4320 THOMSON AVE 2023-08-10 - Laserfiche WebLink

Thomson Residence Stormwater Site Plan Report March 5, 2021 Section IV, Page 1 Section IV — Permanent Stormwater Control Plan Section IV Summary: Narrative On -site Stormwater Management Flow Control Conveyance Sizing WWHM Report 2<3tg The project triggers Minimum Requirements #1-5 of the 22 6E Manual and the Combined Sewer Requirements of the Everett Manual. This section addresses SWMMWW MRS #5: On -site Stormwater Management and Combined Sewer Requirements for Flow Control and Conveyance Sizing. On -site Stormwater Management To meet MR #5, the project must either use On -site Stormwater Management BMPs from List #1 or demonstrate compliance with the LID performance standard per Section 2.5.5 of Volume I of the 2014 DOE Manual. The site will use On -site Stormwater Management BMPs from List #1. A detention system with a flow control structure will be used for all hard surface runoff (see WWHM report later in the section for detention pipe design). The BMPs from List #1 were evaluated for feasibility as follows: BMPs for Lawn and Landscaped Areas: 1. Post -Construction Soil Quality and Depth in accordance with BMP T5.13 in Volume V, Chapter 5 of the Drainage Manual: This BMP will be used for all disturbed landscaped areas. BMPs for Roofs: 1. Full dispersion in accordance with BMP T5.30 is infeasible because 65% of the developed site will not be in a forested or native condition. 2. Downspout Full Infiltration Systems in accordance with BMP T5.10A are infeasible because there is insufficient horizontal space on -site with building and property line setbacks. 3. Bioretention facilities are infeasible because there is insufficient horizontal space on -site to locate a facility while maintaining the necessary building and property line setbacks. 4. Downspout Dispersion Systems in accordance with BMP T5.1013 are Infeasible because the required lengths of vegetated flowpath cannot be met. 5. Perforated Stub -out Connections in accordance with BMP T5.10C are infeasible because this BMP would fail to meet the Flow Control Performance Standard. BMPs for Other Hard Surfaces: 1. Full dispersion in accordance with BMP T5.30 is infeasible because 65% of the developed site will not be in a forested or native condition. 2. Permeable pavement in accordance with BMP T5.15 is infeasible because the 9.5% downslope of the driveway could potentially compromise adjacent impervious pavements. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.7785536 www.cgengineering.com