Transportation | Water and Environment
SR 520 Eastside Transit and HOV Fish Passage Improvements
The SR 520 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 SR 520 corridor east of Lake Washington. 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.
DEA’s role was to prepare permit modifications and final design for all of the fish-passable culverts, wetland mitigation, and stream relocation/enhancements. This work 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. 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.
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