Coastal landslides

Landslides in Washington

Landslides are common in Washington with many occurring annually in the Puget Sound area, especially along the Sound's steep shoreline bluffs. Landslides also occur along the bluffs of the Olympic Peninsula.

Landslide areas are deemed geologically-hazardous, environmentally-critical areas under the state Growth Management Act. We work with local governments and property owners to reduce hazards in slide-prone areas and help assure local Shoreline Master Programs address both safe development and protection of shoreline resources.

What causes landslides?

Most landslides occur after heavy rainfall or rapid snow melt saturates soil. Certain geology and soil conditions increase the likelihood of saturation:

• Where erosion or past slides make it easier for gravity to act on weak soils.
• Where loose sand and gravel overlie dense silt and clay.
• Where upland vegetation clearing and drainage problems lead to excess runoff from cleared spaces, driveways, roofs, and yards.

The most common landslides along coastal shorelines are shallow failures on steep slopes known as debris avalanches. These often pose the greatest danger to development on the slope or at the bottom of the bluff. Less common are larger slides that may cut many tens of feet into the slope, posing a significant risk to upland property.

Our coastal bluffs also include many much larger, deep-seated landslides that may extend more than a mile along the shoreline. These may reactivate periodically with serious consequences for structures, including homes and roads.

Landslide warning signs

Signs of an impending landslide

• Cracks in the ground that get larger
• Sounds of cracking wood or knocking boulders
• Sudden changes in creek water levels.

Signs of potential land movement in the landscape, built environment, vegetation, and water can indicate a higher risk for landslide.

Landscape

• Head scarps or steep cliffs at the toe of a slope
• Large or growing cracks, downslope movement
• Exposed clays uplifted on the beach
• Trees or large blocks of clay partially buried on the beach

Built environment

• Sagging or taut utility lines
• Growing cracks in walls and window corners
• Broken water or sewer lines
• Doors that don’t close properly
• Significant cracking of pavement

Vegetation

• Tilted or curved trees
• Split trunks
• Stretched roots
• Large clusters of trees of similar age

Water

• Small ponds on sloping terrain
• Disrupted natural drainage
• Unusually heavyvy or muddy seepage
• Unusual increase or decrease in flow from springs or in creeks

How to reduce your risks

Lack of recent slides does not mean they won’t occur. Build your house a prudent distance from the top or bottom of steep slopes. Don’t trade safety for a view. Drainage improvements can be the most cost-effective way to reduce slide risks.

Before buying shoreline property on a bluff:

• Check with the local planning department about setbacks, vegetation, drainage, and other regulations regarding vegetation removal, septic placement, impervious surface limits, and other development.
• Check into past slides in the area and review slope stability maps.
• Contact an engineering geologist or geo-technical engineer familiar with coastal slopes and your area.
• Ensure construction contractors are qualified to address geological and slope stabilization issues.

After acquiring property with the potential for landslides:

• Maintain existing vegetation. Trees, shrubs, and ground covers help to anchor soils and absorb excess water. On steep slopes, hydroseeding grasses may be ineffective and hinder the establishment of native plants which are more erosion-resistant.
• Convey runoff water from roofs and impervious surfaces away from the slope in a pipe system.
• Inspect and maintain drainage systems regularly.
• Avoid placing septic drain fields or irrigation systems between your home and the edge of the bluff, as these can exacerbate slope instability.
• Retain natural drainage patterns unless a qualified geologist or geo-technical engineer tells you otherwise.
• Do not place debris, yard waste, or fill material on a steep slope; these can become saturated and precipitate a larger slide.
• Don’t cut into a steep slope or excavate the toe of a slope.
• Check with your insurance agent to find out if your property insurance covers damages from landslides. Most policies will not.

Resources and information

These Ecology resources and publications can help you make sound property development choices:

• Washington State Coastal Atlas
• Surface Water and Groundwater on Coastal Bluffs: A Guide for Puget Sound Property Owners
• Vegetation Management: Guide for Puget Sound Bluff Property Owners
• Slope Stabilization Erosion Control Using Vegetation: A Manual of Practice for Coastal Bluffs

For more information about landslides, how and where they occur, and how to prepare for them, explore:

• Washington Department of Natural Resources: Landslides
• Washington Emergency Management Division: Landslides
• U.S. Geological Survey: Landslide Preparedness
• Local conservation districts and WSU Extension offices may also be able to provide assistance.