Rainstorm brings risk of debris flows to L.A. fire zones soon

Why is California at risk?

Landslides have always been a threat to those living close to the mountains. All it can take is an intense amount of rain in a short amount of time to create deadly and damaging flows of mud and debris. California’s mountains present a particular challenge, as they’re quite tall and the elevation drops rapidly as water drains to the sea. The situation has become more dangerous as residents increasingly developed and inhabited these paths of destruction.

How do wildfires worsen the landslide risk?

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 in its wake, 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.”

What are the 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 non-scientists. The word is a portmanteau of mudflow and landslide, and is commonly used to describe debris flows or mudflows.

How fast does rain have to fall to trigger a debris flow?

Rainfall rates of half an inch per hour is 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.

Can landslides happen after the rain stops?

Yes. A shallow landslide involves a saturated hillside that collapses but does not move very far — maybe burying a neighboring 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.

How damaging or deadly can debris flows be?

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.

How long does it take for hillsides to recover from wildfires?

The highest risk for debris flow following a wildfire is in the first year or two afterward. But burn areas are still 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.

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 can accumulate and build up pressure, Kean 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.

What’s an example of a deep-seated landslide?

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.

Conclusion

As the rainstorm approaches, it’s essential to be aware of the risks associated with debris flows and landslides. While it’s impossible to predict with certainty when and where these events will occur, understanding the factors that contribute to their likelihood can help residents take steps to prepare and stay safe.

FAQs

Q: What is a debris flow?
A: A debris flow is a type of landslide that occurs when water rapidly flows downhill and picks up rocks, branches, and other debris.

Q: What are the warning signs of a debris flow?
A: Warning signs of a debris flow include rapid runoff, loud rumbling or roaring sounds, and the sudden appearance of mud or debris in a stream or river.

Q: How can I prepare for a debris flow?
A: To prepare for a debris flow, residents should stay informed about weather conditions and potential hazards, have an emergency plan in place, and be prepared to evacuate if necessary.

Q: What should I do if I’m caught in a debris flow?
A: If you’re caught in a debris flow, stay calm and try to move to higher ground as quickly as possible. Avoid trying to outrun the flow, as it can be deadly.