SR 520 Eastside Transit and HOV Design-Build
Location: Bellevue, Washington
SR520 Eastside Transit and High-Occupancy Vehicle (HOV) Design-Build Project is a 3-mile-long roadway improvement project designed to reduce transit and HOV travel times and enhance travel time reliability, mobility, access, and safety for transit and HOVs in rapidly growing areas along the SR520 corridor east of Lake Washington. DEA’s role was to prepare permit modifications and final design for wetland mitigation and stream enhancements. This included improving fish passage at the SR520 crossing of Fairweather Creek, Cozy Cove Creek, and Yarrow Creek and its tributaries by replacing existing barriers to fish with seven fish passage structures. Through innovative design that eliminated unnecessary skewed alignments and by combining culverts, DEA was able to reduce the project’s net culvert length by more than 300 feet. In addition to culvert replacements, nearly 3,000 feet of stream channel was restored.
Restoration included daylighting some stream sections and reconstructing the streambed and banks, increasing sinuosity, improving connectivity to the floodplain and wetland, and placing stream gravel and large wood debris. Other major components of the project were hydraulic and hydrologic modeling, designing and implementing temporary stream diversions and bypass systems, developing a phasing plan for completing reconstruction within the in-water work windows, and protecting sensitive areas.