Debris flows and El Niño in southern California: What can happen?
Understanding debris-flows and the damage they cause
Intense storms and prolonged precipitation can cause floods, debris flows, and landslides. All depend to some extent on how much rain has already fallen (saturating the ground), how intense a particular storm is, and how rapidly snow melts in the mountains.
Floods. Flash floods move so rapidly it can be hard to outrun them. Flash floods are all the more alarming because they can destroy property and wash away cars miles from the place where the rain fell. They occur in channels cut into into alluvial fans and in 'dry' washes (arroyos) in valleys during brief periods of intense rainfall.
Debris flows. Debris-flows (also referred to as mudslides, mudflows, earth flows, or debris avalanches) are debris-laden flows of water, often the consistency of wet concrete. Material on steep slopes that becomes saturated with water after prolonged, intense rain or rapid snow melt 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.
Landslides. Landslides are the wholesale movement of rock and soil down-slope by gravity; they can occur quickly or slowly. Landslides are much more likely to occur when the slopes are saturated by water.
How floods, flows, and slides are triggered.
Debris flows and floods generally occur during prolonged heavy rainfall
during individual storms, and generally end when the individual storm ends.
Rapid spring snow melt may trigger floods and debris flows as well. In contrast,
landslides are triggered by excessive rainfall but can continue moving after
storms end. When rainfall totals exceed average rainfall, the number of
landslides increases, and the landslides can continue to move and cause
damage long after the end of the stormy season. Economic losses from landslides
in California greatly increase in seasons with rainfall exceeding the average.
Reducing damage -- in the
past and now
What are debris flows and how
do we anticipate their danger?
Preparing for future El
Niño events
More on floods, flows, and slides: Effects of storms in the desert
Photographs of landslides and debris flows in Southern
California.
LINKS: El Niño pages for Southern California
For further information, contact David Miller
http://geology.wr.usgs.gov/wgmt/elnino/scampen/what.html, 29 December 1997, Contact: El Niño Web Team