Mouth of Duwamish Wet Weather Facilities

Client: King County Wastewater Treatment Division

Environmental review and permit acquisition services for King County’s largest combined sewer overflow (CSO) reduction project to date.

Overview of Floyd|Snider Contributions

Environmental and permit lead for State Environmental Policy Act (SEPA), technical studies, and federal and state permit packages

Detailed permit acquisition strategy and work breakdown structure schedules

Comprehensive permit application packages (U.S. Army Corps of Engineers [USACE], Washington State Department of Ecology [Ecology], Washington Department of Fish and Wildlife [WDFW], Washington State Department of Natural Resources [WDNR])

Collaborative approach with King County, owner advisor, and teaming partners

Project Summary

King County is working to control five CSO outfalls in the area of the mouth of the Duwamish River.

The outfalls are located in the east and west waterways of the Duwamish River. The project will consist of building a new 228 million-gallons-per-day wet weather treatment station (WWTS) on a 9-acre site, and a new 7-million-gallon wet weather storage tank. The Mouth of Duwamish Wet Weather Facilities is the County’s largest CSO project to date and will prevent approximately 430 million gallons of polluted water from entering Elliott Bay and the Duwamish River per year on average. The project is being designed and implemented under a Consent Decree with Ecology, the U.S. Environmental Protection Agency, and the U.S. Department of Justice. The County’s CSO system is addressed in the County’s West Point Treatment Plant National Pollutant Discharge Elimination System (NPDES) permit and the new WWTS will be governed by this permit as well.

King County is working to control five CSO outfalls in the area of the mouth of the Duwamish River. The outfalls are located in the east and west waterways of the Duwamish River. The project will consist of building a new 228 million-gallons-per-day wet weather treatment station (WWTS) on a 9-acre site, and a new 7-million-gallon wet weather storage tank. The Mouth of Duwamish Wet Weather Facilities is the County’s largest CSO project to date and will prevent approximately 430 million gallons of polluted water from entering Elliott Bay and the Duwamish River per year on average. The project is being designed and implemented under a Consent Decree with Ecology, the U.S. Environmental Protection Agency, and the U.S. Department of Justice. The County’s CSO system is addressed in the County’s West Point Treatment Plant National Pollutant Discharge Elimination System (NPDES) permit and the new WWTS will be governed by this permit as well.

Our Approach

This includes supporting agency coordination and preparing SEPA analysis, permit acquisition strategies, and Joint Aquatic Resources Permit Application and other permit application packages. The project also included a mixing zone and reasonable potential analysis to support the new outfall and multiport diffuser and NPDES permit support, including Ecology negotiation support.

Floyd|Snider is leading the environmental review and providing federal and state permitting support to King County, including for USACE Section 404/10 Permit, Section 408 Approval; Federal Services Section 7 and Section 106 consultations; Ecology 401 Water Quality Certification and Construction Stormwater General Permits; WDFW Hydraulic Project Approval; and WDNR Aquatic Use Authorization. This includes supporting agency coordination and preparing SEPA analysis, permit acquisition strategies, and Joint Aquatic Resources Permit Application and other permit application packages. The project also included a mixing zone and reasonable potential analysis to support the new outfall and multiport diffuser and NPDES permit support, including Ecology negotiation support.

Project Team

Floyd|Snider is one of 22 subconsultants on the project team.

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