3535 RIVERFRONT BLVD 2023-02-21 - Laserfiche WebLink

3. STORMWATER MANAGEMENT GUIDELINES Stormwater design was based on Department of Ecology's 2014 Stormwater Management Manual for Western Washington (SWMMWW). The project site has no existing impervious coverage and the proposed development results in more than 5,000 square feet of impervious surface. Therefore, as per Figure 2.4.1 in Volume 1 of the SWMMWW, all of the Minimum Requirements apply to the new impervious surfaces of the project. • Minimum Requirement 1:Preparation of Stormwater Site Plans Stormwater Site Plans were prepared to meet the requirements of the 2014 SWMMWW. Refer to Appendix A of the report for the Drainage Plans. • Minimum Requirement 2: Construction Stormwater Pollution Prevention Plan (SWPPP) Since the proposed project adds more than 2,000 square feet of new impervious surface, a Construction Stormwater Pollution Prevention Plan (SWPPP) in accordance with the 2014 SWMMWW,will be required prior to construction. The SWPPP will be prepared prior to or after obtaining permit approval but will be in place before beginning construction. Erosion and sedimentation that result from construction is addressed through the implementation of Temporary Erosion and Sedimentation Control (TESC)best management practices(BMPs). • Minimum Requirement 3: Source Control of Pollution Steps will be taken to adopt reasonable source control BMPs during construction and after construction. The source control BMPs for use during construction shall be detailed in the SWPPP. Permanent source control BMPs such as street sweeping, maintenance of stormwater conveyance and treatment systems, and landscaping shall be employed by the City of Everett. • Minimum Requirement 4:Preservation of Natural Drainage Systems and Outfalls • In the existing condition, runoff from the site sheets flows to the east, into the Snohomish River. However,the riverbank slope was found to be highly unstable. Therefore,to prevent erosion of the unstable river bank, runoff will be collected via trench and pipe conveyance system.and routed to SDMH #10008. Flow that is routed to the SDMH discharges to the city's storm outfall. • Minimum Requirement 5:On-site Stormwater Management The project discharges to the Snohomish River which is a flow control exempt water body in accordance with Section 2.5.7 and Appendix I-E in Volume 1 of the SWMMWW. However, if feasible, the project must still implement infiltration and dispersion BMPs as per Section 2.5.5 in Volume 1 of the SWMMWW. Although infiltration is not an option because of poor soils in the area, surface dispersion has been employed prior to collection in underdrains as part of On-Site Stormwater Management on this project. The runoff collected from the impervious surfaces of the project site is collected and routed to bioretention facilities in the parking lot then conveyed to SDMH#10008 which discharges to the city's storm outfall. The runoff collected from the trail and surrounding landscape areas east of the project site will be collected by catch basins located at low points or by trench system, conveyed to SDMH#10008, and discharged to the city's storm outflow. KPFF Consulting Engineers 2