Environmental and Regulatory Compliance

SR 520 Eastside Transit and HOV

Location: Bellevue, Washington

The SR 520 Eastside Transit and HOV project was a three-mile-long roadway improvement project designed to reduce transit and high occupancy vehicle (HOV) travel times. The project reconstructed SR 520 and all interchanges and overcrossings from Lake Washington to I-405. DEA led portions of the roadway, structures, and drainage design. DEA also provided survey, traffic control design, and utility design services.

One of DEA’s roles was to prepare permit modifications and final design for all of the fish-passable culverts, wetland mitigation, and stream relocation/enhancements. This included improving fish passage at the SR 520 crossing of Fairweather Creek, Cozy Cove Creek, and Yarrow Creek and its tributaries by replacing existing barriers to fish with eight fish-passage structures. DEA provided innovation throughout the design process to reduce the project’s net culverted length by more than 300 feet through the reduction of skewed alignments and the combination of the culverts. In addition to culvert replacements, nearly 3,000 feet of stream channel were restored.

Restoration included daylighting some stream sections and reconstructing the bed and banks, increasing sinuosity, improving connectivity to the floodplain and wetland, and placing stream gravel and large wood debris. Included in the restoration were hydraulic and hydrologic modeling, designing and implementing temporary stream diversions and stream bypass systems, preparing the phasing plan for completing stream construction within in-water work windows, and protecting sensitive areas.

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