Hydrologic extremes, like heavy rain or drought, can create challenges for people, the environment, and infrastructure in King County. A major cause of heavy rain in King County is atmospheric rivers—long, narrow bands of moisture in the sky that bring large amounts of rain and snow to the West Coast. While these events are a natural part of the water cycle, strong atmospheric rivers can lead to widespread flooding, landslides, and other damage.
The causes of drought in King County are more varied. The Pacific Northwest region is dependent on winter rains and a robust snowpack to fill reservoirs for drinking water and hydropower, and to provide ample cool water in rivers and streams during our dry summers. A low snowpack, or one that melts off too quickly because of a warm spring, can make it difficult to meet water needs for people and salmon through summer. Drought can also be intensified by hotter than normal summer temperatures and a delay in the return of fall rains.
Changes in hydrologic extremes affect many issues that King County is actively working on, including flood management, stormwater management, salmon recovery, and recycled water. Actions in this focus area cover stormwater management and recycled water. Actions related to river flooding and salmon recovery are covered in separate focus areas.
What's at stake
Climate change is expected to lead to more intense heavy rain events and drier summers in King County. While individual model results vary, climate modeling projects that the heaviest two-year, 24-hour rain events in King County will increase 14 percent on average (range 7 to 30 percent) by the 2080s, relative to 1980–2009.18 Many locations could see increases exceeding 20 percent. This increases the likelihood of urban flooding and combined sewer overflows in locations where stormwater control is absent or unable to keep up with runoff volumes.
Stormwater infrastructure is designed and built to operate for many decades, meaning that the infrastructure built today will still be operating as climate change becomes more pronounced. King County research19 finds that stormwater management facilities designed for future (2080s) rainfall will need to be 10 percent to over 100 percent larger in volume than those based on 1990s rainfall, depending on land use, soil infiltration, and facility type. Avoiding overflows in older, more urbanized areas where stormwater and sewage are conveyed in the same pipe (combined sewers) also becomes more difficult with heavier rain events. Understanding and preparing for the range of possible outcomes is critical to ensuring that stormwater systems can meet current and future service loads and mitigate future rainfall projections.
Changes in summer conditions will also have impacts. Climate change is expected to lead to lower snowpack, earlier spring runoff, and hotter summers, reducing the amount of water available instream for summer irrigation and salmon. King County investments in the delivery of recycled water to irrigators in the Sammamish valley are helping to keep more water in streams for salmon during the summer by providing an alternate source of water for growing summer irrigation demands.
A better outcome
King County envisions a future where the impacts of heavier rain events and more intense summer drought are being effectively mitigated. Where stormwater runoff from heavier rain events is controlled using built and nature-based solutions that provide multiple benefits equitably to communities and the environment. Stormwater is cleaner, helping to ensure that groundwater, lakes, and streams are clean, healthy, and accessible. Lives, homes, and properties are protected from stormwater-related flooding and damage, including damage from erosion and landslides. Water bodies that are in or important to predominantly BIPOC communities reach these goals promptly and thoroughly. Additionally, work on recycled water is helping to reduce the impacts of summer drought on water users and salmon.
What we've done to get here
- Opened the Georgetown Wet Weather Treatment Station (2022), the Wastewater Treatment Division’s (WTD) newest combined sewer overflow control project, to treat high stormwater flows from extreme rain events. By January 2024, the treatment station had protected the Duwamish River and Puget Sound from 32 million gallons of polluted stormwater.
- Partnered with the UW Climate Impacts Group to develop hourly rainfall projections for King County through the 2080s. Used those data in preliminary assessments of climate change impacts on stormwater infrastructure, wastewater infrastructure, and combined sewer overflows.
- Via the RainWise rebate program, supported the installation of 379 cisterns and 100 rain gardens on private properties, managing a total of 4,750,000 gallons of stormwater in areas where combined sewer overflows are a concern.
- Integrated climate change into King County’s regional stormwater investment priorities.
- Piloted new technologies to help optimize the performance of existing stormwater facilities during heavy rain events.
- Made progress in establishing relationships with farmers related to recycled water and worked with some of the largest irrigators in the Sammamish Valley to expand the use of recycled water, helping to reduce water withdrawals and benefit streamflow in the Valley.
19 King County, “Stormwater Regulatory Response to Predicted Climatic Impacts on Puget Sound Rainfall,” Prepared by Mark Wilgus and Jeff Burkey, Water and Land Resources Division, Seattle, Washington, 2019.