Click to review the final PEIS

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Click to review the final PEIS 🚨

Protecting the Lower Green River Corridor

The King County Flood Control District (District) protects people, property, and communities from the harmful impacts of floods. Without additional measures to manage floods, large areas of the Lower Green River Corridor (corridor) are at risk of substantial inundation during an 18,800 cubic-feet-per-second (cfs) flood. The District wants to make sure the corridor is a safe place to live, fish, work, farm, and play for generations to come.

Interactive Flood Map

This interactive flood map was developed to help users see how much fl­ooding would occur in the corridor during a 500-year flood event. Review this how-to guide before using the map.

The corridor includes approximately 21 miles of the Green River and its floodplains, including portions of Auburn, Kent, Renton, SeaTac, Tukwila, and unincorporated King County. Flooding puts at risk approximately 14,600 housing units, 42,200 jobs, warehouses, farms, parks, and natural areas that sustain threatened salmon.

Although users can input an address or a place for reference, the tool is intended to show flooding at a corridor level and is not intended to show flooding for individual properties. If you want to see the flood risk for a specific property, please visit the District’s Flood Risk Map webpage.

  • Flood events are characterized in terms of their chance of occurring each year. Two common characterizations are 100-year and 500-year flood events:

    A 100-year flood event has a one percent chance of occurring each year or a 26 percent chance of occurring at least once in 30 years. Local floodplain regulations are typically based on 100-year flood event.

    • A 500-year flood event has a 0.2 percent chance of occurring each year or approximately a six percent chance of occurring at least once in 30 years.

    • A flood event can also be defined as the volume and rate of water flowing through the river. For the Lower Green River, the median peak flow for a 500-year flood equates to 18,800 cfs of water, and a 100-year event equates to 12,000 cfs of water.

Protection Takes Planning

The District is considering developing a Lower Green River Corridor Flood Hazard Management Plan that would guide future investments to reduce flood risks over the next 30-50 years. To inform the plan, a programmatic environmental impact statement (PEIS) under the State Environmental Protection Act was prepared. A PEIS takes a comprehensive approach to analyzing the impacts, benefits, and costs of planning-level decisions that affect a large area.

The District issued the final PEIS on April 30, 2024. The study analyzes three different approaches (called alternatives) to reducing flood risks in the Lower Green River Corridor while providing other important community benefits:

Input is Important

Public input helped shape the Lower Green River Corridor final PEIS.

The District held a public comment period during scoping and on the draft PEIS. Tribes, jurisdictions, agencies, interested parties, and members of the public were invited to comment on types of environmental impacts to be considered as part of the draft PEIS and three alternative approaches for flood hazard management.

The District responded to all comments in the final PEIS. The District Board of Supervisors will identify next steps that could include directing the development of a Lower Green River Corridor Flood Hazard Management Plan.

Project Materials and Translations

All project materials including the final PEIS and translated materials can be found here.

To request additional translated materials, please call 206-349-6361 or email lowergreensepa@kingcounty.gov.

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