Larry Collins reports on the hazards faced by Southern California firefighters when floods and mud and debris flows follow wildfires. Four years after deadly fire storms swept across Southern California and burned thousands of homes, it happened again. In October and November 2007, an unprecedented...
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Conditions that mobilize debris flows often cause multiple simultaneous events in different canyons, confounding rescuers trying to reach trapped victims. If one canyon is flooding, chances are good that adjacent canyons are being hit hard at the same time. Major debris flow events have occurred in California during at least 24 rainy seasons since 1905. It is estimated that one ancient flow sequence in the Wrightwood area (long before it was populated by Europeans) dislodged 18 million yards of material and transported it 15 miles into the desert, burying the future town site under dozens of feet of material.
Mud and debris flows in steep terrain commonly move faster than 20 mph, and they have been known to travel faster than 100 mph. Debris flows have moved rocks measuring 20 by 30 by 40 feet. One debris flow in the Tujunga area of Los Angeles County transported a boulder weighing 15 tons into a residential area two miles from the San Gabriel Mountains.
The extreme heat of wildfires can coalesce plant life and soil into a coating that repels water. Chaparral contains high levels of long-chain hydrocarbons (oils) that help maintain internal moisture in the arid climate. Chaparral litter gives up these complexes to the soil as it piles thick in the years between wildfires. When intense fires occur, the waxy compounds are vaporized, condensing in a layer a few inches below the ground. The layer of soil just below the surface essentially becomes hard and waterproof.
Dry, unconsolidated particles of soil, rock, ashes and other material is left in thick layers on steep canyon walls. Without plants and roots to hold it to the slopes, the particles cascade down hillsides in a steady dry stream of chunks. High winds under these conditions have been known to create dust storms. In essence, the canyons are being "pre-loaded" for debris flows.
Here's how it happens: Rain is the trigger; in even moderate rainfall, topsoil can quickly become saturated, increasing the pore pressure just above the hydrophobic (waterproof) layer. The soil then liquefies and begins moving downhill, seeking the path of least resistance (as in a stream), forming small canals called "rills" across the hydrophobic layer. The rills speed the water downslope, sometimes increasing the velocity threefold (and increasing the transport capacity a thousandfold). As rills develop across the face of the burned slopes, they connect, forming small tributaries leading to the bottom drainages. Massive loads of debris can be mobilized by relatively small amounts of rainfall. With the onset of intense rainfall, the trigger is set, the trap is sprung, and anyone down canyon is vulnerable to the unannounced arrival of huge walls of mud, rock, water, trees, and sometimes homes and cars.
For reasons still not entirely understood, wildfire burn areas seem to attract tremendously intense rain. The burned areas appear to act as separate microclimates, attracting storm cells that concentrate directly over the denuded terrain. This creates one type of worst-case scenario (denuded soil) overlaid by another worst-case scenario (intense rain in steep topography). As noted by McPhee, an inch of rain on an area of mountain 10 by 10 miles is approximately 7.2 million tons of water. Mix that amount of water with millions of tons of rock, soil and other debris, add the runoff factor caused by the hydrophobic layer and the stage is set for disaster.
Heeding the warnings of geologists, soils engineers and the LACoFD's Forestry Division after the 1993 firestorms, Fire Chief P. Michael Freeman and his staff assigned Battalion Chief Terry DeJournet and Firefighter Vern Atwater to develop training to prepare firefighters and other rescuers to evaluate mud- and debris-flow situations, recognize danger signs, request proper rescue resources, and manage rescues with maximum safety and efficiency. Atwater and DeJournet made a "blitz" attack on the project, conducting extensive research with geologists, soil engineers, public works officials, rescue experts, the California Division of Mines and Geology, the U.S. Army Corps of Engineers, the Federal Emergency Management Agency (FEMA), the California Office of Emergency Services and others. Key elements of the program, presented to personnel in the high-hazard burn areas, were as follows: