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Project update

King County is working closely with Seattle Public Utilities to coordinate our projects and complement our community engagement efforts. To learn more, visit the South Park Drainage Improvements project web page .

Summer 2021

Be RainWise! You can help us reduce CSOs into the Duwamish River by participating in RainWise, a rebate program that pays property owners to install rain gardens and cisterns on private property. RainWise has been available in South Park and Highland Park since 2013. Hear from Highland Park and South Park neighbors who have participated in the RainWise program here .

help reduce CSOs, visit RainWise

King County has determined that the best technical solution for this project is to build an underground storage tank to store flows during heavy storms until we can put the wastewater back in the pipes for treatment. Learn more about our process for finding the right solution here .

Project need

Like many cities around the country, older parts of King County’s sewer system use the same set of pipes to carry both sewage and stormwater to a treatment plant. During storms, the pipes can fill with stormwater that runs off roofs, driveways and streets. When the system is overwhelmed, it is designed to overflow. These overflows are called Combined Sewer Overflows, or CSOs.

To keep the sewer system working and to prevent sewer backups, the excess water and sewage is released into our local water bodies through CSOs. However, CSOs pose a risk to public health and the environment. Over the past several years, King County has been planning a project to reduce CSOs from the South Park drainage basin into the Duwamish River. Our requirement is to reduce CSOs to no more than one overflow per year on average.

Project description

Every drainage basin is unique, so over the last year we’ve been focused on learning the specifics of South Park, including studying soils, the groundwater table and how redevelopment has influenced the way water flows in the neighborhood.

We looked at a range of options to reduce CSOs in South Park, including:


Using plants, trees and soil to soak up water is called green stormwater infrastructure (GSI). GSI techniques could include roadside rain gardens or permeable pavement that allows stormwater to soak through to the ground below.


Traditional infrastructure solutions use pipes and underground storage tanks to store wastewater. Tanks and pipes hold extra sewage and stormwater until there is room in the pipes again. If a storage pipe or tank is selected, that project could take place outside of South Park.


A separated stormwater system consisting of two pipes: one that carries stormwater and one that carries sewage from homes and businesses. If a separated system is selected, it would include the design and installation of new pipes underground.

The project team considered technical needs, operation and maintenance requirements, land use and permitting, community impacts and cost. We have determined that an underground storage tank is the most effective solution to minimize CSOs in South Park. We are still working out details on location, but we anticipate that construction will take place near the 1st Avenue South Bridge.

Some of the benefits of a storage tank include:

  • Building a storage tank allows us minimize construction impacts in residential areas of South Park
  • Most storage tank construction will take place off the road, so we can keep traffic moving
  • Lowest construction and operations cost of the options studied

Project schedule

Project schedule 


Contact us

Contact Kristine Cramer, community services lead at:



Project location

West Duwamish CSO control map

Back to the capital projects overview map .

Combined sewer systemWhy does sewage overflow on the rainiest days?

Rain garden planting event at Highland Park Improvement ClubRain garden planting event at Highland Park Improvement Club  

New rain garden at the corner of 12th Avenue South and South Southern Street in South ParkNew rain garden at the corner of 12th Avenue South and South Southern Street in South Park