Reducing CSOs to Protect Public Health and the Environment

The project included siting and design of combined sewer overflow control facilities to meet Washington State Department of Ecology's regulatory requirements for the Windermere Basin by 2020. HDR developed over 20 CSO reduction alternatives, including pipe and tank storage facilities, deep tunnels, pump stations and conveyance systems. We evaluated alternatives using Seattle Public Utilities’s Triple Bottom Line analysis, which includes economic, environmental, and social impacts and risks to determine the preferred alternative. We designed the preferred alternative with a construction cost of $22 million.

The preferred alternative included a 2.05 million gallon CSO storage tank within close proximity to Magnuson Park. Other design components included 2,250 feet of conveyance and associated controls, and mechanical and odor control equipment. The design required coordination between multiple City Departments (Public Utilities, Department of Transportation, Department of Planning and Development, Parks and Recreation, and City Light) and seven subconsultants.

Key Project Characteristics

• Developed over 20 CSO reduction alternatives including storage facilities, deep tunnels, pump stations, and conveyance systems.
• Green Stormwater Infrastructure design using bioretention planters in street right-of-way.
• Extensive public outreach through open houses in dense urban residential neighborhood. Community outreach in diverse multilingual communities.
• Street Improvement Permit required for conveyance piping.