11108 EVERGREEN WAY BASE FILE 2018-01-01 MF Import

pue�ic w�irs MEMORANDUM T�� AI Locke/Gordon Witcher FROM: Jane Zimmern�an DATE: July 19, 1996 SUBdECT: Drainage Calculation Review, Evergreen Way Retail Cenur, 112th and Evergreen Way I have the foilowing comments on the drainage calculations for the subject project: With respect to the wetpond calculadons, the following two methods aze acceptable for detecmining che wetpond design volume: 1) multiply the azea of impervious surface in the contributing basin by the precipitation associated with the design storm (0.99 inches), or 2) deternrine ffie total volume of stormwater runoff from contributing areas , (unpervious �pervious) using the Santa Bazbara Urban Hydrograph method or compazable method. The drainage calculations deternuned the wetpond volume from the storm hydrograph generated using SBUH and only the impervious surface area. The control structure must ensure that storn►water detendon release rates do not exceed allowable release rates� that the detained vo:ume in the pond reaches or exceeds the wetpond design volume for the wetpond design storm. This requirement usually resuits in a multiple oriFce/notched weir arrangem�nt. Orifice calculations must be submitud showing that the control structure is meeting both these requirements. City standards allow a minimum orifice size of 1 inch. This project eliminates the temporary pond provided to detain runoff from the previous construction of the temporary access road. Stormwater runoff from the entire access road must be collected and treated/detained in the stormwater facilities for this site, including that portion of the access road located along the north property li� of this parcel and the Washington Credit Union parcel. The design of the stormwater detendon facilities on Uris site must account for the previous change in conditions to the gravel/impervious surface of the access road. Therefore, when deuntion is calculated for this project, the ground cover that existed beforc the access road was constructed must be used in the detention volume/release calculations. If the applicant hasn't already done so, I would recommend that the Depamnent of Fish and Wildlife be contacted regarding their HPA requirements for stormwater discharges from project sites in the Swamp Creek drainage basin.