9300 W MALL DR 2018-01-01 MF Import

Y TO: FROM: DATE: SUBJECT: Calculations: MEMORANDUM Gordon Wit� .er Jane 2immerman January 'L1, 1992 West Mall Village Apts. Plan Review, SEPA #186-89 Detention calculatio�� must be submitted, showing the required volume and allowauie release rate. Wetpond calculations must be submitted showing the total wet pond volume required. Orifice calculations must be submitted showing that the control structure is meeting both the detention and wetpond requirements. A multiple orifice system should be used. The design wetpond volume must be reached or exceeded for the two year, 24 hour storm. As a first guess, the primary orifice should be sized to release approximately the existing two year storm peak and the second orifice located at the elevation relating to the design wetpond volume. The two year, 24-hour storm should then be routed through the pond to verify that the wetpond design volume is being reached or exceeded for the design storm. Current City standards call for vegetated swales to be designed for the 2 year, 24 hour storm peak discharge, with higher flows diverted around the swale. If possible, the swales should be designed with a bypass system. The swale should then be designed per the following criteria for the 2 year, 24-hour storm: "n" = 0.12 max velocity = 1.5 feet per second max. depth = 0.5 feet If diversion is not feasible, the following additional design criteria will apply for the 25 year, 24-hour storm: "n" = 0.035 max velocity = 4.0 feet per second max. depth = 1.0 feet .