A major rainstorm will hit Southern California this week, bringing the risk of debris flows in fire zones. Rain will begin on Wednesday, but the most damaging rains are expected Thursday.
Here are four things to know about the potential for debris flows:
1. The risks
When hills are verdant and healthy, vegetation can anchor the soil in place, even during heavy storms. But when that protective blanket is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down in a torrent of mud, rocks and dead branches like whitewater rapids, imperiling any homes — or anyone — in their path.
The heat from a fire also makes it harder for water to infiltrate the top soil layer, U.S. Geological Survey hydrologist Jason Kean said in a previous interview.
Ash tends to clog the soil, and oily substances can hamper the ground’s ability to absorb water. Impact from the rain can also help seal the top layer of sediment.
As a result, the soils become repellent to water. Instead of percolating below the surface, water starts flowing downhill — picking up rocks and debris on in its way, Kean said.
It’s like water on a parking lot or the surface of a playground slide, he said: “When the rainfall hits it, it just runs right off. … The rapid runoff from these bare hill slopes can quickly pick up sediment, and that can transform into a really nasty debris flow.”
The highest risk for debris flow following a wildfire is in the first year or two afterward.
But burn areas remain more prone to debris flow for three to five years after a fire — the amount of time usually needed for vegetation to grow back. With each additional year of recovery, the risk decreases, experts say.
2. Preparing
Here are some basic tips to prepare for mudslides from the California Department of Conservation:
- Be ready for debris flows for two to five years after a wildfire. An intense rain (typically about one-half inch per hour — like a thunderstorm) on a recently burned slope can trigger a debris flow.
- Pay attention to official weather forecasts. Just a few minutes of intense rain can start a debris flow. The National Weather Service will issue a flash flood watch or warning for your area when rainfall is anticipated to be intense. Note that it’s the rain in the mountains that will start the debris flow, even if it’s not raining — or only sprinkling — where you live.
- Don’t rely on what you’ve seen in past debris flows. Debris flows can hit new areas or return to previous areas; they might be smaller — or larger — the next time. Whatever happened before, the next time could be different.
- Get out before the storm arrives. Debris flows move fast! If you wait to see if a debris flow is coming your way, it will be too late to leave safely. You cannot outrun a debris flow.
- If you must shelter in place, choose your spot in advance and stay alert. Find the highest point nearby (such as a second-story room or the roof) and be ready to get there at a moment’s notice. Listen and watch for rushing water or mud and unusual sounds. Survivors describe sounds of cracking, breaking, roaring, or a freight train in advance of a debris flow.
- Never underestimate a debris flow. Debris flows can start in places they’ve never been before. They can leave stream channels and plow through neighborhoods. When a debris flow is small, people can control it with walls, K-rails and sandbags. When a debris flow is big enough, nothing can stop it.
- Expect other flood dangers. Storms that can cause debris flows can also cause more common flooding dangers.
- Debris flows can also occur in the absence of fire during wet winters. Be aware that the soil may be waterlogged and that more rain can trigger debris flows.
- Watch for new springs or seeps and excess surface erosion on slopes on and around your property. If there are nearby streams, do they appear muddier than normal?
- Avoid sleeping in lower-floor bedrooms on the sides of houses that face slopes. Debris flows can bury people sleeping in lower-floor bedrooms adjacent to hazardous slopes.
- Sign up for alerts from your local emergency management agency. In Los Angeles County, that’s Alert L.A. County.
3. Different types of landslides
“Landslide” is an all-encompassing term that can describe any movement of rock, dirt or debris downhill.
But there are a few distinct varieties, as defined by hydrologists:
Mudflow: Water rushing down with only mud. This is considered a type of shallow landslide — less than 15 feet deep.
Debris flow: When water rapidly flows downhill and picks up rocks, branches, sometimes even massive boulders and cars. This is also considered a type of shallow landslide.
Hydrologists don’t typically use the term “mudslide,” but it’s widely used and understood by nonscientists. The word is a portmanteau of mudflow and landslide, and is commonly used to describe debris flows or mudflows.
How a debris flow works
4. How it happens
Rainfall rates of half an inch per hour are the typical starting point for when debris flow can be triggered in recently burned areas.
The higher the rainfall rate above that threshold, “that’s where we could get some more appreciable and significant debris flows,” said Ryan Kittell, a meteorologist with the National Weather Service in Oxnard.
A shallow landslide involves a saturated hillside that collapses but does not move very far — maybe burying a roadway with dirt and rocks. They can happen up to an hour after a burst of intense rain.
Debris flow can also occur even if there isn’t a history of recent wildfire.
About a year ago, pounding rainfall triggered damaging debris and mudflows in a number of hillside neighborhoods in Southern California, including Beverly Glen, Studio City, Tarzana, Baldwin Hills and Hacienda Heights.
Some of the state’s deadliest debris flows have struck after wildfires.
In January 2018, 23 people died and numerous structures were destroyed when a river of mud and rock flowed through Montecito, which had been burned less than a month earlier in the Thomas fire. That fire, one of California’s most destructive on record, burned 282,000 acres in Ventura and Santa Barbara counties and destroyed more than 1,000 structures.
In 1982, a debris flow hit Love Creek in the Santa Cruz Mountains. The area hadn’t burned beforehand, but heavy rains destabilized the slopes. Ten people died, including two children, and 30 homes were destroyed.
Moments into New Year’s Day 1934, a major flood and debris flow came sailing down La Crescenta Valley, triggered by heavy rains preceded by a fire in the nearby Angeles National Forest. A 20-foot wall of mud and rocks came thundering out of canyons, killing 45 people and destroying more than 400 homes, according to Los Angeles County archives.
5. A big concern: ‘Deep-seated’ landslides
The type of landslide that can be among the most unpredictable is 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 can accumulate and build up pressure, Kean has said.
The pressure can destabilize an entire chunk of land, causing it to collapse downhill. The landslide can happen slowly and show warning signs such as cracking or subtle movement, allowing people time to escape. But it can also strike rapidly with no warning, even on a rainless day months after the end of winter.
This is called a deep-seated landslide, involving landslides greater than 15 feet deep. Often, deep-seated landslides strike in areas with a history of such events. The USGS has warned that such landslides can become active many months after a very wet winter.
Animated infographic shows how debris flows and deep-seated landslides happen
One occurred on a foggy morning in June 2005 in Laguna Beach’s Bluebird Canyon after heavy rains fell over the previous December through February. No rainfall occurred during or just before the landslide. Seventeen homes were destroyed and 11 seriously damaged.
There has been a history of devastating landslides in Bluebird Canyon. The neighborhood suffered a slide in October 1978 that destroyed more than 20 homes. The California Geological Survey said the heavy rains between December 1977 and April 1978 are believed to have played a role, along with a history of landslides and erosion at the site, and weakness in the rock.
Deep-seated landslides also hit the Ventura County hamlet of La Conchita in 1995 and 2005. The second landslide occurred at the end of an intense 15-day rainy period that saw heavy precipitation throughout Southern California. It came with no warning and buried 10 people, killing them.
Times staff writers Hailey Branson-Potts, Susanne Rust, Joseph Serna and Rosanna Xia contributed to this report.
