A History of Floods
Floods are among the most frequent and costly natural disasters in terms of human hardship and economic loss. Much of the damage related to natural disasters (excluding droughts) is caused by floods and associated mud and debris flows.
Floods can be categorized by what caused them. Dam failure can cause catastrophic flooding. If a dam fails as a result of neglect, poor design, or structural damage caused by an event such as an earthquake, a gigantic quantity of water is let loose downstream, destroying anything in its path. Intense or prolonged storms that drop large amounts of rain within a brief period can cause flash floods. Flash floods occur with little or no warning and can reach full peak in a few minutes.
Southern California's climate makes floods more likely. The area has a distinct wet season. Floods are more frequent during this season. When a very wet winter follows several dry ones severe flooding can occur.
In The Control of Nature, John McPhee writes about the dramatic floods of 1978. But dramatic floods occur regularly in the San Gabriel foothills. La Crescenta, Shields Canyon, La Canada, and Pickens Canyon have experienced several profound and damaging floods during the 20th century.
The year 1934 came in with a bang in Los Angeles County. The "New Year's Day Flood" in the La Canada Valley killed more than 40 people, destroyed about 400 houses, and damaged streets, bridges, and highways. A deadly debris flow killed 12 people who had gone to seek shelter in the Montrose Legion Hall. The debris simply crashed through the middle of the building, leaving holes in the uphill and downhill walls.
Why was the 1934 debris flow so damaging? Two reasons - fire and lack of heavy rainstorms in the years before the flood. First, fire increases the likelihood of landslides after a heavy storm. Fire destroys the vegetation that anchors top soil, and makes it more likely that the soil will slide when saturated. A report on the 1934 storm notes, "About 7.5 square miles of mountain area tributary to La Canada Valley was burned over by a fire in November, 1933, and from this burned-over area came practically all of the run-off that produced the debris movement in the La Crescenta-Montrose district." Second, the 1934 storms produced the heaviest rainfall in years. Sediments of all sizes - including boulders weighing tons - had been building up in the canyons for a long time. The heavy rains and the burned hills caused the debris to flow, and destroy all in its path.
Living in Earthquake Country
Southern California is home to more than 20 million people and is vital to the Nation's economy. Unfortunately, the region is also laced with many active faults that can produce strong earthquakes. The San Andreas Fault is the best known. It runs almost the entire length of California and generates shocks as large as magnitude 8. In Southern California the last magnitude 8 earthquake was in 1857. But smaller temblors, like the 1971 San Fernando and 1994 Northridge earthquakes, occur more frequently. Both of these magnitude 6.7 quakes were very damaging.
An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earth's surface. This shaking can cause buildings and bridges to collapse; disrupt gas, electric, and phone service; and sometimes trigger landslides, avalanches, flash floods, fires, and huge, destructive ocean waves (tsunamis).
More Earthquakes to Come
Southern California has a problem with earthquakes - it hasn't been having enough of them. After the 1994 magnitude 6.7 earthquake in Northridge (northwest of La Crescenta), scientists used new techniques to study seismic activity. They have discovered that Southern California has not had enough large earthquakes to release all the pressure building up underground.
When the tectonic plates under the Earth's crust grind against each other, energy builds. In Southern California, the stresses are distributed along the San Andreas Fault and other smaller faults. When too much stress builds up along a fault, the earth's crust cracks and earthquakes occur. Quakes must occur to relieve the pressure along the faults.
But a look back at the earthquake history of the last two centuries suggests that Southern California should have had seven times as many Northridge-sized earthquakes as it has had. The scientists' conclusion: in Southern California, the probability of a magnitude 7 or greater earthquake by the year 2024 is as high as 80 to 90 percent.
Landslides occur when masses of rock, earth, or debris move down a slope. Landslides may be small or large and can move at slow to high speeds. They are activated by storms and fires and by human modification of the land. New landslides occur as a result of rainstorms, earthquakes, volcanic eruptions, and various human activities.
Mudflows (or debris flows) are rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, such as during heavy rainfall. Mud and debris flows can move rapidly down slopes or through channels and can strike with little or no warning at avalanche speeds. These flows can travel several miles from their source, growing in size as they pick up trees, cars, and other materials along the way.
Landslides are classified in several ways. One way to describe a landslide is by sediment size. Debris flows contain a wide assortment of sediments - boulders, sand, and mud - but gener-ally less than 10 percent silt and clay. Earthflows contain few boulders and contain mostly fine sand, silt, and clay. Debris flows move faster, farther, but earthflows can move over lower slopes. In the mountains, debris flows and mudflows fill up channels and reservoirs. Landslides are also classified by material type and how it moves. Geologists also describe landslides by the percentage of saturation (with water) and the slope inclination necessary for the slide to occur. (See the table "Geologic environments likely to produce earthquake-induced landslides in the Los Angeles region.")
Landslides and mudflows are common events in Los Angeles County because of active mountain-building processes, rock characteristics, intense storms, and earthquakes. In fact, the 1994 Northridge earthquake triggered more than 10,000 landslides. Many landslides have been occurring over the same terrain since prehistoric times. Although they may be stable right now, these landslides can be activated by storms, fires, or inappropriate human modification of the land.
|This figure shows that numerous faults fracture the Earth's crust in southern California. Remember that an earthquake is a release of stored energy that causes the sudden movement of rock on opposite sides of a fault.
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