Intense footage illustrates hidden danger following extinguished wildfires
Even after a wildfire has long been extinguished, its impacts on the terrain continue to create hazards to nearby areas. A recent time-lapse shows just how dangerous it can be downhill of a burn scar.
Time-lapse video shows mud and debris pushing through a burn scar in Yucaipa, California, on Sept. 12 and covering the area in less than an hour.
When moisture from Tropical Storm Kay helped fuel thunderstorms over Southern California, it triggered a dangerous event in a wildfire-ravaged portion of the state.
During the autumn of 2020, a pyrotechnic device at a gender reveal party in Southern California's El Dorado Ranch Park ignited a blaze that would go on to scorch over 22,000 acres and claim the life of a firefighter. The El Dorado Fire, as it would be dubbed, gained national headlines as it burned near the communities of Oak Glen and Yucaipa in San Bernardino County, California.
The fire was extinguished after two months, but it had already created long-lasting damage to the landscape in the form of a burn scar, or an area where a wildfire has cleared the vegetation, in steep mountain terrain in an area already prone to mudslides.
FILE - In this Saturday, Sept. 5, 2020, file photo, a helicopter prepares to drop water at a wildfire in Yucaipa, Calif. A couple’s plan to reveal their baby’s gender at a party went up in smoke Saturday at El Rancho Dorado Park in Yucaipa, when a pyrotechnical device they used sparked a wildfire that has burned thousands of acres. (AP Photo/Ringo H.W. Chiu, File)
Mudslides, also called debris flows, are a type of landslide that can occur during periods of intense rainfall or rapid snowmelt, according to the USGS. Areas around burn scars from a wildfire are typically more susceptible to these, especially if they're on sloped land, as there's no vegetation to stabilize the soil. Here, the water can flow without much interruption, collecting mud, debris and anything else in its path as it picks up momentum.
“Rainfall that would normally be absorbed will run off extremely quickly after a wildfire, as burned soil can be as water repellent as pavement. As a result, much less rainfall [than normal] is required to produce a flash flood, “ according to the National Weather Service (NWS).
AccuWeather Senior Meteorologist Bob Larson provided the analogy of water running down a paved driveway during a storm compared to the lawn next to it. The vegetation provides a natural barrier for the water compared to the less porous surface of concrete or asphalt.
"The same thing can happen to debris that would normally be stopped by tree branches or trunks or whatever may be the case, even just grass. There's nothing to really stop it." Larson said. "And once it gets going, the sheer momentum of it if there's any kind of hill or a slant can take it on its own, and it turns into a real problem."
The debris flow can easily sweep up rocks, burned trees and even cars as it surges downhill, gaining enough force to damage or destroy ridges, roadways and buildings as far as miles away from the burned area.
This Monday afternoon, Sept. 12, 2022, image released by Caltrans District 8 shows maintenance crews on scene cleaning debris from mudslides that closed part of Highway SR-38 in the San Bernardino Mountains, Calif. The mudflows and flash flooding occurred in parts of the San Bernardino Mountains where there are burn scars, areas where there's little vegetation to hold the soil, from the 2020 wildfires. (Caltrans District 8 via AP)
These burn scars and the hazards associated with them can last for years.
Two years after the El Dorado Fire, the leftover moisture from Tropical Storm Kay aided in generating storms over Southern California, delivering staggering rainfall totals and even aiding firefighters in quelling ongoing wildfires. The rain was less welcome near the communities of Oak Glen and Yucaipa.
The nearby locations of Riverside and the city of San Bernardino recorded just under an inch and nearly two inches of rain, respectively. The average amount of rainfall for the month of September in the area is less than a quarter of an inch.
"It takes far less water, far less rain for there to be flash flooding [near burn scars] than compared to ground that's heavily covered in vegetation," Larson said.
Video of a debris flow in Oak Glen, California, following rainfall near the El Dorado Fire burn scar. (Twitter/@RogerSeheult)
In some cases, less than half an inch of rain is enough to trigger a mudslide, according to the NWS, though more rainfall can lead to a more intense flooding event.
The heavy rain resulted in devastating debris flows that killed at least one person and caused significant damage to infrastructure and homes. Evacuation and shelter-in-place orders were issued for communities in San Bernardino County near burn areas.
Video of the debris flow near Oak Glen Steakhouse Saloon, near the El Dorado burn scar, showed mud and debris gushing downhill, rendering the road impassable.
Sudden downpours like those from the thunderstorms that rumbled across Southern California can easily trigger such mudslides, according to Larson.
"It could be as little as 20 to 30 minutes of very intense rain that you would typically find in a so-called cloud burst with a thunderstorm that can lead to flash flooding or steady, prolonged rain over the course of a day or more," Larson said.
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