Widening SR 202 from SR 520 in Redmond to Sahalee Way in rural King County presented many environmental opportunities for our engineers. The road runs through wetlands, flood plains and even an ancient land slide.
We are proud of our efforts to be good neighbors and environmental stewards. We want to leave the environment in a better and more stable condition than we found it.
Now that we are done, we have improved drainage, created new and improved wetlands, and built retaining walls to prevent landslides.
Wetland Mitigation
WSDOT has adopted a goal of "no-net-loss" of wetland acreage and function. In that effort, this project:
- Creates three flood plain and/or wetland mitigation sites including a 16-acre wetland at the corner of SR 202 and Sahalee Way to make up for land we have to displace to widen the highway.
- The first site (called the Raab site), located between 188th and 196th Avenues NE, addresses flood control. Crews excavated material in this area to ensure that, in the event of a flood, the elevation of the creek does not rise and create flooding downstream.
- The second site, at Sahalee Way, provides most of the flood volume storage needed for mitigation of the entire SR 202 widening project. A channel running through this site connects a small tributary to Evans Creek at the northwest corner of the site.
- The third mitigation site is located at 196th Avenue NE, became a backwater channel for the creek.
- Crews built 22 retaining walls totaling roughly 4,000 square feet to help protect Evans Creek, wetlands, floodplains, and properties along the SR 202 corridor.
This is the what the area looked like when we first got started. We cleared out a dairy farm and loads of trash and began the long task of clearing and installing native plants and grasses at the corner of SR 202 and Sahalee Way.
Crews constructed a shady area for fish and other wildlife along Evans Creek in the wetland area in Happy Valley.
Between November 2006 and March 2007 crews planted 37,000 plants and several hundred trees.
The site is now home to several varieties of wildlife.
Improved water quality and erosion prevention
Roadway runoff is an unacceptable source of river and area stream pollution.
- Crews improved how water from the roadway is retained, treated and released back into ground by building two underground wet vaults, a pond for water detention and quality treatment and another pond just for detention.
- We installed two bioswales (landscape structures designed to remove silt and pollution from runoff water).
- Crews built 3/4 mile of Ecology Embankment, a small grass filter strip followed by a trench backfilled with gravel and other materials. The Ecology Embankment is an experimental water quality treatment that is, basically, a linear storm water treatment facility incorporated into the road embankment rather than a separate pond or wet vault.
Reduced noise
- Noise walls were built through a half-mile stretch of SR 202.