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January 25, 2016
Project: Manchester Stormwater Park
Client: Kitsap County
When Parametrix was hired to replace an aging and undersized stormwater outfall pipe on the beach at Pomeroy Park in Manchester, Kitsap County, it didn’t seem like it would be remarkable feat.
But Kitsap County saw an opportunity not only to repair something old, but to make the solution even more creative and useful. With Parametrix’s engineering expertise, the county built the brand new Manchester Stormwater Park, which not only cleans up polluted stormwater in an environmentally friendly way, but also mitigates what has been devastating flooding in the area.
Manchester Stormwater Park is the first of its kind in the Puget Sound area, and one of only a few such combined treatment/recreation parks in the nation. The project site was once an abandoned brownfield a former gas station but through smart new engineering ideas and techniques, it now collects runoff from up to 100 acres of developed land, treats it using soil and plants, and discharges it gently and evenly onto the beach and back into Puget Sound.
A primary design feature of the new park is a spiral rain garden made of bioretention soil mix and plants that intercepts low flows from groundwater and light storms, then distributes the water via valves to alternating zones of the rain garden for treatment. Because the rain garden intercepts the low flows, the life of the more expensive engineered treatment media in the treatment cells will be extended and required maintenance reduced.
The new stormwater system is designed to treat flow magnitudes exceeding 2,000 gallons per minute, removing more than 90 percent of pollutants. An estimated 100,000 pounds of contaminated suspended solids will be cleansed from the upstream stormwater runoff over the next 10 to 20 years. The new park now creates a physical and aesthetic connection between the Manchester Community Center and Pomeroy Park at the community’s waterfront, serving as a setting for concerts, farmers markets, outdoor movies and Manchester Days celebrations.
Andrew Nelson, Kitsap County’s Public Works director, wrote that “Parametrix embraced the innovative nature of the stormwater park concept and was able to come up with numerous features such as the ‘spiral rain garden’ that enhanced the functionality of the facility and also added to the aesthetic appeal and community acceptance.”