Editor's note: The following story details the severity of mud and debris flows. Although not a common occurrence, recent incidents in California as well as the deadly mudslides in Italy (see page 96) graphically showed the dangers and consequences of this natural phenomenon. Photo by...
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Editor's note: The following story details the severity of mud and debris flows. Although not a common occurrence, recent incidents in California as well as the deadly mudslides in Italy (see page 96) graphically showed the dangers and consequences of this natural phenomenon.
Photo by Gene Blevins/CFPA
Los Angeles City firefighters prepare to pull a car out of a river that washed out a road.
Mud and debris flows are among the most destructive forces on the planet. The power of these events was demonstrated in 1985, when an erupting Colombian volcano melted the snowcap. The sudden heating of the thick snow pack unleashed a huge mud and debris flow that rampaged down the mountain, where it wiped one town off the face of the earth and buried a large city in twenty feet of mud. More than 23,000 people perished in the disaster.
Mud and debris flows are also a hazard in many parts of the United States. In 1978, a large mud and debris flow buried part of a neighborhood in LaCanada, an upscale suburb of Los Angeles nestled below the steep south face of the 10,000-foot San Gabriel Mountains. The event occurred late one night during a driving winter rain storm, in an area below slopes that had recently been burned by a 60,000-acre wildland fire. Several homes were buried to their roofs. In one home, an entire family - and most of their furniture - literally floated to the ceilings atop the invading mud, which poured from the cracks from the pressure of the flow. The couple and their two children were trapped for several hours, their faces pressed to the ceiling, with little breathing room.
Personnel from Los Angeles County Fire Department Stations 82 and 19 were confronted with head-high levels of mud and debris that had settled in the canyon bottom, blocking access by fire apparatus. Firefighters used plywood sheeting to spread their weight and to create a path over deep mud to reach the roof of the home, the ridge of which was protruding from the muck. Carefully using chain saws and axes to avoid injuring the victims trapped just beneath the ceiling, the firefighters burrowed through the roof and ceiling to effect a dramatic rescue.
Where Slides Occur
Mud and debris flows are endemic to places where steep mountains and foothills rise above valleys and flood plains. They are common where the mountains have been cracked and fractured by earthquakes and tectonic forces, making them more vulnerable to erosion. If the mountains are covered with highly flammable vegetation, the probability of mud and debris flows increases exponentially because vegetation is sometimes the only thing keeping boulders and soil clinging to the slopes. When fire denudes the vegetation, the rock and soil begin sliding and falling into the canyon bottoms. When intense rain occurs, tremendous amounts of debris can be quickly turned to a slurry and mobilized into a huge flood.
Photo by Gene Blevins/CFPA
The car went over the side of the cliff, killing the driver.
Mud and debris flows can also be associated with the occurrence of landslides and mudslides. This process sometimes occurs where steep slopes are underlain by siltstone bedrock. An analogy can be made to a layer cake, with the frosting representing topsoil. If the layers are tilted (as in the case of geologic upthrust related to earthquake faulting), gravity begins to act upon them, pulling them toward the canyon bottom. If the slope is covered with vegetation that has deep root systems, it can resist slippage. But as the slope becomes saturated, water percolates through the topsoil, eventually reaching the siltstone.
The effect is intensified where recent fires have damaged the vegetation. The moisture begins to act as a lubricant between the layers, increasing the tendency of the soil layers to slide away. Eventually, the bedrock softens and begins to break up. When the weight of the water-laden soil reaches a critical point, gravity can overcome the "holding power" of the root systems. Portions of the slope begin to break free, creating a cascade of mud and rock that is capable of carrying homes away and burying entire neighborhoods.