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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These collapses were a precursor to a huge mudslide the occurred during the El Nino-spawned storm in January 1995. The cliff collapsed and buried a dozen homes, prompting the Ventura County Fire Department to establish a major multi-day search and rescue operation that included regional USAR units, swiftwater rescue teams, heavy equipment, camp crews, ground-penetrating radar specialists, geologists and soils engineers.
The 1995 mudslide, in turn preceded the disastrous mudslide of Jan. 10, 2005, when the same mountainside collapsed once again. This time, the event was filmed in its entirety by news cameras covering mud and debris flows that had trapped motorists on the 101 Freeway, with the Ventura County Fire Department already committed to major rescue operations there. Nearly three dozen homes were buried in a mudslide/debris flow that took only eight seconds to occur. Ten people were killed and dozens injured in that disaster, and the town remains under threat of future mudslides and mud and debris flows because the basic conditions that precipitated the 1889 and 1909 slides are still present.
What Are Mud and Debris Flows?
Author John McPhee, in his book The Control of Nature, describes a mud and debris flow that buried a neighborhood in LaCanada in 1978, following the huge "Mill Fire" that swept across the San Gabriel Mountains: "It was not a landslide, not a mudslide, not a rock avalanche; nor by any means was it the front of a conventional flood...In geology, it would be known as a debris flow. Debris flows amass in stream valleys and more or less resemble fresh concrete. They consist of water mixed with a good deal of solid material, most of which is above sand size. Some of it is Chevrolet size. Boulders bigger than cars ride long distances in debris flows. Boulders grouped like fish eggs pour downhill in debris flows...It was not only full of boulders; it was so full of automobiles it was like bread dough mixed with raisins."
The mud and debris buried several homes to the roofs. One family and most of their furniture was floated to the ceilings by the invading mud. They were trapped for several hours, faces to the ceiling with precious little breathing room, until Los Angeles County firefighters tore through the roof of the house to rescue them.
The mountains of the West are among the most prolific producers of mud and debris flows in North America. The tectonic forces that raise them are still at work. For example, the Northridge Earthquake raised sections of the Santa Susana and San Gabriel mountains 12 inches in seconds. The Northridge quake and its aftershocks sent millions of tons of rock and soil into canyon bottoms all across the San Gabriel Mountains and elsewhere. Every time the earth shook, clouds of dust rose out of the canyons as landslides rained down.
Not only did the Northridge quake increase the amount of debris; it greatly increased the probability of upstream natural dams that might form and burst, sending down huge walls of water, mud and debris. As Southern California dealt with the quake and braced for further winter storms, the possibility of disastrous mud and debris flows loomed large for firefighters in vulnerable areas.
And every day, regardless of earthquakes, rocks break and fall off the steep slopes continuously as the mountains erode. Moderate rain increases erosion and carries the broken rock into streams. Heavier rain begins moving even large boulders. As the hillsides become saturated, mud and rock slides begin filling the canyons with more debris. The heaviest debris normally remains in the canyons, creating rocky streams and occasional natural dams. The debris may pile up in this manner for years.
Intense rainfall can mobilize the accumulated debris with disastrous results. A good example is found in Southern California, where the winters are often characterized by severe storms sweeping in from the Pacific Ocean and stalling when they hit the mountains, dropping millions of tons of water into steep, narrow canyons. Despite the usually arid climate, the San Gabriel Mountains have seen some of the most intense rainfall recorded anywhere on Earth. In 1978 (the year of the "Mill Fire"-related debris flows), 12 inches fell in 24 hours and 1Â½ inches poured down in one five-minute period, causing a debris flow 25 feet high that swept at least 13 people to their deaths in an area off Angeles Forest Highway. In 1933, over 30 people were killed by a large debris flow from the San Gabriels near Glendale/Montrose. Twenty-six inches fell in 24 hours in 1943.