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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 the 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 elevations. 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, vulnerability assessments should be repeated as future sea level rise scenarios are refined.

Project Background

King County covers more than 2,200 square miles and has a population of 1.9 million people. Over 17 cities, 16 local sewer utilities and one tribe rely upon the 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 eustatic sea level 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 climate change, King County commissioned a vulnerability assessment of its 77 wastewater treatment sites and developed recommendations and adaptation strategies to ensure the wastewater treatment facilities are not compromised due to a changing climate.

Project Implementation

To assess the vulnerability of King County’s wastewater treatment facilities three datasets were used:

  1. Sea level rise projections for Puget Sound (University of Washington's Climate Impacts Group) ranged 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 was calculated to be 9.01 feet above NAVD88 (NOAA Tides and Currents) and storm surges 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.

Project Outcomes and Conclusions

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 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. 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, vulnerability assessments should be repeated as future sea level rise scenarios are refined. Sea level rise should be a primary consideration in future wastewater treatment plant retrofits and site selection.

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Information gathered from published resources. Last updated on 3/10/10.

Recommended Citation

Feifel, K. (2010). Vulnerability of King County, Washington Wastewater Treatment Facilities to Sea Level Rise [Case study on a project of the King County Wastewater Treatment Division]. Product of EcoAdapt's State of Adaptation Program. Retrieved from CAKE: www.cakex.org/case-studies/vulnerability-king-county-washington-wastewat... (Last updated March 2010)