Washington State Department of Ecology
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about landslides

About slides on Puget Sound
Causes and common types of slides.
 
deep-seated slides
deep-seated slides
shallow slides
shallow slides
bench slides
bench slides
large slides
large slides
 
slide causes

Landsliding is a significant hazard along Puget Sound shorelines. Many factors contribute to slides, including geology, gravity, weather, groundwater, wave action, and human actions. Typically, a landslide occurs when several of these factors converge.

Geology
Many slides on Puget Sound occur in a geologic setting that places permeable sands and gravels above impermeable layers of silt and clay, or bedrock. Water seeps downward through the upper materials and accumulates on the top of the underlying units, forming a zone of weakness.
Puget Sound geology
Puget Sound geology

Gravity
Gravity works more effectively on steeper slopes, such as the bluffs that surround Puget Sound, but more gradual slopes may also be vulnerable.

Weather
Most slides on Puget Sound occur during or after heavy rains, from January through March.

Groundwater
Groundwater may rise as a result of heavy rains or a prolonged wet spell. As water tables rise, some slopes become unstable.

Waves
Wave action can erode the beach or the toe of a bluff, cutting into the slope, and setting the stage for future slides.

Human Actions
Our own actions, most notably those that affect drainage or groundwater, can trigger landslides. Clearing of vegetation, poor drainage practices, and onsite septic systems can all add to the water affecting the bluff.
 

Landslides in Puget Sound, King County.
General information on landslides in King County from their Office of Emergency Management.

The Liquid Earth, Atlantic Monthly Magazine.
A look at landslides with a special focus on the Rolling Bay slide on Bainbridge Island, 1997.

Geologic Hazard Maps, Department of Natural Resources
Landslides and landforms

Landslides in British Columbia, Govt. of British Columbia.
In British Columbia, Canada, the loss of life and damage to property caused by landslides is greater than losses caused by earthquakes and flooding.
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