905 80TH ST SW FORMOST FUJI 2024-12-03

5.0 PERMANENT STORMWATER CONTROL PLAN 5.1 Existing Site Hydrology Refer to the attached Existing Conditions plan sheet. The existing overall parcel area is 5.64 acres. The site was developed in 2016 which included the construction of a 43,550 square foot building with associated pavement, sidewalk, curb, and utility improvements. The topography of the site is generally moderate slope to the north. Elevations range from 585 to 552 feet. The project site drains to the north with runoff being collected by existing catch basins which discharge into the northern parcels stormwater conveyance system. The northern parcel stormwater conveyance system conveys runoff to an existing vortex separator as supplemental treatment and sediment removal prior to discharging to an existing stormwater detention and water quality pond before flowing into the City storm system (48") within Hardeson Rd. This City system flows northerly emptying into Merrill and Ring Creek, which in turn discharges into Possession Sound. 5.2 Developed Site Hydrology The developed site does not significantly deviate from the existing site hydrology. The proposed building expansion requires a portion of the existing on -site catch basins and underground stormwater conveyance system to be rerouted around the building expansion and tie back into the existing conveyance system. The minor increase in net impervious cover for this project will result in less than a 0.15 cfs increase in the 100-year flow frequency and will have no significant impacts to the hydrology of the site. 5.3 Low Impact Development Features This development triggers minimum requirements #1- #5. In order to meet Minimum Requirement #5 — On -site Stormwater Management, List #3 was evaluated for each surface to determine which LID BMPs could be used on this site. See below for feasibility analysis: • Lawn and Landscape Areas: o Post -Construction Soil Quality and Depth (BMP T5.13) This BMP is feasible and applicable to all disturbed soils which will be landscaped in the final condition. • Roof Areas: o Downspout Full Infiltration is infeasible due to soil composition and shallow underlayment of glacial till soils preventing the use of infiltration facilities. Refer to the geotechnical report for more information (see Appendix A). o Downspout Dispersion Systems of the stormwater from roof area is infeasible due to lack of available space for a native vegetation flowpath. The slow infiltration rate of the native soils is expected to cause ponding issues for yard areas if this BMP is used. o Perforated Stub -out Connections are infeasible due to groundwater separation not being achievable. • Other Hard Surfaces: 22723