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When Mud Flows
Author: Nancy J. Rigg
Copyright: Copyright 9-1-1 Magazine, Feature Content
The Other Side of Firestorms
By Nancy J. Rigg
Originally published in our March 2008 issue.
On October 2, 2007, gale-force Santa Ana winds whipped up wildland fires throughout Southern California, charring 517,267 acres; destroying 2,233 homes, five businesses, and 966 outbuildings; killing 10 people and injuring another 139; and launching the largest evacuation in California history, according to the California Governor’s Office of Emergency Services (OES). More than 11 major fires in seven Southern California counties – Los Angeles, Orange, Riverside, San Bernardino, San Diego, Santa Barbara, and Ventura – were finally contained on November 9, 2007.
On November 23, 2007, when yet another Santa Ana windstorm served as a natural flame thrower in the dry chaparral, a second wildfire roared through the mountainous Los Angeles County seaside community of Malibu, incinerating an additional 4,901 acres, destroying 53 more homes and 33 outbuildings, decimating precious natural habitats, and leaving fire-weary residents stunned. The Corral fire was finally contained on November 27. Had OES not pre-positioned numerous wildland firefighting resources in advance of the forecast windstorm, the destruction could have been worse, with 2,314 residences threatened in this fire-burn area alone.
Burn scar areas throughout Southern California from several years’ worth of drought-enhanced wildfires now resemble lunar landscapes, denuded of precious vegetation, with only an occasional ghostly, charred tree stump or fireplace chimney jutting up through the ash and rubble. Even as vigorous reclamation efforts to heal the land and rebuild communities are under way, with the onset of the rainy season, government officials, home and business owners, and recreational land users are now facing the other side of wildland firestorms – the potential for devastating flash floods and debris flows to wreak havoc on already besieged communities.
Flash Floods and Debris Flows
Flash floods are an awesome force of nature. According to the National Weather Service (NWS), which operates within the National Oceanic and Atmospheric Administration (NOAA), several factors contribute to flash flooding, including topography, soil conditions, and ground cover. But the key elements are rainfall intensity and duration. With sufficient rainfall amounts, flash floods can occur quickly. They often involve dam and levee failures or a sudden release of water caused by ice and debris buildup in rivers and streams. Once unleashed, they can destroy bridges, sweep vehicles off roadways, collapse buildings, and threaten the lives of anyone caught in the flow. For the NWS, flash floods represent the “number one weather-related killers” in the United States.
Flash flood–producing rains can also trigger catastrophic mud slides and other landslide activity, including a powerful and potentially deadly phenomenon called a “debris flow.” According to Dr. Susan Cannon, research geologist with the U.S. Geological Survey (USGS), a debris flow is like a flash flood on steroids, featuring a massive, churning slurry of loose soil and water that resembles wet concrete, moving down mountainsides, washes, arroyos, and along streambeds, with a force that can transport boulders the size of Volkswagens, trees, and other debris, destroying everything in its path. Fire-burn areas are especially at risk for flash flood and debris flow activity.
“A wildland fire removes all of the vegetation, dries out the soil, and can create a water repellent or ‘hydrophobic’ soil layer that can enhance the flow,” Dr. Cannon explains. “But we can’t blame debris flows on water-repellent soil only. Vegetation plays an important role in mitigating the impact of rainfall. Instead of the rain hitting trees and plants, falling onto leaves and other material on the ground, and getting absorbed gradually into the soil, the rain pounds down on the bare slopes, dislodging the material, which runs off and concentrates in the channels. In fire-burn areas, it doesn’t take much rain to get the material moving. The first rains on a burned hill slope can be the worst, so preparedness is essential.”
Innovative Debris Flow Warning System
In an attempt to protect people and help emergency responders better prepare to manage flash flood and debris flow emergencies, state and federal Burned Area Emergency Response (BAER) teams have been deployed to evaluate the impact of fire damage to the watersheds and help disaster officials create comprehensive remediation and restoration plans. The Federal Emergency Management Agency (FEMA), OES, and a network of fire-rescue, flood control, and other agencies are working together to address the risk of potential floods and debris flows in burn areas.
One of the key elements in the preparedness process involves an innovative NOAA-USGS Debris Flow Warning System unveiled in Southern California in 2005. According to NWS Senior Hydrologist Jayme Laber, the warning system grew out of the 2003 Christmas Day debris flow tragedy that killed 16 people living below a fire-burn area in San Bernardino County. “Southern California is prone to fires, has steep slopes that are prone to debris flows, and has another key ingredient, urban development up against the mountains,” Laber says. “So when we get fires that burn right up to the urban interfaces, with houses that are sitting right next to steep slopes that have been burned, all the ingredients are in place for another disaster.”
“The USGS is working closely with the weather service, providing them with detailed information about rainfall thresholds, or what rainfall amounts we expect to cause debris flows and floods in recently burned areas,” Dr. Cannon explains. “Then they study the forecast, radar, and real-time precipitation data and compare it to those threshold values, using this information to issue a debris flow watch or warning.”
Dr. Cannon stresses the importance of this scientific research project, which is designed to study and refine the debris flow warning system. “The current warning system is based on comparing the rainfall thresholds,” she says. “But we’ve also developed a technique for mapping which basins can create the biggest events and show the greatest possible hazards, and we’re making this available to the weather service. This will help agencies further refine their evacuation, warning, and response plans.”
