Jan 06, 2020

Whispering Firs Stormwater Park

Kitsap County Public Works

The Kitsap County Public Works Department (KCPW) in Washington state has transformed a vacant former trailer park into a fully operational storm water facility to improve downstream water quality and hydrologic conditions in Clear Creek and Dyes Inlet. 

What at first was supposed to be a large storm water retention pond has been turned into a multi-benefit regional storm water retrofit facility called Whispering Firs that uses green treatment techniques to treat runoff in a way that meets current Washington State Department of Ecology water quality standards.

Project Manager Chris May said the facility treats total suspended solids (TSS), total petroleum hydrocarbons, metals and nutrients and reduces flows to Clear Creek, which is the main salmon-bearing stream in the north Dyes Inlet. 

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Coming off the successful implementation of the Manchester Stormwater Park, KCPW decided to use a similar retrofit design for Whispering Firs to expand it beyond just a retention pond. Both facilities now use Filterra filter media manufactured by Contech Engineered Solutions for their tree-box filter systems. The team said the engineered-soil mix provides enhanced water quality treatment at high-flow rates by using high-performance treatment cells.This allows for the treatment of large drainage areas – more than 100 acres each for this project. 

Keeping water quality treatment and flow control at the forefront of the plans, the team minimized water depth by using a pressure head so that dirty water would be intercepted from the basin and pushed into the treatment system through a closed conduit flow. In the end, all dirty water rises to the park surface at approximately the same elevation, which allows for shallow treatment cells, according to the team. Additionally, they created a flow control splitter configuration that isolates treatment cells.

“As with all retrofit projects, construction in areas with existing infrastructure are challenging due to engineering and site constraints, as well as utility conflicts,” May said. 

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The $4 million project treats 113 acres of high-use roadway and residential areas. It was made possible by a team of engineers, landscape architects and permitting officials and the Washington Department of Ecology, who provided $1.5 million in grant funding. 

The design choice allows the property to not only treat storm water to current standards but also provides public access. In addition to its four large, high-performance treatment cells, two bioretention cells and two wetland ponds, the facility has walking paths, picnic areas, bird and wildlife habitats and offers views of the Olympic Mountains. Additionally, native plants are located throughout the park that naturally cleanse the water. The plants are small now, but eventually, they will grow and should make the park look like a wetland.

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“This system will have direct positive benefits for Clear Creek, Dyes Inlet and Puget Sound,” May said. 

The team said the park conveyance system is designed to accommodate a future connection that would permit additional water from a separate developed drain basin with inadequate water quality treatment to enter the park and be treated. 

Overall, the team is most proud of the fact that they were able to provide water quality treatment at current Washington state standards for more than 100 acres of older development, while also providing a park-like amenity for the community. 

Project Year:
Contractor: Sound Excavation Inc.
Designers: Contech, N.L. Olson & Assoc. and Parametrix
Owner: Kitsap County
Location: Silverdale, Wash.
Cost: $4 million
Size: 113 acres
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