After a wildfire in California, or anywhere, the danger is not over. In heavy rain, the scorched landscape can be especially susceptible to landslides, mudslides and debris flows that can cause further devastation to communities.
Here’s what to know.
What’s the difference between landslides, mudslides and debris flows?
“Landslide” is an umbrella term to describe rocks, mud or any debris that moves down a slope, whether a hillside, a canyon or a mountain. Landslides are set off by rainstorms, earthquakes, volcanic eruptions or any disruption to the land, even the construction of a road. Mudslides and debris flows are types of landslides.
A mudslide is a mass of mud that moves down a slope, such as a portion of a hill with soil saturated by rains that falls onto a highway. The term mud flow is sometimes used to describe a river of liquid mud.
While the terms mudslide, mud flow and debris flow are used interchangeably, scientists describe a debris flow as its own beast. These torrents of water, mud, sand, rocks, trees and boulders can throw homes off foundations, blow out bridges and carry away cars.
Debris flows occur during heavy rain on steep slopes that have been altered, including by road cuts, excavation and wildfires. During heavy rain, a flood of water can pick up sediment as it moves down a hillside, creating a thick milkshake-like mixture that can come roaring down canyons into valleys and mow down anything in its path.
Jason Kean, a research hydrologist with the U.S. Geological Survey, compared the consistency of a debris flow with that of wet concrete poured out of a cement truck.
“It flows down that chute, and you can move it all over the place,” Mr. Kean said. “It doesn’t flow like water. It’s different like that.”
Debris flows carry mud and water, and their power, speed and thick consistency allow them to also transport heavy objects like a boulder the size of a Volkswagen Beetle.
“It becomes a flood on steroids,” Mr. Kean said. “It’s all bulked up and it can move very fast.”
Debris flows can travel faster than a person can run and are sometimes deadly. Twenty-three people died in 2018 when a debris flow that had formed in the Thompson fire burn scar crashed into Montecito, Calif., a coastal town near Santa Barbara.
Burned landscapes especially prone to debris flows.
When a wildfire disrupts the soil and destroys vegetation, it leaves an area especially susceptible to debris flows. Soil is usually porous and good at absorbing rainfall, but soil cooked by fire can turn hard and develop a waxy consistency that repels water.
In burn areas where it’s raining heavily, the debris flows start more like a flood as water easily runs off the hardened soil and then starts to pick up sediment, moving quickly with less vegetation to stop it.
“This all makes it take a lot less rain normally to trigger, a mudslide or a debris flow or whatever you want to call it,” Mr. Kean said. “And so it’s just a garden variety storm, one that might happen every year, that can trigger that. And in unburned areas it usually takes more intense rainfall.”
The common threshold for a debris flow in a wildfire burn scar is a half an inch of rain an hour.
In a burst of heavy rain, a debris flow can happen very fast in a wildfire burn area — “within minutes of intense rain,” Mr. Kean said.
In an unburned area, the ground has to soak up a lot more rain and the soil become fully saturated for it to become unstable and then flow. A ballpark threshold for a debris flow in an unburned area is one inch of rain per hour after at least 10 inches of rain has already fallen over the winter storm season, Mr. Kean said.
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