NE 40th Street Stormwater Trunk Extension Redmond, Washington

Uphill of Lake Sammamish, the 40th Street stormwater trunk was originally built in the 1980s to intercept stormwater runoff from commercial properties and send it to the lower reach of Villa Marina Creek. The complete master plan includes a unique water quality facility. After years of erosion threatened water quality downstream, upgrades were designed to accommodate a 50-year flood event and redirect peak flows with the addition of a new outfall dissipation structure.

Diverting Peak Flows of Stormwater Runoff to Improve Water Quality and Erosion Control

In an area experiencing high growth, the City of Redmond aimed to put in place more sustainable stormwater management infrastructure and practices for future development. Stakeholder outreach provided a forum for feedback and built consensus around the project’s importance throughout development of the lake outfall plan. Upgrades to the trunkline includes over 1,400 linear feet of 42-inch HDPE pipe and 300 linear feet of 12-inch water line relocation. The design also accounts for the restoration of the roadway and sidewalk including signal modifications, the relocation of utilities. To divert damaging peak flows from Villa Marina Creek to the outfall at Lake Sammamish, an interactive flow splitter progressively increases peak flow rates as a new submerged channel is formed and shoreline plantings establish healthy root systems.