What’s Next For Southern California Burn Scars? Debris Flows

Areas near Los Angeles have been devastated by wildfires, namely the Palisades Fire and the Eaton Fire which started on Jan. 7 and burned a combined 58 square miles.

As firefighters continue to beat back these blazes, another threat takes hold of burn scar areas.

Why Are Debris Flows So Dangerous?

Debris flows are a type of fast-moving landslide that carries water. According to weather.com Senior Meteorologist Dina Knightly, they’re “more like a river of mud, debris, rock and earth, but filled with water.”

They can reach speeds of more than 35 mph when rushing downhill. The debris can then flatten out and spread even farther once it reaches the bottom of a hillside. They can strike without warning, and flatten or bury homes and buildings within minutes.

Burn scars are vulnerable to landslides and debris flows for several reasons. When vegetation is removed by flames, nothing is holding the soil firmly in place. Land and debris can move easily in burn scars even during short-lived rain events.

is flows can be deadly. At least 20 people were killed when mudslides sent walls of debris as tall as 30 feet into Montecito, California, in 2018. On average, 25 to 50 people are killed by landslides every year in the U.S., according to the USGS.

Brad Johnson, Professor Of Environmental Studies At Davidson College, told weather.com:

• Very hot fires can create a crust on top of the soil, sealing the soil from absorbing rainfall, leading to dry land that is susceptible to more wildfires, as well as landslides. Burning vegetation can also create a waxy film on top of the soil, leading to more runoff.
• “Recent research shows that debris flows almost always happen in Southern California after wildfires, and given that there are multiple wildfires and each of them is pretty large, really we’re in a situation where the next large rainfall event is almost certain to generate some sort of debris flow.”
• “Those debris flows can go miles, they get so wet, and they’re in the stream channel a lot of times. They can go down the stream channel and then outside the stream channel and they can travel miles at a time.”
• “If you think you're going to get a huge rain event, and you live on a steep slope, maybe that's the day you go get a hotel room down the road or stay with a friend because they happen really quickly.”

More From Senior Meteorologist Dina Knightly:

• “The threat of debris flows is always a threat after a fire. If any rains come too quickly, it could cause a flash flood. These flood threats are increased exponentially because there is not only water but sediment and there is now more volume than just the water. Trees, brush, mud and debris get mixed together to flow like concrete moving at a fast pace downhill. They can destroy anything in their path. After a fire, trees and structures are gone and there is nothing stopping or slowing these landslides or debris flows.”
• “The Eaton burn scar areas are much steeper than the Palisades fire area, so anything at the bottom of the mountains needs to be monitored. Debris flows are not guaranteed, but are much more likely.”

What Else To Know

Those in areas vulnerable to landslides and debris flows should pay attention to their local forecast, and be on alert if any rain develops. Any threat of debris flow should be taken very seriously.

Sign up for local emergency alerts and know your evacuation routes so that you can get to safety quickly. Develop a plan with your family for a spot to meet in case you get separated.

Don’t wait for an evacuation order to leave if heavy rain develops in a burn scar area.

The Eaton Fire and the Palisades Fire are now respectively the second and fourth most destructive wildfires on record in California. Combined, they're estimated to have burned an estimated 12,000 homes and other structures. At least 27 people were killed.

Increased and larger wildfires due to climate change are expected to lead to more frequent debris flows, according to a 2023 study.