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Manchester Stormwater Retrofit

Manchester, Washington


Kitsap County, WA


Parametrix Inc.


Northwest Cascade 


May 2015

Technical Description:

  • Filterra® Bioscape®

Manchester is a small community of about 4,600 people that is on an east-facing slope 400 feet above Puget Sound. The downtown area lies at the foot of this slope and is prone to seasonal flooding and ponding. The causes of these issues are high groundwater, an undersized storm drain system, and sheet flow from roadways. This runoff allows surface pollutants to enter Puget Sound which ultimately degrades the natural habitat.

Kitsap County received a grant from the WA Department of Ecology and funding from the Department of Public Works to address these issues. The funds were used to build an environmentally friendly, state-of-the-art stormwater treatment facility that would also serve as a park with community meeting space. The County worked with engineers from Parametrix to evaluate more than 30 potential retrofit options. Because the drainage basin selected drains directly to the Puget Sound, only water-quality treatment was required (no flow control). The goal for the project was to achieve full compliance with current stormwater volume treatment requirements for new development (91% treatment standard), as well as meeting enhanced water quality treatment objectives (i.e., removal of total suspended solids and dissolved metals).

A 0.5-acre parcel located just uphill from the beach outfall was selected as the site of the new water quality treatment facility. Because of the small size of the available parcel, the relatively large contributing drainage area, and the high level of water-quality treatment desired, a conventional pond, vault, or even sand-filter design would not meet treatment goals. This pushed the design team toward a more innovative approach, which integrated both treatment requirements and preferred ”green” stormwater solutions. Therefore, using Filterra Bioscape soon became the preferred treatment method. Filterra Bioscape is similar to bioretention in its function and application but has been optimized for high volume/flow treatment and high pollutant removal. Its small footprint allows it to be used on highly developed sites. The Filterra Bioscape configuration is very flexible and can be designed to integrate seamlessly with site amenities and aesthetics.

Polluted runoff enters the facility via conveyance piping. Water pressure forces the untreated stormwater up into 18 distribution channels that are positioned to uniformly and simultaneously deliver the water into all four Filterra Bioscape vaults that are located around the perimeter of the park. The Filterra Bioscape vaults contain Filterra media (a specific gradation of washed aggregate and organic material), mulch and plants that aid in the treatment process by uptake of pollutants and by mediating microbial degradation of other pollutants. These plants also provide aesthetic elements to the overall stormwater park design.

Treated water is collected in an underdrain system and subsequently discharged to the outfall at the beach. The system is designed to treat flows well exceeding 2,000 gallons per minute, equating to around 100 million gallons of polluted runoff treated annually. It is estimated that more than 100,000 pounds of contaminated suspended solids will be cleansed from the upstream stormwater runoff over the next 10 to 20 years. Additionally, the project was awarded “Project of the Year” in 2016 by the Washington Chapter of APWA, and the City of Manchester has a beautiful park in the center of downtown for the community to enjoy year-round. 

Technical Description:

  • Filterra® Bioscape®

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