An avalanche of boulders spilling onto a road, chunks of highway sliding down a hill and residents cleaning feet of mud off driveways after a landslide are scenes that Californians have seen over the past three weeks. A parade of atmospheric river storms dropped an entire year’s worth of rain on some California cities in those weeks resulting in jaw-dropping videos.
FOX Weather calculated that statewide, an average of 10.56″ of rain fell since Christmas Eve. One area in the northern California coastal mountains got 64.1″ of rain.
And even though the Golden State will get a break from the onslaught of heavy rain and snow, landslides are still a threat.
All that water soaks into the earth or runs off. Unfortunately, rock and soil slopes are weakened when saturated, according to the U.S. Geological Survey. Then gravity takes over on steep terrain.
A layer of heavy, water-soaked soil breaks free of its foundation and starts sliding. The rain runoff further erodes the now loose dirt and rocks. The new depression also channels the water and could result in a debris flow.
“Debris flows are very dangerous and have been known to destroy roads, homes, and other structures and result in fatalities if people are not able to evacuate,” said Daniel Horton, professor of Earth and Planetary Sciences at Northwestern University. “Debris flows speeds vary depending on the event, but can reach up to 35 mph.”
While landslides can happen in every state, California is particularly at risk.
Burn scars from wildfires are particularly susceptible to debris flows. The USGS says as little as 0.3″ of rain in 30 minutes to trigger a debris flow during an area’s first post-fire burn. A burn scar can take several years to recover. Most debris flows occur in the first two years after a fire.
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Earthquakes create stresses in the rock, which weakens slopes too. And erosion from waves creates steep slopes primed for landslides.
“Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides,” states the USGS.
The state’s coastal mountains are steep slopes. Add in the high population density living in those ranges. Streets, homes and parking lots are all manmade structures that can stress weak slopes, according to the USGS.
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The drought compounds landslides, flash flooding, and debris flow dangers. Western states are currently in a mega-drought, the worst in 1,200 years, according to scientists.
“Drought can cause a hyper-dry condition in the soil, which induces ‘soil-water repellency,’” Chuxuan Li, Northwestern Ph.D. candidate, said. “This means the soil is so dry that it cannot absorb any water when the first big storm arrives, which in turn increases the likelihood of flash flooding and debris flows.”
Even as storms taper off, the water is still seeping into hyper-dry soils slowly. The weight of the water will stress steep slopes until the filters deeper and the earth stabilizes as it compacts again.
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Landslides kill 25 to 50 Americans and cause more than $1 billion in damages yearly, says the USGS.
The FOX Forecast Center calls for one more storm, the last of the parade, on Wednesday for northern California. Over a week of dry weather will give the state some time to process all the rainfall before the next round of rain.