Slides line many stretches along Puget Sound's bluffs. Maps from the Coastal Zone Atlases, Dept. of Ecology 1978-80, show a total of 71 miles of recently active landslides and 65 miles of dormant slides. Many more unmapped slides exist along Puget Sound, hidden under trees, shrubs, or eroded materials.

This massive slide occurred in 1979 on Whidbey Island...

Landslides can range in size from tiny to the size of subdivisions. Notable examples of large slides occur at Ledgewood Beach on Whidbey Island, along Perkins Lane in Seattle, and the vicinity of Hunter's Point near Olympia.

What Causes Slides?

Slides along Puget Sound bluffs are caused by a number of factors. A mix of gravity, geology, weather, water, and development often combine to create slides.

Factors Leading To Slides

  • Gravity
    Steep slopes and bluffs are more prone to slides.
  • Geology
    Glaciers left a legacy of steep, unstable bluffs; many shore bluffs will slide when soaked.
  • Weather
    Most slides occur during or after heavy rains, from January through March.
  • Groundwater
    During early spring, the ground is often saturated from winter rainstorms. As additional moisture builds up, high groundwater pressures contribute to slope failures.
  • Waves
    When waves attack the base or "toe" of a bluff, materials can tumble down.
  • Development
    Clearing and building on bluffs and slopes can cause slides. Excess water from paved areas, roofs, road cuts, excavations, and septic systems can destabilize slopes.

Heavy Rains & Slides: Winter 1999

  • Hunter Point, Thurston County
    Heavy winter rains renewed a large ancient slide near Olympia...
  • US 101, Hood Canal
    Two large slides slammed into US Highway 101 near Lilliwaup...

    Clearing Can Lead To Erosion & Slides

    Clearing trees and shrubs for a homesite can increase erosion and slides. While clear-cutting trees to improve views is a common practice, it carries the risk of increasing slope stability problems.

    Slide below clearing.

    Before Building, Be Aware Of Hazards

    • Property owners often increase bluff erosion and landslide problems by clearing vegetation, disturbing soils, carving road cuts, and creating poor drainage conditions.
    • Before buying, building or making changes on a bluff or slope, evaluate the site for potential slide hazards. Consider hiring a geologist to provide an assessment of site stability.

    Related Topics

    Buyer Beware, Evaluate slope hazards before buying property.
    Construction, Preventing slides and erosion.
    Understanding Erosion, What to consider on land loss.

    Related Link

    Geologic Hazards: Landslides, USGS. National landslide hazards program and national landslide information center. 

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