Shallow landslides can be triggered by rainy weather–Do you know the warning signs?

Heavy rain this weekend can cause more than just localized flooding and high rivers. Prolonged or intense rainfall increases the chances of shallow landslides on steep slopes. During these rain events some rain will flow on the surface to streams and rivers, some is captured by vegetation, and some rain infiltrates into the ground. The rain that infiltrates the ground may saturate the soil to the point the soil strength decreases, especially during prolonged or intense rain events.

Think of building sand castles with buckets on the beach–the right amount of water and the sand binds together to form a near-perfect cast of the bucket, but if too much water is added, the sand cannot hold its form and the sand collapses under its own weight. The saturation of soil is a similar concern on steep slopes. If the rain intensity or duration is sufficient to saturate the soil to a point where the soil begins to lose strength, the likelihood of a landslide increases. This is further accentuated by our steep slopes and geology in western Washington and can be intensified when drainage systems fail or when development increases surface water runoff near steep slopes.

Image of a shallow landslide that initiated during a prolonged and intense rain event in Thurston County. (Image Courtesy of Stephen Slaughter, DNR)
Image of a shallow landslide that initiated during a prolonged and intense rain event in Thurston County. (Image Courtesy of Stephen Slaughter, DNR)

Warning Signs of an Impending Landslide

If you live on or near a steep slope, here are some warning signs of potential slope instability:

  • Formation of cracks in your yard, driveway, sidewalk, foundation or other structures.
  • Tilting of trees, especially evergreens, on slopes.
  • Sudden difficulty in opening or closing doors and/or windows.
  • A hillside that has increasing springs, seeps, or saturated ground, especially if it has been dry.

If you observe one or more of these signs, you should immediately contact your city or county.

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