Landslide Safety

5 months ago

Mercer Island can be prone to natural hazards including flooding and landslides.

To report a non-life-threatening landslide, call our regional, non-emergency dispatch center (425) 577-5656, open 24 hours a day, 7 days a week. In the case of an emergency always call 9-1-1.


What is a Landslide?

Landslides develop during intense rainfall, runoff, or rapid snowmelt, changing the earth into a flowing river of mud. They can flow rapidly, striking with little or no warning at avalanche speeds (faster than a person can run). In a landslide, masses of rock, earth or debris move down a slope. Debris and mud flows are rivers of rock, earth, and other debris saturated with water. Landslides can travel many miles from their source, growing in size as they pick up trees, boulders, cars and other materials. Debris flows don’t always stay in stream channels and they can flow sideways as well as downhill.

Recognize the Warning Signs

Fast-moving landslides and debris flows pose threats to life. Warning signs include:

  • Listen and watch for rushing water, mud, unusual sounds.
  • Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
  • A faint rumbling sound that increases in volume is noticeable as the landslide nears.
  • Fences, retaining walls, utility poles, k-rails, boulders, or trees move.
  • Huge boulders in the landscape can be signs of past debris flows.

Slow-moving landslides pose threats to property. Warning signs include:

  • Changes occur in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees.
  • Doors or windows stick or jam for the first time.
  • New cracks appear in plaster, tile, brick, or foundations.
  • Outside walls, walks, or stairs begin pulling away from the building.
  • Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
  • Underground utility lines break.
  • Bulging ground appears at the base of a slope.
  • Water breaks through the ground surface in new locations.
  • Fences, retaining walls, utility poles, or trees tilt or move.
  • The ground slopes downward in one direction and may begin shifting in that direction under your feet.


City Response

As soon as the City learns of such an event, the City's Building Official travels directly to the affected site to assess the damage firsthand, and determine whether the buildings involved are still safe for occupancy. Unsafe buildings are "red-tagged" and may not be occupied until cleared by a geo-technical engineer, hired by the owner. You can help by making sure storm drains are clear in your neighborhood to help rain water flow away.

More Information

The City has consolidated links to more information, landslide hazard maps, and more. Click the links below for more information.

Click here for a City informational handout on landslides and Mercer Island.

Click here to view the City's landslide hazard map.

Click here to view the USGS local website monitoring current rainfall and forecasting landslide potential.

Click here for a factsheet on how landslides occur from the State's Geologic Hazards Program.

Click here for general landslide information from FEMA.
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