Nature's cruel rejoinder to summer wildfires common in California is sometimes heavy rains that trigger floods and sudden, devastating mudslides and debris flows on charred slopes denuded of brush and roots that bind the soil.
With heavier than usual rainfall this winter in Southern California on the heels of extensive wildfires in the 2003 and 2004 seasons, officials and residents bracing for the worst have a new tool -- maps that show for the first time where debris flows are most likely to occur in recently burned areas in three counties. [Click here to download .TIF format of map.]
"These maps will be very useful for planning erosion stabilization and flood-fighting efforts," said Douglas Isbell, Deputy Director of Public Works for San Diego County.
They can also help officials make decisions about evacuations and escape routes when storms arrive.
The maps did not specifically address the small coastal California town of Conchita, north of Los Angeles, where a huge mudslide crashed down on homes with terrifying force Monday, killing at least one person and leaving up to 12 missing. The mudslide occurred during a rainstorm that hammered Southern California for a fourth straight day.
The maps for fire-denuded basins in Ventura, San Diego, and San Bernardino counties are based on statistical models that predict where debris flows are likely, and how big they might be, after rainstorms of varying magnitudes that occur about every 2 years, 10 years, and 25 years. Such storms can dump more than an inch of rain per hour.
Mudslides are just one type of debris flow, which can entrain all sizes of soil and rock along with cars, litter, and plant and animal life.
"This is a new technique," said Susan Cannon of the U.S. Geological Survey, who is part of a team that "worked like dogs" to finish the maps as soon as possible after the Cedar and Paradise fires that were especially devastating in 2003 in two areas north of San Diego.
Wildfires leave landscapes vulnerable to debris flows that are triggered differently than those from unburned slopes. Rather than acting like a sponge for precipitation, burned slopes turn into water slides when heavy rain comes down. Trees that once served as soil umbrellas are gone, and fire-dried soil is stripped of decayed leaves and twigs. During storms, rainfall then can run wild and erode sediment from slopes and channels.
"At some as-yet-unknown threshold point, sufficient eroded material is incorporated into the runoff to result in a debris flow," Cannon told LiveScience.
As of Jan. 1, 2005, more than 100 Californians had been killed by debris flows in the past 25 years. Winter storms in 1969 triggered debris flows from steep basins burned the previous summer above Glendora, California. More than one million cubic meters of rock, mud, and swept-up debris raced downhill destroying nearly 200 homes.
Debris flows moving faster than 10 mph qualify as "debris avalanches." Speeds of more than 20 mph are not uncommon, and speeds above 100 mph are rare but have been recorded. A January 1982 rainstorm triggered tens of thousands of debris flows and avalanches in the San Francisco Bay Area, killing 14 people and damaging and destroying several hundred structures.
The most important way to survive a mudslide or debris flow is not to sleep on rainy nights in lower-floor bedrooms near hazardous hillsides, according to the California Geological Survey.
The usual pattern after a fire is winter flooding, but Cannon and her colleagues wanted to study what causes the unusual case that results in debris flows. Her team started by studying 398 burned basins from 15 fires throughout the western United States. They found that the probability of a debris flow, and how big it will be, depends on the severity of the fire, the shape of the basin, soil properties, and rainfall intensity.
For the San Diego County maps, the scientists applied these models to more than 400 basins in the Cedar and Paradise fire areas to produce maps that show the probability of debris flows and an estimate of peak intensity for each of the basins. The team also generated maps for areas to the north in San Bernardino (basins affected by the Grand and Old Prix fires) and Ventura (affected by the Piru, Simi and Verdale fires) counties.
The results can be used to set priorities for post-fire hillside stabilization efforts and to guide decisions for evacuation, shelter, and escape routes before or during a threatening storm.
The 2003 wildfires in San Diego County burned 281,000 acres, destroying 2,200 homes and killing 22 people. It was the largest individual fire in California history. To control erosion after the fires, officials and volunteers distributed sand bags, straw mats, straw rolls, and grass seeds to homeowners to protect hillsides. The county also established an erosion protection plan to stabilize hillsides and water courses seen as posing serious risks to homes and businesses.
Most debris flows occur within about two years of a wildfire, making the new maps obsolete three years after their completion.
"A very rapid response is what is necessary for the situation," Cannon said. "Anything else would be a wasted effort."
In fact, Isbell, the San Diego County official, said more use could have been made of the USGS maps if they had been available earlier following the fire.
Testing of the USGS models has shown that they are best suited for burned areas in the inter-mountain West, so the scientists plan to collect data to develop models that better represent conditions in Southern California where there are frequent fires and long-lasting rainstorms.