Tolt River Flood Hazard and Habitat Corridor Master Plan

King County's River and Floodplain Management Section retained WSE and Herrera Environmental Consultants to develop a Corridor Management Plan for the lower six miles of the Tolt River near Carnation, WA. The plan will provide an informed, strategic approach to guide implementation of a comprehensive suite of floodplain management and habitat enhancement actions intended to:

• Reduce risks to life and property from flood and channel migration/avulsion hazards;
• Avoid or minimize environmental impacts of flood hazard management;
• Protect and restore salmon habitat through restoration of riverine processes;
• Reduce the long-term cost of public flood hazard management.

Role / Services: The Corridor Management Plan is being developed in two phases. Phase 1, which is in progress, consists of an extensive science and engineering evaluation to examine the hydrologic, hydraulic, and fluvial processes that created the lower Tolt River valley, to determine how these have been altered by human activities, to assess current flood and erosion risk, to document existing habitat conditions, and to predict how these conditions will change 30 to 50 years in the future if no actions are taken. In Phase 2, this knowledge will be used to help identify and prioritize management actions to restore natural river processes, reduce flood and erosion hazards, restore and reconnect priority habitats, and reduce long-term costs and environmental impacts of hazard management activities. Once the Corridor Plan is complete, the County has the option to extend the contract to include a third phase in which identified flood reduction / habitat enhancement actions would be designed and implemented.

Outcome: Baseline hydrologic and hydraulic evaluations along with geomorphic and ecological assessments are complete. This information is now being used to identify hazards and risks within the corridor. In Phase 2, this knowledge will be used to help identify and prioritize management actions to restore natural river processes, reduce flood and erosion hazards, restore and reconnect priority habitats, and reduce long-term costs and environmental impacts of hazard management activities. Once the Corridor Plan is complete, the County has the option to extend the contract to a third phase to identify flood reduction / habitat enhancement actions to be designed and implemented.