Seattle is unique in that it has a rich record of landslides that dates back as far as 1890. A database was created with 1,326 landslides, revealing that landslides are more likely to occur in the month of January. People in our area should be aware of what they can do to mitigate the risk of landslides.
Two of the most common landslides are-
1. High Bluff Peel Off – block falls of soil from the high bluffs that are found primarily along the cliffs of Puget Sound.
2. Shallow Colluvial (Skin Slide) - shallow rapid sliding of the outer rind of a hillside slope, sometimes also resulting in a debris flow.
The most common type of slide is the shallow colluvial slide, particularly in response to an intense, short-duration storm. The largest and commonly most destructive are deep-seated landslides; however, they are not activated as frequently as the other types of slides. The preponderance of landslides occurs in January after the water table has risen in the previous months, although destructive landsliding can sometimes last until March.
All of the steep slopes on the hill margins are susceptible to sliding. Contributing causes of land sliding may be myriad, but water is involved in nearly all of the cases. Interestingly, consistent studies in the Seattle region show that 84 percent of the reported landslides may have had some factor of human influence associated with them.
Landsliding in Seattle is caused by a combination of geologic conditions, steep topography, concentration of rainfall in the winter months, and the influences of an urban environment. The geologic conditions are primarily a legacy of repeated glacial incursions during the past 2 million years. The topography is the result of mass wasting in the past 13,000 years, since the disappearance of the last glacial ice. Although Seattle does not receive a large volume of precipitation, concentrations of rainfall in the winter months can be significant enough to saturate the glacial and colluvial soils. Overlying this natural setting is the human pattern of residential, commercial, and industrial development, and the infrastructure that binds it together
The greater Seattle area is second only to the coastal region of Southern California as the most significant region to experience landslides. (the above notes are copied in part from the WA State Study of Landslides) http://www.seattle.gov/DPD/Landslide/Study/master_report.pdf
* Do mitigate water run off.
* Try not to remove trees.
* Have proper geological tests run on the property and structural engineering consultations prior to building or remodeling.
* Empty hot tubs on the decks during this season.