Landslide

On a quiet Saturday in March 22, 2014, the small, tight-knit community of Oso, Wash. was shaken with one of the deadliest landslides in our nation’s history. The slide killed 43 people, destroyed dozens of homes and cabins, covered a mile stretch of State Route 530 and shook the rural community and its surrounding neighbors.

A landslide is the movement of rock, soil and debris down a hillside or slope. Landslides take lives, destroy homes, businesses, and public buildings, interrupt transportation, undermine bridges, derail train cars, cover marine habitat and damage utilities.

Ground failures that result in landslides occur when gravity overcomes the strength of a slope. While gravity is the primary reason for a landslide, there can be other contributing factors, including:

  • Saturation, by snowmelt or heavy rains, that weaken rock or soils on slopes.
  • Erosion by rivers, glaciers, or ocean waves that create over-steepened slopes.
  • Topography of a slope – its shape, size, degree of slope and drainage.
  • Stress from earthquakes magnitude 4.0 and greater can cause weak slopes to fail.
  • Volcanic eruptions that produce loose ash deposits and debris flows.
  • Excess weight, from accumulation of rain or snow, from stockpiling of rock or ore, from waste piles, or from manmade structures, may stress weak slopes to failure.
  • Human action, such as construction, logging or road building that disturbs soils and slopes.

Commonly, landslides occur on slopes and in areas where they have taken place before, as well as in areas where they have not been previously documented. Areas historically subject to landslides include the Columbia River Gorge, the banks of Lake Roosevelt, the Interstate 5 corridor, U.S. 101 Highway corridor along the Pacific Coast and from the coast to Olympia, the Cascade and Olympic mountain ranges and Puget Sound coastal bluffs.

Tips

Landslides and Mudflows

Landslides and mudflows usually strike without warning. The force of rocks, soil, or other debris moving down a slope can devastate anything in its path.

Before a landslide

  • Get a ground assessment of your property.
  • Your county geologist or county planning department may have specific information on areas vulnerable to landslides. Consult a professional geotechnical expert for advice on corrective measures you can take.

Insurance

  • Mudflow is covered by flood insurance policies from the National Flood Insurance Program. Flood insurance can be purchased through a local insurance agency.

Minimize home hazards

  • Plant ground cover on slopes to stabilize the land, and build retaining walls.
  • Build channels or deflection walls to direct the flow around buildings.
  • Remember: If you build walls to divert debris flow and the flow lands on a neighbor’s property, you may be liable for damages.

Make evacuation plans

  • Plan at least two evacuation routes since roads may become blocked or closed.

Learn to recognize the landslide warning signs

  • 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.
  • Underground utility lines break.
  • Bulging ground appears at the base of a slope.
  • Water breaks through the ground surface.
  • Fences, retaining walls, utility poles or trees tilt or move.
  • You hear a faint rumbling sound that increases in volume as the landslide nears. The ground slopes downward in one specific direction and may begin shifting in that direction under your feet.

During a landslide:

   If inside a building

  • Stay inside.
  • Take cover under a desk, table or other piece of sturdy furniture.

   If outdoors

  • Run to the nearest high ground in a direction away from the path.
  • If rocks and other debris are approaching, run for the nearest shelter such as a group of trees or a building.
  • If escape is not possible, curl into a tight ball and protect your head.

   After a landslide

  • Remember that flooding may occur after a mudflow or a landslide.
  • Stay away from the slide area; there may be danger of additional slides.
  • Check for injured and trapped persons near the slide area. Give first aid.
  • Remember to help your neighbors who may require special assistance – infants, elderly people, and people with disabilities.
  • Listen to a battery-operated radio or television for emergency information.
  • Check for damaged utility lines. Report any damage to the utility company.
  • Check the building foundation, chimney, and surrounding land for damage.
  • Replant damaged ground as soon as possible. Erosion caused by loss of ground cover can lead to flash flooding.

This document was produced in cooperation
with the state Department of Health and Emergency Management Division of the Washington State Military Department.

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