Landslide Risk & Prevention

  1. 1
    Vashon's Risks
    What areas of the island are at risk?
  2. 2
    Triggers & Prevention
    Know the possible triggers and work towards prevention
  3. 3
    Consequences & More Information
    When slides do strike, what can we expect? And where can we get more information?

Many island properties are located on the waterfront, atop slopes, or in other areas that make them prone to landslides. Awareness of the risks that are associated with our properties is critical to keeping them safe.

Step 1

Vashon's Risks

About 80% of Vashon’s 54 miles of shoreline slopes show up on this King County map of potential landslide hazard areas. Geologists have identified the purple-shaded areas on this map as potential landslide locations based on several factors such as historical landslides, steep slopes, and soil types. You can look for your property online by going to the King County iMap hazard map page.

The Marine Emergency Response Team (MERT) page contains more information about Vashon’s shoreline communities at risk of being cut off by landslides and how MERT can help.

(map from King County iMap system, July 2024)

Larger version of risk map (at right)

Signs of Increased Landslide Risk

 

If you spot any of these signs, contact Vashon Island Fire & Rescue, or the King County Sheriff’s Office. Consider consulting a licensed geotechnical expert. Slides, slumps, rock falls, and other forms of earth movement generally happen on steep slopes, especially in historical land slippage areas. That’s why geologists have identified so many miles of Vashon’s shoreline as potential landslide hazard areas. Look for these signs that the earth might be in motion on your neighborhood slopes.

  • Cracks in soil
  • Tilted or bent trees
  • New water springs
  • Hummocky or uneven soil
  • Tilting utility poles
  • Sunken or cracked roadways
  • Soil pulling away from building foundations
  • Leaking or broken utility lines

This slide of dirt, rocks, and mud was triggered by heavy rain that saturated the soil. The slide blocked Vashon Highway at the north end of the island. Any steep slope could slip when affected by one of the landslide triggers. (John Cornelison photo, January, 2021)

Step 2

Landslide Triggers

Be alert for these conditions that could trigger landslides:

  • Wave erosion at the base of shoreline bluffs
  • Earthquake shaking of steep slopes
  • Intense or prolonged rainfall saturates the soil
  • Construction, excavation, or tree removal on slopes
  • Overwatering of gardens at the top of bluffs

 

Landslide Prevention

 

In a nutshell, water is your enemy, and plants are your friends. On bluffs and slopes, do what you can to reduce water and maintain soil-stabilizing vegetation.

  • Maintain or plant vegetation with deep roots, such as trees
  • Encourage native groundcover plants that absorb water and prevent erosion
  • Direct surface water and downspout runoff away from or below slopes
  • Fix plumbing leaks before they can saturate the soil
  • Get professional geotechnical advice before significantly altering slopes on your property, including significant changes to water drainage or vegetation

Step 3

Landslide Consequences

Land can move suddenly in a devastating slide, or slip gradually over months or years. Even a few inches of earth movement over time can break septic, sewer, or water pipes. Land slippage can crack a foundation and destabilize a home. A severe landslide might block neighborhood street access, destroy buildings, or even kill.

Five homes in the Bunker Trail neighborhood were damaged by a landslide following a period of heavy rain. (John Cornelison photo, January, 2021)

Get More Information

 

Washington & Oregon 12-page homeowner’s guide to landslides.

 

Investigate hazards on your property using the King County iNet map system.