Hazard maps and data
King County Flood Control District Hazard Mitigation Plan
King County hazard maps are for reference use only. The information included in these maps was compiled from a variety of sources using the best available data. These maps are intended as a general guide to delineate hazard-prone areas, but are not a substitute for site-specific investigation.
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- Culmback Dam - A dam failure map for Culmback Dam, located in Snohomish County, is not available but the data has been used to assess extents of potential hazard. Contact Snohomish County (external link) for more information.
- Howard Hanson and Mud Mountain Dams - The U.S. Army Corps of Engineers (external link) owns and operates these dams. These maps are not available for publication but their data has been used to assess extents of potential hazard.
- South Fork Tolt and Cedar River Dams - Flood inundation limits resulting from failure of Cedar Falls or Tolt dams are provided to Emergency Response Agencies and other stakeholders by the City of Seattle (external link). These inundation areas reflect events of an extremely remote nature, and are not in any way intended to reflect on the integrity of the dams.
- King County Earthquake Peak Ground Acceleration 100-year Probabilistic Scenario, May 2010 (2.5 MB, PDF)
A map of the Earthquake Peak Ground Acceleration 100-year Probabilistic Scenario depicts data using information from all historic earthquakes, plus geologically inferred earthquake sources (faults, locations and magnitudes), and computes the probable ground shaking levels that may be experienced during a 100-year recurrence period.
- King County National Earthquake Hazard Reduction Program Soil Site Classes, May 2010 (2.6 MB, PDF)
The National Earthquake Hazard Reduction Program Soil Classes is a dataset compiled by the Washington State Department of Natural Resources, Geology and Earth Resources Division (DGER) and is also based on surficial geology published at a scale of 1:100,000.
- King County Seattle (Fault) ART Scenario - Peak Ground Acceleration 7.2 Magnitude Scenario Shakemap, May 2010 (2.4 MB, PDF)
- King County South Whidbey Fault - Peak Ground Acceleration 7.4 Magnitude Scenario Shakemap, May 2010 (2.5 MB, PDF)
Earthquake scenarios for the Seattle Fault and South Whidbey Fault are based upon peak ground acceleration data published by the United States Geological Survey.
A ShakeMap is designed as a rapid response tool to portray the extent and variation of ground shaking throughout the affected region immediately following significant earthquakes. Ground motion and intensity maps are derived from peak ground motion amplitudes recorded on seismic sensors with interpolation based on both estimated amplitudes where data are lacking, and site amplification corrections. Color-coded instrumental intensity maps are derived from empirical relations between peak ground motions and Modified Mercalli intensity. (United States Geological Survey)
- Earthquake impacts data for King County Flood Control District facilities
- King County Liquefaction Susceptibility, May 2010 (2.7 MB, PDF)
Liquefaction is a phenomenon in which strong earthquake shaking causes a soil to rapidly lose its strength and behave like quicksand. Liquefaction typically occurs in artificial fills and in areas of loose sandy soils that are saturated with water, such as low-lying coastal areas, lakeshores, and river valleys. When soil strength is lost during liquefaction, the consequences can be catastrophic. Movement of liquefied soils can rupture pipelines, move bridge abutments and road and railway alignments, and pull apart the foundations and walls of buildings. (Washington State Department of Natural Resources, Division of Geology and Earth Resources)
Liquefaction data were based solely on surficial geology published at a scale of 1:100,000 by the DNR DGER and is subdivided on the basis of regional geologic mapping for the purpose of providing an estimate of the likelihood that soil will liquefy as a result of earthquake shaking.
A liquefaction susceptibility map provides an estimate of the likelihood that soil will liquefy as a result of earthquake shaking. This type of map depicts the relative susceptibility in a range that varies from very low to high. Areas underlain by bedrock or peat are mapped separately as these earth materials are not liquefiable, although peat deposits may be subject to permanent ground deformation caused by earthquake shaking. This map is based solely on surficial geology published at a scale of 1:100,000 by the DGER.
- King County Landslide Hazard Areas, May 2010 (3 MB, PDF)
Two Landslide datasets were used for this study, one developed by King County and another published by the DGER which represents landslide data previously mapped by a variety of sources at all scales and assessed for reliability by the DGER.
King County Landslide Hazard Areas are areas subject to severe landslide risk identified in the Sensitive Areas Ordinance as:
A. Any area with a combination of:
- 1. Slopes greater than 15 percent; and
- 2. Impermeable soils (typically silt and clay) frequently interbedded with granular soils (predominantly sand and gravel); and
- 3. Springs or groundwater seepage.
B. Any area that has shown movement during the Holocene epoch (from 10,000 years ago to present), or that is underlain by mass wastage debris of that epoch.
C. Any area potentially unstable as a result of rapid stream incision, stream bank erosion or undercutting by wave action.
D. Any area that shows evidence of, or is at risk from, snow avalanches.
E. Any area located on an alluvial fan, presently subject to or potentially subject to indundation by debris flows or deposition of stream-transported deposits.
DNR Landslide Hazard Area: The DGER landslide dataset is a compilation of landslide data previously mapped by a variety of sources at all scales, and is assessed for reliability. The DNR actively identify, assess and map landslide hazards using modern geotechnical and geophysical methods. This hazard data is critical for land-use and emergency-management planning, disaster response, and building-code amendments. As the state population grows, there is increasing pressure to develop in hazardous areas, thus delineating these areas is imperative. Washington is also prone to landslides triggered by intense rainfall or earthquakes.
- King County Lahar Inundation Zone, May 2010 (3 MB, PDF)
The Lahar Inundation Zone is a delineation published by the USGS Cascade Volcano Observatory and depicts various lahar innundation zones around Mount Rainier, which is a dormant volcano that is the highest peak in the Cascade Range and carries a larger load of glacier ice than any other mountain in the contiguous U.S. The mountain's large ice mass and great topographic relief pose geologic hazards during both future eruptions and time periods without eruptive activity.
Severe weather is characterized by “microscale” atmospheric systems including tornadoes, thunderstorms, windstorms, ice storms and snowstorms. Severe weather generally results in immobility and loss of utilities, but seldom loss of life. The following data was obtained from the United States Geological Survey; Washington State Department of Natural Resources, Division of Geology and Earth Resources; and the United States Department of Agriculture/Natural Resources Conservation Service, National Cartography and Geospatial Center.
NOTE: The information included on these maps has been compiled for King County from a variety of sources and is subject to change without notice. King County makes no representations or warranties, express or implied, as to accuracy, completeness, timeliness, or rights to the use of such information. King County shall not be liable for any general, special, indirect, incidental, or consequential damages including, but not limited to, lost revenues or lost profits resulting from the use or misuse of the information contained on this map. Any sale of this map or information on this map is prohibited except by written permission of King County.
For questions about the Hazard Mitigation Plan, please contact Katy Vanderpool, Project/Program Manager, River and Floodplain Management Section.