When the Ground Slips: A Common Devastating Natural Hazard in the Pacific Northwest

The cause of the recent landslide that pushed a Bellevue home off its foundation this week is not yet known. But the regularity with which homes and lives are lost in landslides in Washington is undeniable.

It is the nature of the state’s geology and topography to slip, slide and crumble during the wet months of the year. Take our slopes, just add water – and gravity.

Landslides are one of the most common and devastating natural hazards in the Pacific Northwest – and the damage they cause is almost never covered by insurance, warns the Department of Natural Resources of the United States. State in its Owner’s Guide to Landslides, at st.news/slide.

That there is even such a guide should be taken as fair warning: we all live in and around landslide country.

This is the time when the risk increases, because the heavy rains saturate the soil. What happens next, says Washington State geologist Casey Hanell of the DNR, is predictable — and in some cases, preventable.

The principles at work are familiar to anyone who has ever built a sandcastle. Those grains of sand that clump together to form vertical walls with just the right amount of moisture? They dissolve into liquid as water is added.

So it goes with many slopes, given the right combination of soil, water, slope and gravity.

There are two main types of landslides, Hanell explains.

One is the shallow or rapid landslide – basically a debris flow that happens quickly. In this case, the water permeates the soil and the rock and accumulates on the impermeable bedrock. The layer of saturated soil and debris will eventually slide into a rapid detachment of material.

Any traveler on Sounder trains traveling at the base of cliffs along the Puget Sound shoreline is all too familiar with these types of landslides, which disrupt service on muddy tracks.

The other type of landslide is called a deep landslide. These occur much deeper underground in the earth. These slides usually move slowly – over decades or even centuries.

There are hundreds of these slides across Washington, Hanell notes, that have moved through different climatic conditions and are currently dormant. But they can be triggered again by human activities, such as excavating at the toe of a slope or adding water, Hanell said.

The water is not only heavy, increasing the driving force of a slope, but it pushes the grains of the soil apart, insinuating itself between the tiniest particles. If enough water saturates the soil, it will not hold together on a slope. It will slip.

Washington has experienced many large and deadly landslides.

The Oso landslide in Snohomish County on March 22, 2014 killed 43 people and destroyed 49 homes and other structures when an unstable hillside collapsed, sending mud and debris south across the North Fork of the Stillaguamish River and Highway 530.

Over the past year, scientists have gained new insights into the circumstances surrounding the Oso landslide. Research by the US Geological Survey indicates that the landslide’s average speed was around 40 mph, with maximum speeds likely even higher. The area invaded by the landslide was approximately half a square mile, and the landslide displaced approximately 18 million tons of sand, till, and clay. This amount of material would cover approximately 600 football pitches 10 feet deep.

Precipitation in the region in February and March 2014 was 150% to 200% of long-term average and likely contributed to landslide initiation and mobility, according to USGS analysis.

The hill’s history of landslides dates back more than 60 years, and some experts said they were shocked when home construction was allowed after a large landslide in 2006. A 1999 report filed with the U.S. Army Corps of Engineers warned of the potential for catastrophic failure. Two creeks in the area are known as Slide Creek and Mud Flow Creek. Experts have also warned against logging on a plateau above the slope, due to the risk of catastrophic landslide.

The Aldercrest-Banyon Landslide was a large, slow-moving landslide in East Kelso, Cowlitz County that began in 1998 and continued for nine months in 1999. The disaster caused 110 million dollars in damage and destroyed 138 homes.

On October 11, 2009, a massive landslide in Yakima County’s Nile Valley demolished half a mile of State Highway 410 and redirected the flow of the Naches River.

A 1997 landslide on Perkins Lane in the Magnolia neighborhood caused houses to collapse on the beach.

Another massive landslide in 2013 on Whidbey Island sent a 1,000ft wide strip of hillside cliff into the Ledgewood Beach development on the west side of the island. The mighty slide pushed a house down the cliff about 200 feet into the water and washed away nearly 400 feet of Driftwood Way, the road to the shore.

Incredibly, there were no injuries.

A family of four from Bainbridge Isand were killed in 1997 when an avalanche of mud hit their home as they slept on Rolling Bay Walk. The crushing fall toppled the third story of the house, sending it into Puget Sound, and buried the rest of the house under tons of mud. Five other houses in the neighborhood have been declared unsafe due to the risk of slipping.

The cause of the slide was determined to be precipitation and snowmelt, which saturated the slope. The area was historically a slippery area, with the winter months regularly sending mud to the beaches.

There are steps that can be taken to live more safely with our state’s landslide risk, advises the DNR.

Do not add water on steep slopes. Avoid placing fill, yard waste or debris on steep slopes or digging under them. Always drain surface runoff from slopes, repair plumbing leaks that could saturate the ground, and plant and protect ground cover and native trees. Consult professionals if you think you are on a landslide.

The MRN’s geological information portal, searchable by address, is a starting point.

For Seattle residents living in a home on hilly terrain or considering buying one, the city maintains an online resource page as well as survey maps showing areas with a 40% or greater slope. Don’t get me wrong, the landslides are us, in a city that has recorded over 1,300 landslides.

But it’s up to the owner to do the research and, if necessary, take preventative measures, including hiring a geotechnical engineer for a site survey.

As the Northwest’s rainy season takes its usual toll, the Northwest Insurance Council, a trade group, is once again issuing the warning on its website that homeowners, renters and standard commercial insurance policies do not include landslide coverage.

Special coverage called a “difference in condition” policy is available for stand-alone purchase, but is expensive and usually obtained from a broker in specialist lines of insurance.

The incessant rain again this fall and winter – sometimes breaking records – is a good reminder that water is heavy, soils are saturated and gravity prevails. Don’t ignore the ground motion sign, Hanell said.

Cracks in the walls, bulging walls or cracks in the floors. Tilting of retaining walls, cracks in foundations and twisted trees. These are clues that the ground is moving.

“Be aware of your surroundings,” Hanell said. And always, evacuate any structure that begins to slip.

Material from the Seattle Times archives was used in this story.

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