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The Sammamish River Outfall Retrofit Project at Wilmot Gateway Park involved the installation of water quality treatment filters in an existing stormwater vault. This provides for the treatment of stormwater runnoff from a 52-acre urban drainage basin in the City of Woodinville's Downtown core.

The project was partially funded by the State of Washington Department of Ecology's FY2011 Stormwater Retrofit and LID Competitive Grant Program.

Storm Water Vault
The storm water vault is a large concrete box that lies below the ground. When it rains or snows, some of that precipitation flows toward the Sammamish River as stormwater runoff carrying pollutants to the river. This vault contains filters that help clean that water.

How Does it Work?
The filter contain multiple layers of silica sand encapsulated in geotextile fabric. During rainstorms, runoff is directed toward catch basins and pipes that flow into this large vault. Sediment, along with pollutants, is removed as stormwater passes through the filters. Dislodged sediment falls to the base of the vault where it can be removed by a vacuum truck. The filter cartridges are routinely checked and are replaced when they meet thier service life.

Why Does Stormwater Runoff Need Cleaning?
Precipitation that does not soak into the ground, such as rain falling on pavement, becomes stormwater runoff. Stormwater runoff contains pollutants such as phosphorus, oil and grease, fecal coliform bacteria, and metals such as copper and zinc, which are harmful to aquatic organisms and fish. Many of these pollutants are attached to sediment―sand and silt particles that transport the pollutants as the sediment is washed into streams and rivers. The filters in this vault remove almost 90% of the sediment that is suspended in the stormwater runoff.

How this Helps Our Environment?
Many people think that most water pollution is caused by factories dumping pollutants from a pipe. However non point source pollution―contained in stormwater runoff from agricultural fields, lawns, roads, and parking lots is actually more of an environmental problem than factories.

The Sammamish River is home to chinook, coho, sockeye, and kokanee salmon, as well as steelhead and cutthroat trout. The river provides a critical migratory corridor for juvenile and adult salmon. Great blue heron, various ducks, geese, and beavers also use the river habitat. The Sammamish River flows from Lake Sammamish through Redmond, Woodinville, and Bothell to Lake Washington in Kenmore and has a drainage basin covering 170 square miles. Clean water is essential to the health of the entire river ecosystem from the smallest to the largest organisms.

Additional information:

Public Works Storm Water Management Plan

Department of Ecology Stormwater Grant

How the filters work Video  

On-site Interpretive Panel 


City Contact: Project Manager, 425.489.2700, ext. 2295

Sammamish River Outfall Project Archived Documents



Page last modified: March 28, 2018

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