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.
"Residents of California are still not out of the woods when it comes to debris flow and landslide threats," said Northwestern University's Daniel Horton. "Given the saturated soil, any additional heavy precipitation has the chance to form debris flows when enough of the surface runoff becomes channelized."
How do landslides happen?
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.
"And that's when the ground is really getting saturated. So there's less place for the water to go. And also the creeks are filling up. And we've also had a lot of fires in California recently as well. And that makes the ground much less permeable, much less able to absorb all that extra water," said Roering. " And so that combination of just already soaking rains and more coming and more coming starts to make it really, really dangerous conditions.
The state identified more than 500 landslides since the storm series started. See the blue areas which also correspond to seem mountainsides.
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."
Why is California so at risk for landslides?
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.
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.
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"California has a couple of things going for it when it comes to landslides. They've got a lot of young mountain ranges, "Professor Josh Roering of the University of Oregon explained why the state has so much steep terrain. And those things create a lot of steep terrain. There's a lot of loose soil and debris that can be mobilized really easily by these rainstorm events."
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.
"High river discharges reported across the state also have the potential to undercut banks which could further destabilize slopes along waterways, said Horton. "Tt should be noted that active precipitation is not a pre-requisite for landslide occurrence. Just a few years ago, the Mud Creek landslide on Highway 1 let loose several days after the rains stopped."
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.
"We've learned a lot in the last couple of decades about these atmospheric river events. And they are the number one reason for causing rainfall induced landslides in the west all the way from Alaska down to Baja, Mexico," Roering explained. "So they are the mechanism that delivers these really intense bursts of rain. And we have steep topography and all of this loose debris and all that intense rain. It takes a lot of energy to drain it and it just prevents the water from getting out soon enough. And it makes all that stuff fluid and can start to flow downslope."
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.