4918 24TH AVE W 2023-06-22 - Laserfiche WebLink

Terra Vista NW r i( Stormwater Management Report BMP T7.30 is infeasible as there is no location on the site where Bioretention can be reasonably designed on a slope less than 8%. Also, there is inadequate space on the site for bioretention. 5. Sheet Flow Dispersion in accordance with BMP T5.12: Sheet Flow Dispersion,or Concentrated Flow Dispersion in accordance with BMP T5.11: Concentrated Flow Dispersion BMP T5.12 is infeasible as a minimum 10-foot vegetated flow path per every 20 feet of contributing surface cannot be provided. Per the List 1 approach,the only BMP that is feasible for the roof area is BMP T5.10C: Perforated Stub- out Connection. The as-built from the original plat and recent survey show that a storm stub-out was provided to this lot from a stormwater detention system in the adjacent private street. This project proposes a tightline from the existing storm stub-out to the proposed Perforated Stub-out Connection. Stormwater runoff from the roof area will be conveyed to the existing detention system before release to the City's stormwater system. As there is no feasible BMP for the proposed driveway,the stormwater runoff from the driveway will be allowed to sheetflow into the adjacent private street and be collected into the existing stormwater detention system via existing catch basins before release to the City's stormwater system. BMP T5.13: Post-Construction Soil Quality and Depth BMP T5.13 is required as part of Minimum Requirement#5. Although the Contractor has the option of stockpiling existing topsoil material, it is anticipated that the Contractor will import topsoil material to meet the requirements of BMP T5.13. Upstream Analysis Stormwater from offsite areas does not flow through the area of disturbance,therefore there are no impacts from upstream areas. • Downstream Analysis Existing drainage patterns based on topographic contours, local drainage features, and the City's stormwater system, indicate the surface water from the site first drains to an onsite detention system. Runoff from the detention system is then conveyed to the City's stormwater system which converges with Narbeck Creek and eventually Puget Sound. This occurs within a single threshold discharge area (TDA). The local drainage features include a stormwater detention and control structure system that was installed under a previous version of drainage code to mitigate stormwater runoff for the plat of which this lot is part of. The proposed stormwater management system will take advantage of this existing system and its ability to attenuate the release of runoff prior to entering the City's stormwater system. There are no impacts to downstream areas. (210314)4900-3 Everett Page 8 May 19,2021