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The most common type of landslide on Puget Sound. |
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Shallow landslides and debris avalanches are frequent and widespread along Puget Sound's shoreline, typically occurring during prolonged periods of heavy rainfall. They involve a relatively thin layer (less than five feet) of wet soil and vegetation, but can be extremely dangerous, as mud and debris travel fast and with destructive force. In a major disaster, such as the Holiday Storm of 1996-1997, many hundreds of landslides may occur throughout the region in a very short period of time, and without warning.
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Danger at slope toe
Shallow landslides are common along the steep slopes that line Puget Sound. Such slides are usually small and rarely result in serious damage. They may be scary and disconcerting, however, because they come without warning and can devastate a hillside. The greatest danger is to homes or other structures built close to the toe of the slope, where they may be struck or buried by rapidly moving mud and debris.
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Destructive debris flow
A shallow landslide high on a slope or in the upper portion of a ravine can turn into a debris flow as it moves downslope and incorporates more water. Such flows can be highly destructive and travel far from the base of the slope.
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Vegetation can hide slides
At any given location along a steep bluff, a shallow landslide may only occur every thirty or forty years, allowing vegetation to become well established and hiding signs of previous slides. Steep slopes that lack older vegetation may indicate past landsliding. |
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Reduce risk by managing drainage
Because water is usually the trigger of a landslide, managing the drainage on your site is a key way of reducing the risk.
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Slides build beaches
Much of the sand and gravel that forms the beaches of Puget Sound was eroded from the surrounding bluffs during periodic landslides. These beaches in turn buffer the shoreline from wave action, reducing erosion. Ironically, were it not for continued landsliding, rates of erosion along the shoreline might increase. |
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