Geologic processes and El Niño storms

The 1997-1998 El Niño climatic event is predicted to bring heavy rain and snow to the west coast and inland areas. Damage caused by moving water, rock, and mud, geologic processes that U.S. Geological Survey studies, are described below.

Landslides
Landslides are the wholesale movement of rock and soil down-slope by gravity; they can occur quickly or slowly, but they most often are aided by excessive rainfall or snowmelt.. The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors:

• rock and soil slopes are weakened as they are saturated by snowmelt or heavy rains
• erosion by rivers or ocean water at the base of slopes creates oversteepened slopes
• excess weight from accumulation of rain or snow, stockpiled rock or ore, or waste
• earthquakes create ground shaking that makes weak slopes fail
• volcanic eruptions produce loose ash deposits, heavy rain, and debris flows
• vibrations from machinery, blasting, and even thunder may trigger failure of weak slopes

Slow movement of landslides and gradual creep of rock and soil downslope can be as destructive as fast slides but are less likely to cause loss of life. Photograph: Madison landslide, MT, 1959; white area is new landslide deposit damming the river to form a lake.

Debris flows and mudflows
Debris flows (also referred to as mudslides, mudflows, or debris avalanches) are debris laden flows of water, often the consistency of wet concrete, that move surprisingly quickly down steep slopes. They move so rapidly (to speeds of 35 miles per hour or more) that they are generally unexpected, destroying property and taking lives. Material on steep slopes that becomes saturated with water after prolonged, intense rain may develop a debris flow or mud flow. The resulting slurry of rock and mud may destroy homes, wash out roads and bridges, sweep away cars, knock down trees, and obstruct streams and roadways with thick deposits of mud and rocks. Alluvial fans close to mountain fronts commonly are mostly made up of debris-flow deposits, indicating that this is an area of potential debris-flow hazard.

Photograph: Debris flow path in the Sierra Nevada, CA, January 1997; flow destroyed huge trees in a path in places over 100 feet wide , and carried huge boulders as wide as 15 feet. It resulted from heavy rains that rapidly melted snow.

Flash floods
Flash floods take place in normally dry country as a result of intense cloudbursts that cause water to wash across the surface, collecting in rivulets and washes. By the time the water reaches valley-bottom washes, walls of water several feet deep can be moving rapidly downstream. Flash floods are all the more alarming because they can destroy property and wash away cars miles from the place where the rain fell.

Photograph: Flash flood on the Kaiparowits Plateau, UT. The rapids indicate the water ismoving very fast; at one point, the flood waters were 8 feet deep.

For further information, contact David Miller

http://geology.wr.usgs.gov/wgmt/elnino/examples.html, 30 November 1999