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Jun
30

Mouth of Duwamish Wet Weather Facilities

Learn about King County’s efforts to develop wet weather facilities to control combined sewer overflows in the area of the mouth of the Duwamish River.

JUNE 2024 UPDATE

Our online open house is live!

Visit our online open house to explore information about our plans for wet weather facilities in the SODO neighborhood and share your insights by completing our survey: publicinput.com/mouthofduwamish

  • Haga clic en esta jornada de puertas abiertas en línea para obtener más información sobre los planes para las Instalaciones para Clima Húmedo de la Desembocadura de Duwamish en el vecindario de SODO: publicinput.com/mouthofduwamish-es
  • 点击此在线开放日，了解有关南市区中心 (SODO) 的 Mouth of Duwamish Wet Weather Facilities 计划的更多信息: publicinput.com/mouthofduwamish-zh-cn
  • 點選此線上開放日，瞭解有關南市區中心 (SODO) 的 Mouth of Duwamish Wet Weather Facilities 計劃的更多資訊: publicinput.com/mouthofduwamish-zh-tw
  • Nhấp vào sự kiện tham quan tự do trực tuyến này để tìm hiểu thêm về kế hoạch cho Các Cơ Sở Xử Lý Nước Khi Có Mưa Lớn tại Cửa Sông Duwamish ở khu vực Trung Tâm Thành Phố Phía Nam (SODO): publicinput.com/mouthofduwamish-vi
  • ចុចតាមរយៈសិក្ខាសាលាបើកចំហតាមអនឡាញនេះ ដើម្បីស្វែងយល់បន្ថែមអំពីគម្រោងសម្រាបបរិក្ខារគ្រប់គ្រងអាកាសធាតុសើមនៅមាត់ទន្លេ Duwamish ក្នុងទីប្រជុំជនប៉ែកខាងត្បូង (SODO)។: publicinput.com/mouthofduwamish-km

Looking for other ways to participate?

Register for one of our virtual public meetings later this month:

Find our team at the community event below:

As we progress through planning, design, and construction, we will continue to provide opportunities for you to ask questions and share thoughts and concerns. For project updates, join our email list using the link in the “stay informed” section of this webpage or by scanning the QR code below.

Can’t make it to one of our virtual or in-person events? Contact Bibiana Ocheke-Ameh at bibiana.ocheke-ameh@kingcounty.gov or 206-477-5604 with questions or to request a briefing for your organization.

Previous updates

King County is working on an effort called the Mouth of Duwamish Wet Weather Facilities that will reduce overflows from 5 combined sewer outfalls where the Duwamish River meets Elliott Bay.

We are evaluating options that control combined sewer overflows (CSOs) to comply with environmental laws. The CSOs were designed many decades ago to be overflow relief points in our sewer system during periods of heavy rain. Today, to better protect local water bodies, King County is investing in solutions to limit the number of overflows that occur and to ensure we are meeting regulatory requirements to protect water quality.

What we’re doing

King County is working to control 5 CSO outfalls in the area of the mouth of the Duwamish River. The outfalls (called Chelan, Hanford #2, Lander, Kingdome, and King Street) are located in the east and west waterways of the Duwamish River on both sides of Harbor Island and along the Seattle shoreline of Elliott Bay in the SODO neighborhood. This work fulfills legal requirements (called a consent decree) we have with the U.S. Environmental Protection Agency and the Washington State Department of Ecology.

King County has made great progress in controlling CSO outfalls in Puget Sound and other local water bodies. As of 2023, we've controlled 18 outfalls to reduce overflows to reduce pollution into local water bodies. But our work is not done yet. Controlling CSO outfalls at the mouth of the Duwamish River represents a large portion of the remaining CSO outfalls we strive to control.

The Mouth of Duwamish Wet Weather Facilities is one of the most complex efforts we will undertake. The effort will help King County be a good environmental steward by improving water quality. We must control CSO overflows and limit pollution to Duwamish River and Elliott Bay to do our part in creating healthy water bodies where communities and wildlife thrive.

About combined systems and CSOs 

Much of Seattle was built using a combined sewer system. Combined sewer systems carry sewage and stormwater in the same pipe. They prevent sewer backups in homes, businesses, and streets during heavy rain. When heavy rains fill the pipes, CSO outfalls release stormwater and sewage into local water bodies.

Today, King County is modernizing a system that’s been in place for more than a century to ensure sustainable operations into the future by investing in more wet weather facilities. Since our CSO Control Program began in the 1980s, we’ve reduced the average CSO discharge volume from an estimated 2.3 billion gallons to about 1 billion gallons per year. Controlling CSO outfalls at the mouth of the Duwamish River represents a large portion of the remaining CSO outfalls we strive to control.

Key terms

Stormwater is the rain that falls on our roofs and streets and flows into storm drains. The water picks up what it touches, like oil from the road, yard chemicals, and pet waste. Then the polluted water runs off into local bodies of water — like the Duwamish River.

CSO outfalls are relief points designed in combined sewer systems that carry sewage and stormwater in the same pipe. They prevent sewer backups in homes, businesses, and streets during heavy rain. When heavy rain fills the pipes, CSO outfalls release sewage and stormwater into rivers, lakes, or Puget Sound.

Wet weather facilities are designed to treat a combination of stormwater and wastewater that fills pipes to overcapacity during periods of heavy rain.

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