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.
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.
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.
In some cases, mudslides and landslides can block the flow of streams, creating natural earthen dams that might not be detected downstream (especially if the slide occurs at night, far upstream from populated areas). The danger lies in the potential for the stream to breach the dam, loosing a flood of mud and debris upon populated areas downstream.
Intense rainfall can act as the trigger to "mobilize" mud and other material in areas that have been "pre-loaded" by the continuous fall of rocks and debris into the bottoms of canyons and drainages. In 1978, a foot of rain fell in 24 hours in the San Gabriel Mountains above Los Angeles, including 10 inches in one five-minute period. The result was a mud and debris flow 25 feet high that swept at least 13 people to their deaths. In 1933, over 30 people were killed by a large debris flow from the San Gabriel Mountains, near Glendale. One mud and debris flow sequence in the Wrightwood area of the San Gabriels dislodged 18 million yards of material from the mountain and transported it 15 miles into the desert.
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 boulders as large as five-story buildings. A debris flow in the Tujunga area of Los Angeles County carried a 15-ton boulder into a residential area nearly two miles "downhill" from the edge of the San Gabriels.
The Role Of Wildfires
Major wildfires greatly increase the incidence of mud and debris flows. One of the chief factors in this phenomenon is the heat, which can coalesce plant life and soil into a coating which repels water. In Southern California, for example, chaparral contains high levels of long chain hydrocarbons (oils) which 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 sweep the area, fed explosively by the oil-laden plants, 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 during this reaction.
After the fire, dry unconsolidated particles of soil, rock, ashes, and other material are left in thick layers on steep canyon walls. Without plants and roots to hold it to the slopes, it begins cascading down hillsides in a steady dry stream. High winds under these conditions have been known to create dust storms comparable to a bad day in the Sahara. Thus, the canyons are "pre-loaded" for debris flows.
Rain is the trigger that can move large portions of mountains. The topsoil quickly becomes saturated, increasing the pore pressure just above the hydrophobic (waterproof) layer. The soil liquefies and begins moving downhill. It seeks the path of least resistance, forming small canals called "rills" across the hydrophobic layer. The rills speed the water down slope, 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. Massive loads of debris can be moved by relatively small amounts of rain. With the onset of intense rain, the trigger is set, the trap is sprung, and anyone down canyon is vulnerable to the unannounced arrival of mud, rock, water, trees, cars and even large buildings that have been swept into the flow.
Larry Collins
LA City Firefighters Pull Horse From Muck
The Los Angeles City Fire Department was called to rescue a horse that had become stuck in a flood-control basin in the Pacoima area. The first units on the scene reported that the horse was up to its belly in mud. The rider had tried to take the animal through the muck, thinking it was stable enough to support its weight. Firefighters dug a space under the horse and placed tarpaulins beneath it, then moved it onto pieces of plywood and more tarpaulins. The animal was then placed in a sling and harness. A helicopter was called in and after a failed first attempt was able to airlift the horse to a drier area (after a veterinarian administered a sedative). The horse was not injured. This was the second such rescue within two weeks.
Mike Meadows