Laber notes that the terminology related to flash floods, debris flows, and mud slides is commonly used interchangeably, despite vigorous efforts to educate the media and the general public, as well as emergency responders. Even within the weather service, there is no separate warning product for debris flows. “The only thing we have at our disposal at this time is a flash flood warning product,” he explains, “so we’ve been issuing flash flood watches and warnings, noting in the text that the warning is for debris flows in a burn area. We’ve been discussing the need to create a new product that would be specifically for debris flows, reserving the flash flood warnings specifically for flash floods. This would help clarify these warnings. But at the current time, we don’t have a separate product, so there’s some confusion about what distinguishes a debris flow from a flash flood.”
Dr. Cannon explains that there are clear distinctions between flash floods and debris flows. “Because debris flows can carry rocks, trees, and other large debris in the ‘wet concrete’ slurry, their impact forces are greater than normal flash floods,” she notes. “You can get powerful surges and pulses of material moving downstream. The debris can dam up and then burst out of the channels, so they’re less controllable. Because the burn areas are so reactive, if you’re living or hiking in a dangerous area and you know it’s going to rain, or it’s starting to rain, or it’s already raining upstream, you should leave the area. A debris flow can move faster than you can run.”
Jayme Laber urges emergency responders and government agencies to pay heed to weather “watches” and “warnings” and to understand these distinctions as well. “A flash flood or debris flow ‘watch’ typically is issued 6 to 12 hours in advance of a storm,” he says. “It’s meant to give emergency managers, police and fire departments, cities, homeowners, and everyone else in the area a heads-up that they need to pay attention to this storm and take precautions ahead of time. A watch notes the potential that something is likely to happen in the near future. A ‘warning’ means that there is a life-endangering situation happening right now. Typically a warning is issued from 30 minutes to just a few minutes from when something is happening right then. So if people wait until a flash flood or debris flow warning is issued, they may not be giving themselves time to get to safety.”
To date, the NOAA-USGS Debris Flow Warning System has been available in Southern California only. But the early success of the program indicates that it would be of value nationwide.
Emergency Response Training
Captain Larry Collins of the Los Angeles County Fire Department is a man on a mission to prepare fire-rescue personnel to safely respond to incidents involving flash floods and debris flows. “We have to make sure that our personnel are all updated on flood and debris flow problems, including rescue protocols and dispatch procedures,” he says. “Communications personnel know that they’re the hub of the whole system. Their decision-making tree affects how incidents like this go down. Because flash flood and debris flow incidents don’t happen that often, each individual dispatcher may or may not have run an incident like this in the past. So we provide everyone with special training that allows them to see in their mind’s eye how an incident like this is managed.”
Collins notes that both swiftwater rescue and urban search and rescue (US&R) resources need to be deployed to debris flow rescues. “Because there is currently no specific ‘mud and debris flow’ matrix, if we dispatch a debris flow as a swiftwater incident, at least we’ll have enough resources to get working on it, with five engines, a truck company, paramedic squad, two helicopters, a swiftwater rescue team, US&R task force, and a battalion chief on the first alarm, plus automatically notifying the appropriate search and rescue teams through the sheriff’s department, as well as downstream agencies per the preplan,” he explains.
For comm centers, Collins says, “We talk about what you’re going to hear if you get a call for this kind of emergency and the response matrix you need to use, including how many victims, what age they are, what’s the point last seen, and other information we’re going to need in the field.” Understanding the NOAA-USGS Debris Flow Warning System and other weather watch and warnings provides communications and command staff with important preplanning and response tools to integrate into the flood and debris flow response matrix. “The more accurate scientific information we can process, the safer we’re going to be in the field,” Collins explains.
Following the 1993 firestorms in Southern California, the Los Angeles County Fire Department let surrounding agencies know that their special mud and debris flow rescue and dispatch training programs were available for use. “We didn’t get much of a response,” Collins says. But since the 2003 fires, interest has increased statewide and within FEMA to expand the agency’s debris flow programs and use them as a training standard.
Collins notes that the unique dangers of debris flows cannot be overstressed. “If you’re in the path of a debris flow, your survivability potential is very low, whether you’re a rescuer or the initial victim,” he says. “For the burn areas this year, we’re emphasizing precautionary evacuations.” If this means that homeowners whose houses survived the fires but are now at risk for debris flows have to evacuate every time it rains,” Collins adds, “our advice is that it’s better to get out of the way of flash floods and debris flows, rather than have us come in and try to rescue someone. Everyone needs to pay attention to the weather watches and warnings and assess the danger. For people who may have stayed behind to fight the fires, this is not a good idea with floods and debris flows. By the time you recognize the danger and realize you can’t fight it off, it may be too late. Your only warning may be a rumbling sound and you may not have time to get out.”
The Los Angeles County Office of Education and Department of Public Works produced a pioneering debris flow safety education program, Danger! Debris Flow, in 1994, following a debris flow in Bailey Canyon that killed a young father and his nine-year-old son in the aftermath of the Kinneloa firestorm near Altadena. “For responders in the field, we show Danger! Debris Flow, which really makes the danger clear,” Collins says. “It would be good for comm centers to have copies of this video and our debris flow dispatch PowerPoint program as well.”
With careful preplanning, updated training, accurate information, and education, emergency responders in Southern California, including communications personnel, will be better able to weather oncoming storms and help their communities stay safe, even in the shadow of devastating firestorms.
Nancy J. Rigg is a writer, filmmaker, and nationally recognized flood safety education specialist. In 2000, she was asked to testify before the Subcommittee on Oversight, Investigations, and Emergency Management Hearing on Flood Water Rescue. She is a frequent contributor to 9-1-1 Magazine.