Photo credit: Washington State Department of Transportation (Maple Valley landslide following the 2001 Nisqually earthquake) provided by King County Executive's Office
Landslides can be a dangerous secondary hazard when initiated by severe storms. The power of these land movements was exemplified in the severe winter storm that hit the Puget Sound region in December 1996 and January 1997. During this storm, heavy snowfall was followed by a warming trend that caused quick melting, runoff and flooding, followed by a period of rain. This led to over 100 slides in King County over the subsequent two-month period.
Severe storms are not the only cause of landslides. The Nisqually earthquake in February 2001 caused a portion of hillside near Jones Road in the Renton area to slide into the riverbed of the Cedar River. The flow of the river was partially blocked resulting in several homes along the river being damaged by the dammed waters. Evidence of slide activity can still be seen along the eastern side of Interstate-5 from King County Airport all the way to the Interstate-90 interchange where portions of hillside collapsed carrying trees and debris downhill, falling just short of impacting Interstate-5.
Although landslides can and do occur in almost any part of the state, King County and the Puget Sound Basin are especially vulnerable due to our urban environment and unique geological conditions. Because of our high population density and the fact that many structures are built either on top of or below bluffs and slopes subject to landslides, more lives are endangered during these land movements and there is a greater potential for damage or destruction to private and public property. Many of the major valleys and shoreline bluffs of Puget Sound are bordered by steeply sloping unconsolidated glacial deposits that are highly susceptible to landslides. Other vulnerable areas include the Cascade Mountain passes leading to eastern Washington. As a result, allowing for the possibility of a landslide occurring in our area is an essential component to your family emergency plan.
Hazard-specific preparedness steps
- Get a ground assessment of your property.
- Your county or city geologist or planning department may have specific information on areas vulnerable to landsliding.
- Seek advice of geotechnical experts for evaluating landslide hazards or designing corrective techniques to reduce landslide risk.
- Plant ground cover on slopes to stabilized the land and build retaining walls.
- 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 foundation.
- 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.
- 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 that direction under your feet.
- Sinkholes - a sinkhole occurs when groundwater dissolves a vulnerable land surface, such as limestone, causing the land surface to collapse from a lack of support.
- Make arrangements for housing in the event you need to evacuate your home.
- Plan for "earthquakes" and "severe storms" that can trigger a landslide.
- If inside a building, stay inside. Take cover under a desk, table, or other piece of sturdy furniture.
- If outdoors:
- Try to get out of the path of the landslide or mudflow.
- 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.
- Remember that flooding may occur after a mudflow or landslide.
- Stay away from the slide area. There may be danger of additional slides.
- Once it's safe, check for injured and trapped persons near the slide area. Remember to help neighbors who may require special assistance.
- Listen to your weather radio and local radio/TV stations for current information.
- Check for damaged utility lines. Report any damage to the utility company.
- Check the building foundation, chimney and surrounding land for damage.