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7. ON -SITE STORMWATER MANAGEMENT <br />As documented in Section 1 of this report this. project. is required to meet Minimum <br />Requirements 1-5.. As a result, this project is required to use the On -Site Stormwater <br />Management BMP's from the 2019 DOE SWMMWW. <br />Per the DOE SWMMWW the BMP's must be evaluated in the order they are listed. <br />BMP's for roofs: <br />1. Full Dispersion: Not feasible due to the 65:35 requirement. <br />2. Downspout Infiltration Systems: <br />As previously mentioned in this report, test pits taken by the geotechnical engineers <br />found very sandy silt/clay to approximately 42 inches below grade. However, due to <br />site constraints and the amount of grading to take place on this property, infiltration is <br />infeasible. This is due to setback requirements, slopes, and the fact that any otherwise <br />viable areas will consist of fill material. <br />3. Downspout Dispersion Systems: <br />Minimum flow paths cannot .be..achieved; therefore, downspout dispersion is infeasible: <br />4. Perforated Stub -out Connections <br />Selected for all NPGIS for this this project. As per the city 'of Everett stormwater <br />manual, a 10 foot long perforated stubout is sufficient for up to 5,000 sf of contributing <br />drainage area. The NPGIS area proposed for this project total 2,273 sf, therefore a <br />single 10 foot long perforated stubout connection will be used for this project. The <br />perforated stubout connection (BMP T5.10C) will be placed west of the proposed SFR, <br />at least 10 feet from the proposed retaining wall and home foundation. This perforated <br />stubout ..will tightline runoff :to ..a. proposed. catch ..basin to be .installed to the city <br />stormwater system within Tulalip Ave to the west. <br />BMP's for other hard surfaces: <br />1. Sheet Flow Dispersion: <br />A total of approximately 722 sf of PGIS is proposed for this project. The runoff <br />generated by the driveway surfaces will be dispersed via sheet flow dispersion. As per <br />BMP 12.70, a minimum 10 ft flow path over amended or existing vegetated soils, <br />meeting ..BMP .42,20-will-be .provided -for.. the PGJS...runoff before the dispersed sheet <br />flows leave the site, are collected by any critical area or pass over any additional hard <br />surfaces. <br />Finally, any area of the site not proposed to be covered. with impervious .surfaces but <br />graded or compacted will be treated to meet the criteria for BMP 12.20. Please refer to <br />the engineering plans. for basic soil amendment notes. <br />Kikuchi SFR 21-0104 <br />September 2021 Page 9 <br />