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� <br />Casper Creek <br />4/05/93 <br />Revised 6/04/93 <br />88-4555 <br />CRO <br />STORM WATER GIUALITY ENHANCEMENT SYSTEM <br />Baffle-Type Oil/Water Separator in series w/ a Vegetated Swale <br />A bafiie-type oil/v✓ater separator will be used in series with a vegetated swale will be used <br />for stormwater quality enhancement. The ofl/water separator and vegetated swale will <br />be designed per 1993 City of Everett Design and Construction Standards and <br />Specifications effective 4/15/93. Since the oil/water separator and biofiltration swale will <br />foliow detention, the design ;lov✓ rate will be based on the 10-year peak discharge for <br />existing conditions (see R/D Analysis & Design - Basin "B"). A ilow diversion manhoie <br />will be used to route all larger flows around the water quality enhancement system. The <br />proposed slope of the vegetated swale is 0.5 percent and the swale will be located in a <br />pessible high groundwater area so the vegetated s�vale will be designed as an emergent <br />swale. A 5.0 foot gravel path for maintenance access will be provided along one side of <br />the swale. <br />1. Vegetated Swale Desiqn: <br />Oad51e„ = 0.52 cfs <br />Bottom Width = 2.5' <br />Slope = 0.50% <br />"n" = 0.20 <br />m = 3 <br />_ _ _ _ _ > V = 0.27 fps < 0.90 ips maximum <br />y = 0.50' = 6.0" = 6.0" max for <br />emergent swales <br />Check residence time using a 150 lineal foot long vegetated swale. <br />V = L/T <br />T= L/V =(150 ft)(second/0.27 ft)(min/60 second) = 9.26 min > 5.0 minute min <br />A minimum 0.50' of freeboard is required above the design water suriace depth. <br />Therefore, the minimum total swale depth required is calculated as follows: <br />Total Depth = 2-year Flow Depth + Freeboard =(0.50'}+(0.50') = 1.0' <br />UseTotalSwaleDepth=l.0'<_________________________-__ <br />2. oilJWater Separator oesign: <br />Odes;a„ _(0.52 ft3/sec)(60 sec/min)(7.481 gal/ft3) = 233.41 gpm <br />Calculate the required separator volume to achieve a minimum 15 minute residence time. <br />(233.41 gallon/minute)(15 minutes; = 3,501.11 gallons <br />Therefore, use Utility Vault Co. oil/�vater separator vault �/814-SA ��dhich has a 4,200 <br />gallon capacity at 5'-0" �vater depth. <br />