902 PINE ST NORTH SOUND BEHAVIORAL HEALTH TREATMENT CENTER 2025-11-04

i 4.4 Flow Control Flow control is required to match developed discharge durations to the existing condition durations for the range of pre-developed discharge rates from 50 percent of the 2-year peak flow up to the full 50 year peak flow. The project site is achieving the flow control standard implementing OSM methods, as follows: • Reducing impervious percent from existing to proposed. • Converting existing lawn to pasture by providing BMP T12.20 Post-Construction Soil Quality and Depth for all proposed landscaping. • Providing bioretention cells, which attenuate flows, allow for biological processes, provide evapotranspiration, and provide some infiltration. Refer to Table 1 above for inputs into the MGSFlood model. Refer to Appendices A-2 and A-3 for delineation of areas. Refer to Appendix C-1 for flow control calculations showing that the project meets the flow control standard at the point of compliance. 4.5 Stormwater Conveyance Analysis and Design Conveyance analysis and design is performed using the Autodesk Storm and Sanitary Analysis and the outputs from the MGSFlood model. Delineation of subbasins and modeling results are provided in Appendix D-1 and D-2. The system conveys the 25-year event and contains the 100- year event. 5.0 Stormwater Pollution Prevention The project does not require a Construction Stormwater General Permit (CSGP)from the DOE because the project site is less than 1 acre. The contractor will provide temporary BMPs per the Construction Stormwater Pollution Prevention Plan (CSWPPP) report and engineering drawings. A separate CSWPPP report will be provided in the final report. 5.1 Source Control of Pollution BMPs In addition to the temporary construction BMPs outlined in the CSWPPP report, the project will provide the following permanent Source Control of Pollution BMPs per Volume IV of the City Manual. Source Control BMPs: • Landscaping and Lawn/Vegetation Management. • Maintenance of Stormwater Drainage and Treatment Systems. • Parking and Storage of Vehicles and Equipment. • Roof/Building Drains at Manufacturing and Commercial Buildings 6.0 Special Reports and Studies Refer to Appendix B-1 for the following information: • Geotechnical Report—North Puget Sound Behavioral Health—Treatment Center Addition by GeoTest Services, Inc., dated November 16, 2018 Stormwater Site Plan Report North Sound Behavioral Health Treatment CMOS AHBL No.2170857.10