Levee Breach Mapping and Risk Assessment

Project overview

The King County Flood Control District together with King County, as a service provider, assessed levee systems on the Raging, Tolt, South Fork Snoqualmie, South Fork Skykomish, and Cedar rivers. Project goals were to:

Background

For levees built in the 1960s or earlier, there is little information available about when and how they were built. This makes it difficult to determine the potential for breaching.

King County conducted an initial Levee Breach Analysis in 2019 to identify where more information was needed about existing levee systems. The Levee Breach Mapping and Risk Assessment Project is collecting missing data, completing specific analyses on physical levee conditions, and evaluating potential community impacts in the event of a levee breach.

After completing the mapping and risk assessment on each river, King County will share steps to ensure public safety. Next steps may include recommendations for further evaluation, updated plans for monitoring, maintenance, emergency preparedness or education programs, and potential repairs to the levee system. King County will share results when available with local groups, agencies, and the community.

Lower Raging River Project and Results (complete)

The levee system on the lower Raging River consists of four levees totaling 2.85 miles, from River Mile (RM) 1.5 (at 328th Way SE) to the confluence with the Snoqualmie River along both banks of the river. These levees are near the town of Fall City.

Low risk of levee breach on the Raging River. The project had several key findings:

• The likelihood of a levee breach on the lower Raging River is very low. The levees are tall, wide, and built on solid ground. The levee slopes are stable.
• Some risk remains:
  • Floodwaters can overtop the levees on the town side of the lower Raging River when flows are high on the Raging and Snoqualmie rivers.
  • Climate change is expected to make very large floods more frequent, which would increase levee breach risk.
• Ongoing levee inspection, maintenance, and education are important to keep levees in good condition. This will help reduce the risk of a levee breach in the future.

Project documents:

View the Lower Raging River Map (171 KB)

Lower Tolt River (in process)

The levee system on the Tolt River totals 4.27 miles, from River Mile (RM) 2.2 to the confluence with the Snoqualmie River on both banks of the river. The system is near the city of Carnation and recreational and agricultural property.

View the Lower Tolt River Map (268 KB)

South Fork Snoqualmie River (in process)

The levee system on the South Fork Snoqualmie River totals 6.25 miles from RM 5.4 (upstream of I-90) to RM 2.1 (Snoqualmie Valley Trail crossing) on both banks of the river. This system is near rural residential property and portions of the city of North Bend.

View the South Fork Snoqualmie River Map (268 KB)

South Fork Skykomish River (in process)

The levee system on the South Fork Skykomish River consists of the Town of Skykomish Levee (on the south side of river), which is approximately 0.5 miles in length, and the Town of Skykomish Revetment on the north side of the river. This system is near the town of Skykomish.

View the South Fork Skykomish Map (3.72 MB)

Lower Cedar River (not yet started)

The levee system on the Lower Cedar River consists of a system of levees and floodwalls that extend about 1.2 miles from Interstate 405 to the river mouth where it discharges into Lake Washington on both banks of the river. This system is near commercial, industrial and residential areas within the City of Renton.

View the Lower Cedar River Map (3.22 MB)