Vulnerability of King County, Washington Wastewater Treatment Facilities to Sea Level Rise

Kirsten Feifel Kathryn Braddock
Posted on: 3/10/2010 - Updated on: 11/24/2021

Posted by

Kirsten Feifel

Project Summary

Forty of King County’s 77 major wastewater facilities are located adjacent to tidally-influenced water bodies. Sea level rise due to thermal expansion and melting glaciers coupled with storm surge could make low-lying treatment facilities prone to flooding in the future. The county assessed the vulnerability of its wastewater treatment facilities using sea level rise projections, historical tidal data, and site elevation data. The report concludes that based upon current sea level rise projections, the probability of imminent damage to King County’s wastewater treatment facilities is low. However, more recent studies have found that saltwater intrusion is corroding wastewater infrastructure and investments are needed to better prepare the system.


King County covers more than 2,200 square miles and has a population of over 2.2 million people. Over 17 cities, 16 local sewer utilities, and one tribe rely upon King County wastewater treatment facilities to treat waters and thereby protect human health. The King County Wastewater Treatment Division manages 77 major wastewater treatment facilities, 40 of which are located adjacent to a tidally-influenced water body. Climate change is expected to raise sea levels through thermal expansion and increased freshwater input from melting glaciers. Sea level rise coupled with storm surges could flood low-lying wastewater treatment plants in King County. To help plan for the effects of sea level rise, King County commissioned a vulnerability assessment of its 77 wastewater treatment sites and developed recommendations and adaptation strategies.


Three datasets were used to assess the vulnerability of King County’s wastewater treatment facilities:

  1. Sea level rise projections for Puget Sound (University of Washington's Climate Impacts Group), ranging from three to 50 inches by 2100;
  2. The elevation of the center of wastewater treatment plants located adjacent to tidally-influenced water bodies; and
  3. Mean higher high water (MHHW) for the last 19-year epoch of the tidal cycle, calculated to be 9.01 feet above NAVD88 (NOAA Tides and Currents), and storm surges, which were calculated to be 1.48 feet annually with a range up to 3.19 feet for a hundred-year storm.

Projected increases in sea level were added to the average MHHW to infer future sea levels; vulnerabilities were assessed based upon the cumulative impact of storm surges. The results indicate that MHHW could vary from 9.26 to 32.2 feet above NAVD88 by 2050.

The 2008 assessment determined that none of King County’s wastewater treatment facilities will be damaged if regional sea level rise is less than 0.8 feet. However, if sea level and storm surge exceed 3.87 feet relative to the current MHHW, some wastewater facilities could become vulnerable to flooding. If sea level increases by seven feet or more, over 14 King County wastewater treatment facilities could flood. Based upon the sea level rise projections used in the 2008 assessment, the probability of imminent damage to King County’s wastewater treatment facilities was determined to be low. However, more recent studies in 2011 and 2012 indicate that saltwater intrusion is already posing problems for Wastewater Treatment Division, corroding equipment and pipe linings, and using system capacity needed during critical overflow periods.

In 2011, the agency quantified saltwater intrusion into the wastewater system by monitoring inflow into facilities during periods when high tides were between 10.80 and 12.95 feet between spring 2007 to fall 2009. Findings indicated that about 3–6 million gallons of saltwater enter the King County wastewater system each day, resulting in 1–2 billion gallons each year. A 2012 study identified the hydraulic impacts of sea level rise on the county’s wastewater treatment system, concluding that 20 facilities were at risk of saltwater intrusion due to weir elevations being lower than the highest predicted tides. Eight of these facilities were not protected by flap gates, making them at the greatest risk to saltwater inflow.  

Outcomes and Conclusions

The King County Wastewater Treatment Division evaluated the vulnerability of its facilities to flooding from sea level rise and intense storms in combination with concerns about aging infrastructure and capacity requirements. While approximately twenty facilities are at risk, the probability of imminent damage was determined to be low according to the projections used at the time. The agency is currently involved in a cross-departmental effort to prepare King County-owned assets (e.g., wastewater treatment facilities) for the impacts of sea level rise, using updated sea level rise projections from the Washington Coastal Resilience Project. In addition, the county engaged the University of Washington Climate Impacts Group to assess how climate change may alter rainfall patterns and identify the potential impacts of heavy rain events on wastewater conveyance and stormwater management. The 2020 King County Strategic Climate Action Plan includes priority strategies for the Wastewater Treatment Division to:

  • Establish a Climate Adaptation Work Group to mainstream climate change into agency programs and operations;
  • Provide an updated assessment on the hydraulic effects of saltwater intrusion on wastewater conveyance;
  • Update the assessment on how heavy rain events may affect the wastewater system; and
  • Collaborate on the development of the U.S. Geological Survey’s Puget Sound Coastal Storm Modeling System.

The Wastewater Treatment Division has recognized that much of the adaptation decision-making to date has taken place on a project-by-project basis, and that mainstreaming climate science and adaptation into operations and programs will provide more consistency in how climate change is being factored into decision-making.


Feifel, K., K.N. Braddock, and R.M. Gregg. (2021). Vulnerability of King County, Washington Wastewater Treatment Facilities to Sea Level Rise [Case study on a project of the King County Wastewater Treatment Division]. Version 2.0. Product of EcoAdapt’s State of Adaptation Program. Retrieved from CAKE: (Last updated October 2021)

Affiliated Organizations

The King County Wastewater Treatment Division’s mission is to create resources from wastewater. It oversees the operation of 77 major wastewater treatment facilities serving over 1.4 million people.

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