Thornton Confluence Restoration, Seattle Public Utilities, Seattle, WA
NSD led a five-firm team through the preliminary engineering and detailed design of the restoration of the confluence of the north and south branches of Thornton Creek, located in North Seattle on the most productive salmonid habitat in the City. The project site is surrounded by residential properties, municipal facilities and roadways, has multiple utilities crossing through the site including sewer mains, stormwater mains, gas and water and is located in a low-gradient area that was formerly wetland.
The 6-acre project, constructed in summer 2014, consists of the expansion of a 3-acre stormwater management facility, re-construction of over 1,000 ft. of channel and installation of a new 30-ft span culvert under an arterial roadway. Over 160 logs were installed within the channel to create spawning and rearing habitat for Chinook and coho salmon as well as resident cutthroat trout and peamouth. The channel was designed to allow large amounts of incoming sediment and debris to transport through the system and relieve localized flooding. The pond expansion was completed in 2013 and the stream was restored in 2014 at a combined cost of approximately $5.5 million.