As wind and flames continued to batter Los Angeles, small teams began to creep into the scorched dirt left behind.
Approximately a dozen members of the California Watershed Emergency Response Team and the U.S. Forest Service are surveying the edges of the Eaton and Palisades wildfires to identify the most heavily burned patches of land. It will soon publish hazard maps to help people prepare for what comes next: the near-certain threat of flooding and landslides that loom over the coming days, months, or even years as cities recover.
“Even after a wildfire, the danger to the public doesn't end,” said California geologist Jeremy Lancaster. He and his team spent Wednesday hiking in rugged canyons on either side of the San Gabriel and Santa Monica Mountains. If it rains hard enough, the sediment on these slopes can quickly roll down onto houses downhill, pushing it further up the fire-prone hills.
The two main hazards after a wildfire are flash floods and post-fire debris flows. Spongy soil normally absorbs water, but burnt soil is hard, like concrete, and repels water like a raincoat. The water then flows down the slope after the fire with little or no vegetation left to contain it.
Hazard maps combine satellite imagery and soil field testing to show where patches of moderately to severely burned soil can increase post-fire risk. Attached to the map are recommendations for emergency services to erect barricades against hazards.
Maps for Los Angeles have not yet been released, but experts say most of the Palisades fires had mild to moderate burns, while the Eaton fire is likely to have moderate to severe burns. said.
Debris flows require three ingredients: steep slopes, burnt soil, and rain, as the debris they draw claws into the terrain, creating a snowball effect that disrupts trees, vegetation, soil, and rocks. Often more dangerous than flooding. and everything else along the way.
“Debris flows are like floods on steroids,” said Jason Keene, a hydrologist at the U.S. Geological Survey. “It's all bulked up with rocks and mud and wood.” Floods often have longer reach, while debris flows swirl water faster and are less frequent but more destructive.
After the 2017 Thomas Fire, debris flows occurred in Montecito, California, killing 23 people and damaging or destroying more than 400 homes.
Neither homeowners insurance nor federal flood insurance covers the impact on your property from debris flows, which are defined as landslides by the Geological Survey.
The fire and flood cycle is a long-studied relationship, but scientists say global warming is making post-fire threats more likely. Fires burn bigger and more intensely. Rain falls heavier and more frequently. These changes could expand the area of post-fire hazards and increase the magnitude and frequency of flooding and flooding.
“Fire scientists tell us that wildfires are increasing in size and severity,” Dr. Keene said. “That fact alone means more terrain is exposed, more terrain is vulnerable to post-fire problems.”
Los Angeles faces extreme danger due to a dangerous combination of extremely steep terrain, large amounts of sediment, intense fire activity, and large numbers of people forced into the mountains.
“The Los Angeles area and Southern California are world centers for post-fire debris flows,” Dr. Keene said.
Debris flows occur so frequently in Los Angeles that the state has excavated a debris flow basin on the edge of the San Gabriel Mountains, where the Eaton Fire occurred, to collect waste from large debris flow events. Lancaster said more than 2 million people in Southern California live in alluvial fans, which are prone to flash floods and debris flows.
The National Weather Service in Los Angeles, in collaboration with the Geological Survey, launched the nation's first early warning system for these post-fire hazards in 2005. Geological surveys set thresholds for rainfall that can cause landslides, and the Bureau of Meteorology issues warnings if: The amount of rainfall they predict is close to or exceeds that amount.
Jamie Lover, a senior hydrologist at the Los Angeles Forecast Bureau, has been issuing such warnings for the past 20 years. There's still no sign of rain in his seven-to-10-day forecast Wednesday, but the garden-type storms that Angelenos see at least once or twice a year are the next wave of danger. may be enough to cause Minutes.
“In burnt areas, the only type of rain that doesn't cause problems is a really light drizzle that lasts a long time,” Lover said. But it added that at some point this winter, “we will experience rain that could cause flash flooding and debris flows in newly burned areas.”
Video of a small debris flow that occurred in 2016 near the current site of the Eaton Fire shows how quickly large objects, such as 6-foot-tall rocks, can move downhill. .
Lover advised residents to prepare for future evacuations, monitor the forecast and alert local emergency officials in the event of a warning.
