RESIDENCE MAINTENANCE INSTRUCTIONS FOR SOIL AMENDMENT Your property contains an on-site BMP (best management practice) called “soil amendment,” which was installed to mitigate the stormwater quantity and quality impacts of some or all of the pervious surfaces on your property. Soil amendment is a method of regaining greater stormwater functions in the post development landscape by increasing treatment of pollutants and sediments, and minimizing the need for some landscaping chemicals. To be successful, the soil condition must be able to soak water into the ground for a reasonable number of years. This on-site BMP shall be maintained per Appendix A of the City of Renton’s Surface Water Design Manual. MAINTENANCE RESTRICTIONS The size, placement, and composition of these devices as depicted by the site plan and design details must be maintained and may not be changed without written approval from the City of Renton or through a future development permit from the City of Renton. INSPECTION FREQUENCY AND MAINTENANCE GUIDELINES To be successful, the soil must be able to soak water into the ground for a reasonable number of years. • Return leaf fall and shredded woody materials from the landscape to the site when possible in order to replenish soil nutrients and structure. • On turf areas, “grasscycle” (mulch-mow or leave the clippings) to build turf health. • Maintain 2 to 3 inches of mulch over bare areas in landscape beds. • Re-seed bare turf areas until the vegetation fully covers the ground surface. • Avoid using pesticides (bug and weed killers) which damage the soil. • Where fertilization is needed (mainly turf and annual flower beds), a moderate fertilization program should be used which relies on compost, natural fertilizers, or slow-release synthetic balanced fertilizers. RECORDING REQUIREMENT These on-site BMP maintenance and operation instructions must be recorded as an attachment to the required declaration of covenant and grant of easement per Requirement 3 of Section C.1.3.4 of the City of Renton Surface Water Design Manual. The intent of these instructions is to explain to future property owners, the purpose of the BMP and how it must be maintained and operated. These instructions are intended to be a minimum; the City of Renton may require additional instructions based on site-specific conditions. See the City of Renton’s Surface Water Design Manual website for additional information and updates.