Geology experts in Washington and Oregon have produced an easy-to-read brochure that can help people understand landslide risks, the underlying geology of slides and precautions that could avoid a disaster.
I have written a lot of words about landslides through the years, often relating stories of people involved in a catastrophic slope failures. But this new publication excels as a concise discussion of what people need to know if they live on or near a steep slope.
After the Oso landslide in the Stillaguamish Valley three years ago, I wrote a piece in the Kitsap Sun to help residents of the Kitsap Peninsula understand the risks they could be facing. Now I can point people to this graphically rich pamphlet, called “A Homeowners Guide to Landslides for Washington and Oregon” (PDF, 3.8 mb). It was produced by the Washington Department of Natural Resources and the Oregon Department of Geology and Mineral Industries.
“Our job is to understand Washington’s complex geology and how it impacts the people who live here,” Washington State Geologist Dave Norman said in a news release. “We want to make sure we put that information into their hands.”
Water is often the key ingredient in determining whether the soils on a slope will hold together or become a dangerous mass of moving earth. A cubic foot of water — just 7.5 gallons — weighs 62.3 pounds. Water not only weighs down a slope, it also pushes the grains of soil apart, making for a more slippery material.
To prevent water from increasing the risk of slides:
- Maintain healthy vegetation,
- Use drought-resistant plantings,
- Fix leaking plumbing immediately,
- Direct downspout runoff well away from slopes, and
- Plant trees and shrubs, which uptake water more efficiently than lawns.
Actions that people should avoid are mostly related to water and the pressures exerted from piling up dirt. The publication advises against:
- Adding water to steep slopes,
- Placing fill soil on or near steep slopes,
- Placing yard waste or debris on steep slopes, and
- Excavating on or at the base of steep slopes.
An appropriation by the Washington Legislature in 2015 allowed DNR to extend its hazard mapping with the use of high-resolution lidar. See the Washington Lidar Portal for locations where high-res imaging has been applied, or learn more about LIDAR and the mapping project.
A 2007 map of geologic hazards in Kitsap County (PDF 16.7 mb) can be found on the county’s website. A study by federal geologists identified 231 landslides of significant size in Kitsap County, potentially affecting 1 percent of the county’s total area. Read the report or download the map (PDF 34.1 mb) from the U.S. Geological Survey website.
I can’t sign off on this topic without mentioning that landslides may occur more often in the future because of the patterns of rainfall expected in the Northwest as a result of climate change. Check out my story, “Shifting ground: Climate change may increase the risk of landslides,” in the Encyclopedia of Puget Sound.
If you are looking for an easy-to-understand report about climate change, I highly recommend “State of knowledge: Climate change in Puget Sound” by the University of Washington’s Climate Impacts Group.