COSTCO STORMWATER FACILITY SIZING
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Client: Costco Wholesale, Issaquah, WA

Costco's development of a new store in south Snohomish County, just north of Woodinville, required innovative strategies and analysis to protect adjacent Little Bear Creek. The Costco underground stormwater storage facilities reduce peak flows in Little Bear Creek.

The Costco store site is located on 14.38 acres between SR 522 and Highway 9. An additional 2.35 acres of Highway 9 improvements was planned along the store's frontage. Drainage from the site crosses SR 522 in WSDOT culverts and enters Little Bear Creek. The WSDOT permit required the Costco stormwater system to meet Department of Ecology flow control standards.

Doug Beyerlein used WWHM to size the two stormwater systems for Costco. On-site stormwater runoff (from the store and parking lot drainage) are routed to an underground storage facility consisting of 5240 linear feet of 96-inch diameter pipe (6.04 ac-ft of storage). The runoff from off-site Highway 9 improvements are directed to a separate underground storage system with 1140 linear feet of 96-inch pipe (1.31 ac-ft of storage). Both systems include 6 inches of dead storage for initial water quality treatment. Additional water quality treatment will be provided by Stormwater Management, Inc.'s Stormfilter units.

The stormwater runoff impact to Little Bear Creek was analyzed using HSPF and the Little Bear Creek watershed model developed by Mr. Beyerlein for King County. The proposed Costco land use conditions were modeled and compared with the existing land use (a warehouse facility and adjacent gravel auto yard). Stormwater runoff is reduced from 23 cfs to less than 1 cfs for the 100-year flood. Little Bear Creek peak flows are reduced by approximately 1%.