Counties in Washington rank high in new study on landslide vulnerability. Find your county
Landslide season has just started, spanning from November to March most years. Washington state is one of the most landslide-prone states in the U.S., according to the state Department of Natural Resources.
In fact, the entire west coast is much more likely to experience a landslide than other parts of the country. Experts have long said this is due to the climate and geology of the region.
Recently, the home service resource site Gutter Gnome analyzed and ranked the landslide-prone counties in the U.S. to determine which are the most vulnerable to landslides. All of the top 25 rankings are in Oregon, California and Washington.
Counties most vulnerable to landslides
Gutter Gnome is a resource for gutter services and information, part of the larger family of websites, Home Gnome. The study used three categories of factors, with varying weights, to evaluate 743 counties identified by the Federal Emergency Management Agency as having relatively moderate to very high landslide risk. Each was given a score out of 100 based on the weighted factors.
Top ten most landslide-vulnerable counties:
Douglas County, Ore.
Lincoln County, Ore.
Lane County, Ore.
Coos County, Ore.
Santa Cruz County, Cali.
Multnomah County, Ore.
Chelan County, Wash.
Snohomish County, Wash.
Mendocino County, Cali.
Clackamas County, Ore.
Further down the list, Washington counties Yakima and Cowlitz ranked 14 and 16, respectively.
Idaho has the first mention outside of states on the western coast, with Elmore County at 22. Besides that mention, there are no counties outside of Washington, Oregon and California until 30, McDowell County in North Carolina.
The three categories of factors were landslide risk, landslide triggers and financial risk. Landslide Risk includes the score from FEMA. Landslide Triggers includes scores on earthquake risk, volcanic eruption risk, hurricane risk, drought risk, wildfire risk, historical average yearly precipitation in inches and the 20-year change in tree cover, in kha (kilohectare) and percentage. Financial Risk includes the expected financial loss from landslides. Information was gathered from FEMA as well as the Global Forest Watch and the National Oceanic and Atmospheric Administration.
“Oregon, California, and Washington face the highest risk of landslides, with many counties expecting to lose millions of dollars annually to landslide damage,” state the study findings. “Some counties — like Clackamas County, Oregon (No. 10), Snohomish County, Washington (No. 8), and Mendocino County, California (No. 9) — also finished in the top 10 of Landslide Triggers, due to more frequent earthquakes and higher tree cover loss over the past 20 years.”
Landslides in Washington
Landslides are very common in Washington with hundreds to thousands of landslides happening yearly due to heavy rainfall, earthquakes, water-level changes, human activity and geology, according to the Department of Natural Resources.
Landslides are unpredictable and often occur rapidly and can be dangerous as they can “easily destroy or bury a car or house,” according to the DNR.
Although some landslides may happen rapidly and without warning, others might leave signs that they are going to happen.
According to the DNR’s website, “If you notice these signs or observe a landslide in progress, leave the area immediately if it is safe to do so!”
Active landslides and any signs of a potential landslide should be reported to your county Emergency Manager.
Warning signs of impending landslides
Cracks growing in the ground; downslope movement of rock, soil, or vegetation.
Sudden changes in creek water levels, sometimes with increased sediment, especially during or right after large or protracted storm events.
Sounds of cracking wood, knocking boulders, groaning of the ground, or other unusual sounds, especially if the sound increases.
Warning signs of a potential landslides
A hillside that has increased spring and (or) seep activity, or newly saturated ground, especially if it was previously dry.
New or developing cracks, mounds, or bulges in the ground.
Sagging or taut utility lines; leaning telephone poles, deformed fences, or or tilting of trees on a hillside.
Sticking windows or doors; new and (or) growing cracks in walls, ceilings, or foundations.
Broken or leaking utilities, such as water, septic, or sewer lines.
Separation of structures from their foundation; movement of soil away from foundations.
Changes in water well levels or water wells that suddenly run dry.
Movement of the ground.
Types of landslides in Washington
The most common types of landslides in Washington are known as shallow landslides, which typically occur in western Washington during the winter months and in eastern Washington during the summer months.
Shallow landslides come in various forms such as mudslides, flows, slides or rockfalls and topples, according to the DNR.
Flows: A mixture of water, soil, rocks and debris that rapidly move down a slope.
Slides:The downslope movement of soil or rock on a narrow failure surface.
Rockfalls and topples: The downslope movement of large pieces of rock or debris that are sometimes large enough to cover a road or block a stream. These are most common in the mountain areas in Washington.
“Most shallow landslides and flows occur during or up to several days after a heavy rainfall or rapid snowmelt event. Deep-seated landslides can occur at any time. Earthquakes can also initiate landslides, so be alert if you feel the ground shake and you are near a slope,” according to the DNR.
Safety and prevention
The Washington Geological Survey has developed their own landslide inventory protocol for the state that includes hazard maps that include up-to-date listings of landslides in the state.
“As additional high-quality lidar is obtained, and as mass movements occur, the inventory will continue to be updated; because of these caveats, the inventory may not contain all the mass movements within an area where mapping appears to be complete. This dataset is not intended as a substitute for a detailed investigation of potential slope instability by a qualified practitioner,” according to the Washington Geologic Information Portal.
Checking the hazard maps for known landslide areas can help reduce the risk of being caught in a landslide, but it is not all that people should do to remain safe.
In order to stay safe, the DNR advises people to:
Educate themselves about the risk of landslides.
Make a landslide emergency plan.
Consider evacuating before storm events that could cause flooding.
Avoid living above or below steep areas or in known landslide areas.
Consult a licensed engineering geologist or licensed geotechnical engineer if you would like a site-specific evaluation.
Control runoffs from buildings and roads so it flows into natural drainages or storm drains.
During dangerous weather, the DNR advises people to:
Seek out advisories and warnings during and after intense rainfall events.
Don’t assume that highways are safe—watch for collapsed pavement, mud, fallen rock, or other debris on the roadway.
Listen for loud or unusual sounds, which could be signs of an imminent landslide.
Keep away from landslide-prone areas.
