705 80TH ST SW 2018-01-02 MF Import

Because there is no addition to the total impervious area on the loading dock area there will be no changes made to the existing runoff control system. Runoff from the parking area will be directed into a wet pond that will be located at the southeast corner of the project. This flow will be carried to the pond through a system of catch basins and pipes. An oillwater separator will be placed in a catch basin that will treat the water prior to entering the wet pond. The truck parking area will not flow through this catch basin but rather through a coalescing plate separator system. A bypass structure will be installed which will not allow flows greater than the 2 year flow rate to pass through the c.p.s. A control structure will be placed in a catch basin that will restrict flow leaving the pond. The control structure will have 2 orifices, the first will detain flow from the 2 year storm event and the second will detain flow from the larger storm events. Runoff from these storm events will be stored in the wet pond above the permanent pool elevation. The modeling program used was Hydra, Version 5.12 by Pizer, Inc. A continuous model of a Type ]A, 2411r. scorer was used. The computational method for the calculations is a modified (urban) Soil Conservation Service (SCS). The storm events modeled were 6 month (24 hr.), 2 year (24 hr.), 10 year (24 hr.), and 100 year (24 hr.). The storm Hyelographs are based on data found in the Department of Ecology Stormwater Management Manual for the Puget Sound Basin. The existing flows off the property are summarized in the following table: 0.08 c.f.s. 10 YR. EX 1 0.28 c.f.s. 100 YR. EX 0.63 c.f.s. (All flow amounts taken from link 1, of the attached Existing Storm Analysis, pages 18, 20, and 22) The allowable ofTsite flow for the two year storm event after re -development is half the existing two year storm event flow, or 0.04 cfs. The pond will also be designed such that the 10 and 100 year stone events release at or below existing levels. The wetpond will have a permanent pool volume equal to the volume of runoff from the 6 month storm event using developed conditions. This storage would be 5,138 cubic feet (link 9 page 16) The welpond will consist of 2 cells the open water cell and the wetland treatment cell. The open water cell will contain about 85% of the permanent pool volume at a depth of 4 feet. The welland treatment cell will contain the remaining portion of the permanent pool at a depth of 2 feet. An earthen baffle will be constnrcted to separate the two cells and it will have a top elevation equal to the permanent pool elevation. The outflow of the pond will be controlled via a control structure with circular orifices inside a catch basin. All calculations for sizing will Le the circular orifice formula: Q=CA 2gh Where Q=now, cfs C = discharge coefficient, 0.62 A = orifice cross -sectional area, sq. tl g= gravitational acceleration, 32.2 it/sec = h = liquid head above center oforifice, ft