Providing a Reliable CSO Outfall for Decades to Come

Located on the shore of Lake Washington, Chinook Beach Park provides crucial habitat for juvenile salmon. However, an eroding slope and crumbling 40-foot-long timber bulkhead not only threatened access to the scenic viewpoint at the park entrance, but undermined a 36-inch-diameter combined sewer overflow outfall and allowed pipe bedding to migrate into the lake. 

Habitat restoration and conservation work have been completed at the park over the last 15 years, and the Water Resource Inventory Area 8 Salmon Recovery Plan designated this Lake Washington shoreline south of Seward Park as an area of significance to juvenile salmon coming into the lake from the Cedar River. A gravel beach was created in 2004, and ongoing invasive species removal and bank re-vegetation have been instrumental to juvenile salmonid life in the Lake Washington ecosystem near the park. 

Using innovative ways to reduce the construction duration, several key materials were acquired in advance, such as sheet piles for the new bulkhead, the 36-inch concrete pipe, a precast concrete vault and a fish-exclusion Tideflex check valve. These innovations saved months of schedule and allowed construction to be completed in just 90 working days following notice to proceed.

The project team overcame a steep, narrow driveway that needed to remain open, a shoreline location and a limited construction window to provide a cost-effective, structurally sound bulkhead and lakefront overlook that improved the lake habitat. Further, the CSO Outfall 171 project restored the integrity of the pipe, removed the rotten timber bulkhead, installed shoreline revetment to eliminate future scour, filled in-ground voids, re-paved the scenic overlook area, and provided new fish habitat along the structure.