The fire-and-flood cycle has returned to Southern California, as predicted.
No injuries or property damage had been reported Thursday, but the slides are a reminder of the dangers that lurk during the rainy season.
Why do landslides happen during a storm?
An intense wildfire can make the surface of the soil repellent to water. So during heavy rain, runoff can create a floodlike flow that picks up rocks and debris, U.S. Geological Survey hydrologist Jason Kean said.
Where the ground has not burned, the soil can become saturated. Pressure builds underground and the soil starts moving, picking up mud and debris as it flows downhill.
Water rushing down a slope with only mud is called a mud flow. If the flow picks up rocks and branches — or sometimes even massive boulders — that’s called a debris flow.
Mud and debris flows are types of shallow landslides, generally defined as less than 15 feet deep.
Another type of shallow landslide involves a saturated hillside that collapses but does not move very far, such as one that buries a roadway with dirt and rocks from a neighboring slope. They can happen up to an hour after a burst of intense rain.
What’s the easiest type of landslide to predict?
Landslides can be expected when heavy rain hits recently burned areas where roots of trees and vegetation are no longer there to keep the soil in place.
Sometimes, authorities have accurately predicted when debris flows will occur based on forecast rainfall rates, and have called for evacuations of homes before the rivers of mud and debris begin flowing.
Are all debris flows predictable?
No. In 2010, the winter after the Station fire charred 160,000 acres in Los Angeles County, a debris flow — which one resident described as like “Niagara Falls” — surged through La Cañada Flintridge’s northernmost neighborhood after ta 10-ton boulder clogged a critical basin, plugging up the drain like a giant stopper. More than 40 homes were damaged.
It came as a surprise because the storm was forecast to be fast moving with moderate rainfall, so no evacuations were ordered. But it unexpectedly stalled and dumped rain at an abnormal rate.
How much rainfall is needed to trigger mud or debris flows?
In Southern California’s unburned areas, 10 inches of rainfall during the winter is needed to nearly saturate the ground. After that point, a burst of rain of just one-quarter of an inch an hour can trigger widespread shallow landslides, including debris flows, Kean said.
As of Wednesday at 4 p.m., downtown Los Angeles had received 2.19 inches of rain since July 1, the traditional start of the rainy year. That’s 97% of the average at this point of the season. Camarillo had received 1.57 inches of rain, 67% of average; and Santa Barbara 3.18 inches, or 103% of average for this time of year.
In Northern California, Sacramento airport has received 2.95 inches of rain, 77% of the average; Redding, 7.22 inches, or 86% of the average; and San Francisco, 4.3 inches, or 83% of the average. Precipitation in the northern Sierra Nevada, calculated over eight weather stations in the region and a key index, was 68% of average.
In the burned areas, though, mud and debris flows can be triggered by just a single intense storm, even if the ground is not saturated.
What’s the least predictable type of landslide?
The kind that can strike on a dry day.
In areas where the bedrock is very deep, rainwater can seep deep underground during multiple rainstorms. During a series of repeated heavy storms, water eventually can start to accumulate and build up pressure, Kean said.
The pressure can destabilize an entire chunk of land, causing it to collapse. The landslide can happen slowly and show warning signs, like cracking or subtle ground movements, giving people time to escape. But it also can strike rapidly with no warning, even on a rainless day months after the end of the rainy season.
This so-called deep-seated landslide involves slides greater than 15 feet deep. These types of slides often strike in areas with a history of such events. The USGS has warned that such landslides can occur many months after a very wet winter.