8925 AIRPORT RD MUKILTEO SCHOOL DISTRICT 2018-01-02 MF Import

of the separator, the applicant may wish to avoid a bypass system by designing both the separator and the swale for the 25 year storm peak flow. oil/water separator/qrit removal vault: City standards call for oil/water separator vaults to have a . / minimum design flow determined by using the rational formula V with a rainfall intensity of 0.5 inches per hour over the contributing area. The vault is required to provide a 15 minute detention time for the design flow. Highflow bypass piping is required £or the vault per City standards, unless, in this case, the vault is sized for the peak flow from the 25 year design flow. If a bypass is � preferred, the vault design flow should be reached at the point that the water elevation reaches the invert of the highflow bypass pipe. The flow through the separator should not exceed the maximum process flow (the largest flow the separator can handle befote accumulated oil begins to be flushed out) when the head needed to pass the 25 year storm is reached in the hiqhflow bypass pipe. Overflows would need to be split and half routed directly to each wetland. The following information is required for City review and approval of oil/water separators: 1) design flow calculations � 2) design inlet capacity calculations 3) orifice sizing calculations 4) oil/water separator sizing calculations 5) bypass sizing calculations (if applicable) 6) copy of site plan showing the cont;�buting area veqetated Swales: The C�ty classifies vegetated swales as either grass swales or emergent swales, based on the longitudinal slope of the � swale. The swale shawn on the site would be classified as i an emergent swale. All notes and callouts to the swale area should refer to the swale as an emergent swale. � Current City standards call for vegetated swales to be designed to provide a minimum 5 minute residence time for V, the peak discharge from the 2 year, 24-hour storm, with higher flows diverted around the swale. Developed site conditions need to be used in determining the s�ale de�ign flow. As discussed above, the high flow bypass can be avoided by designinq the swale to meet City design criteria for the 25 year flow instead of the 2 year flow.