Levee Breach Analysis Mapping and Risk Assessment
King County, Washington
This project will identify areas where levee systems on five rivers in King County may be vulnerable to breaching. If this were to happen, local communities and nearby infrastructure could be at risk. The five rivers are the lower Raging, the lower Tolt, the South Fork Snoqualmie, the South Fork Skykomish and the lower Cedar.
The King County River and Floodplain Management Section (RFMS) is overseeing the Levee Breach Analysis on behalf of the King County Flood Control District. Based on the results of the analysis, RFMS will make recommendations regarding potential repairs, capital improvement projects, and/or the need for on-going monitoring and emergency planning.
River flooding is a natural process in which floodwaters spread across a river’s floodplain. To reduce the frequency and extent of flooding and its impact on people and property, levees were built on many rivers to contain the spread of floodwaters. Levees are intended to keep floodwaters contained so that water, sediment, and debris can move downstream during a flood.
Levees constructed throughout King County were built for different purposes and vary in their height and materials. Over time, some levees have been repaired to address flood damages or re-aligned to increase flood capacity. Historically, many levees were built to protect farmland and small communities from frequent flooding. The construction approach for these early 20th-century levees was to excavate sand and gravel from the riverbed and place these materials on top of the riverbanks to create a berm or levee. Areas behind these levees were viewed as safer and encouraged land development. Levees constructed solely of sand and gravel can be highly porous and susceptible to water seepage as well as bank erosion during high river levels. To increase structural integrity of levees, current-day design standards include criteria for building them with engineered materials such as compacted, fine-grained fill as well as surface erosion protection measures. Even with more contemporary, well-engineered designs, levees can have breaches which could put people, property, and economic activity at risk.
Levees can reduce the risk of flooding, but no levee system can eliminate flood risk. A levee system is generally designed to contain a certain amount of floodwaters. If a larger flood occurs, floodwaters can flow over or around the levees. Flooding also can damage levees, creating a breach, which is a break or gap through which floodwaters may pass after a levee is overtopped or part of a levee has given way due to bank erosion.
For levees built before the 1990s, there is little information available about when and how they were built. This makes it difficult to determine the potential for breaching. In 2019, King County completed an initial Levee Breach Analysis that:
reviewed available information on existing levee systems;
identified data gaps; and
recommended conducting further investigation to better understand the risk of potential levee breaches.
The current Mapping and Risk Assessment project will fill information gaps, collect additional data, and complete specific studies such as seepage and failure analysis, inundation mapping, hydraulic modeling, and risk assessment.
Study Areas included in the Levee Breach Analysis. (Click to expand image. (PDF, 2.18 MB)
Study Area Locations
The Assessment will occur on five different rivers:
- Lower Raging River: The levee system on the Raging River consists of four levees, totaling 2.85 miles, from River Mile 1.5 (at 328th Way SE) to the confluence with the Snoqualmie River on both banks of the river. Raging River Map (PDF, 171 KB)
- Lower Tolt River: The levee system on the Tolt River consists of 11 levees, totaling 4.27 miles from River Mile (RM) 2.2 to the confluence with the Snoqualmie River on both banks of the river. Tolt River Map (PDF, 268 KB)
- South Fork Snoqualmie River: The levee system on the South Fork Snoqualmie River consists of 11 levees, totaling 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. Snoqualmie Map (PDF, 268 KB)
- South Fork Skykomish River: 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. Skykomish Map (PDF, 3.72 MB)
- Lower Cedar River: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. Cedar River Map (PDF, 3.22 MB)
|Gather data - field and desktop analysis||Early 2021 - Early 2022|
|Computer modeling and risk assessment||Spring 2021 - Spring 2022|
|Draft study reports||Summer 2021 - Fall 2022|
|Finalize study reports and share with public||Fall 2021 - Fall 2022|
The project is sponsored by the King County Flood Control District.
Floodplain: Low-lying areas along a stream or river channel where water naturally overflows during high river flow.
Levee: A raised structure designed and constructed to contain, control, or divert the flow of water to provide protection from flooding.
Levee Breach: An opening through which floodwaters may pass after part of a levee has given way. A breach may occur gradually or suddenly.
Revetment: Rock or other material placed along a riverbank to reduce erosion or channel migration along a riverbank.
For more information about the Levee Breach Analysis, please contact Judi Radloff, project manager, Water and Land Resources Division, 206-263-6844.