10521 19TH AVE SE 2022-05-23 - Laserfiche WebLink

PUBLIC WORKS downstream flow control facility will not cause backwater conditions or otherwise impede the proper function of the StreamFilter. • MR #7 — Flow Control o Modeling to size the detention tank does not appear to account for stormwater runoff from the driveway that bypasses the tank. The modeling must properly account for both tributary drainage areas and bypass areas, and the Drainage Report must also clearly document these areas and how they were accounted for in the sizing of all facilities. o Document the required horizontal and vertical setback standards for the detention tank and the actual setbacks provided in the proposed plans. o Section 3.2.2, Vol. III of the SWMM requires calculations to demonstrate stability of the tank against floatation. Please provide those calculations (i.e., buoyancy calculations) and a narrative discussion of the methods, assumptions, and resulting design impacts in the Drainage Report. o Document the access requirements in Section 3.2.2, Vol. III of the SWMM and how those requirements are met by the proposed designs. o The proposed designs include a 6-inch sedimentation depth between the detention tank and access pipe. This sedimentation depth is inconsistent with Standard Drawing 4418. o Outlet structure — See comments below regarding Sheet 6 of 7. • MR #8 — Wetlands Protection o Address MR #8 in the Drainage Report. • MR #9 — Operation and Maintenance o Provide an Operation and Maintenance (O&M) plan per Section 2.5.9, Vol. I of the SWMM, outlining the party (or parties) responsible for maintenance and operation. Specify that a copy of the O&M plan shall be retained on -site or within reasonable access to the site and shall be transferred with the property to the new owner. Refer to the above -referenced section of the SWMM for additional detailed requirements. o The O&M manual shall be submitted as a separately bound document for both the property owner's use and for the City's records. o Include reference to the 2013 Guidance Document: Western Washington Low Impact Development Operation and Maintenance developed by Washington State Department of Ecology for LID facilities added to satisfy MR #5, as appropriate based on above comments. o Provide the latest available O&M requirements and guidance by the manufacturer of the selected water quality treatment BMP. Conveyance System Design, o Document the conveyance design standards that were used as the basis of design in the Drainage Report. o Provide sizing calculations performed, including narrative discussion of methods, assumptions, and results and model output. Also include discussion of the hydraulic grade line through the water quality treatment and flow Page 4 of 6