The heavy rains forecasted this weekend may cause more than just localized flooding and higher river levels. Prolonged or intense rainfall increases the chances of shallow landslides on hillsides and other steep slopes. During these rain events, some of the rainwater flows across the surface to nearby streams and rivers, some is captured by plants and other vegetation, and some of the water infiltrates the ground. With enough rainwater infiltrating the ground, soils can weaken and slide.
Think of building sand castles with buckets on the beach–with the right amount of water, the grains of sand bind together to form a near-perfect cast of the bucket, but if too much water is added, the sand cannot hold its form and collapses under its own weight. Soil saturation has a similar result on a steep slope. With enough rain, the soil becomes saturated and begins to lose strength, increasing the chances of a landslide.
The geology of western Washington — steep slopes and soils — make this landslide country but with the right conditions, steep slopes in eastern Washington are vulnerable, too. Lots of rain, combined with failing drainage systems and development that increases surfacewater runoff near steep slopes, can be landslide triggers on both sides of the Cascades.
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:
- Cracks forming in your yard, driveway, sidewalk, foundation or in other structures.
- Trees on slopes, especially evergreens, start tilting.
- Doors and windows suddenly become more difficult to open or close.
- Water begins seeping from hillsides, even during dry weather.
If you see any of these early signs of a potential landslide, immediately contact your city or county.
This blog is reprinted from the Washington State Geology News, a free e-newsletter from DNR.