Debris flows and El Niño in southern California:

Why El Niño events trigger debris-flows and what can happen

As a result of the 1997-1998 major El Niño condition in the tropical Pacific Ocean, precipitation forecasts call for considerably increased storms. These forecasts give rise to concern that this winter's rainy season in southern California will pose a much increased potential for debris-flow hazards during severe rainstorms.

The increased precipitation is expected to result in larger storms than normal, including storms covering larger areas, carrying greater moisture content, and the storms may take place more frequently. If weather patterns develop that carry exceptionally severe storm cells over the populated hillside and canyon areas of southern California, a sizable number of people and homes may be at increased risk from debris flows. Potentially damaging debris flows can develop from shallow soil slips, triggered by prolonged heavy rainfall in any winter rainy season. They are most abundant during large storms and in areas where storm cells yield heavy rainfall for several hours...as a result, they are of great concern during a major El Niño condition.

Most (but not all) El Niño events bring damaging storms to the Southern California coast, as experience from past El Niños shows. The maps below show summarize the evidence for increased precipitation during El Niño events.

The December through February rainfall map was constructed from rainfall data for 11 strong El Niños. Coastal Southern California was strongly affected by storms during seven of the El Niño years, less affected by three, and was drier than ususal during only one of the eleven. Full U.S. map with dates of ENSO events. Data from NOAA.	The December through March rainfall map was constructed from rainfall data for 9 strong El Niños. Coastal Southern California was strongly affected by storms during six of the El Niño years, less affected by two, and was drier than ususal during two of the nine. Full U.S. map with dates of ENSO events. Data from NOAA.

The map to the right shows two kinds of information: Increasing shades of blue indicate increasing likelihood of above-average precipitation during the months of November to January. Increasing shades of yellows and reds indicates increasing landslide incidence in the past. Much of coastal California will likely have increased precipitation and much of the area also has had a high incidence of landsliding.

Geologic mapping in southern California
Photographs of landslides and debris flows in Southern California.
What you can do to prepare for landslides and debris flows.

For further information, contact David Miller

USGS El Niño Home page
Geologic mapping and El Niño Home Page
Southern California El Niño Hazards Mapping HomePage

http://geology.wr.usgs.gov/wgmt/elnino/scampen/methods.html, 29 December 1997, Contact: El Niño Web Team