Mouth of Duwamish Program Management and Owner Advisory Services

Much of Seattle’s original sewer system was built as a combined sewer system. Controlling combined sewer overflow (CSO) outfalls at the mouth of the Duwamish River represents a large part of the remaining CSO outfalls that King County strives to control, ensuring no more than one untreated release per year over a 20-year average as required by Washington State law.

King County is modernizing its wastewater treatment system so that it can improve water quality while withstanding larger storms and other impacts from climate change.

The Mouth of Duwamish CSO Control Program is anticipated to be King County’s largest CSO control project, controlling up to eight CSO outfalls. This program will plan, design, and construct facilities to control five CSO outfalls located in the east and west waterways of the Duwamish River, including Chelan Avenue, Hanford #2, Lander Street, Kingdome, and King Street. Three Seattle Public Utility CSO outfalls, SPU 99, SPU 107, and SPU 111, are being evaluated for potential incorporation into the Mouth of Duwamish project.

King County is analyzing options to control overflows from these outfalls and meet regulatory and environmental requirements. These alternatives will likely include a combination of the following methods: store it, treat it, or move it.

This program is vital to the County’s compliance with the Clean Water Act and meeting its obligations in a 2013 Federal Consent Decree. It will also make major advances in equity and environmental quality for the area. King County promotes equity and social justice in planning, design, construction, and management of infrastructure projects so communities can thrive. The project includes work with community groups in the area to ensure that the project reflects neighborhood values.

Parametrix is providing program management and owner advisory services, including: 

  • Overall program management
  • Master program schedule development
  • Project delivery planning including assisting King County with alternative delivery contracting
  • Independent cost estimating and program cost estimating
  • Program equity planning
  • Preparation of bridging documents for future design-build contracts

As part of the owner’s representative team for this multi-decade separation of wastewater and stormwater systems, Parametrix is guiding the program through the Envision Sustainability process, ensuring design efforts are informed by proper climate science to determine design values, determining how to decarbonize the systems involved such as pump stations, and identifying how to decarbonize the construction process and the materials themselves.

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