Debris flows and rainfall patterns

Debris flows and El Niño in southern California

The effects of excess rainfall on debris-flows

Comparing historic rainfall records and times of debris flows for southern California provides information on how much rainfall is needed to trigger debris flows and what kinds of storms most often trigger them. For areas of natural chaparral, sage, and annual vegetation cover, debris flows are most likely to occur when:

Rain falls at more than one-fifth of an inch in an hour for a sustained period of more than three hours, but sometimes as little as one hour. However, not all storms that rain this hard cause debris flows.

The slope soils must generally have received at least 10 inches of prior total seasonal rainfall.

Cumulative rainfall in and near the Santa Monica and San Gabriel Mountains. Steeper parts of curves indicate intense rainstorms (some over 2 inches per hour). Red parts are times that debris flows occurred near rain gage. Debris flows occur during intense rain storms (storms with more than 1/4 inch per hour), but only after about 10 inches of rain have fallen.

The fire-flood history of southern California wildfires shows that recently burned areas have many more debris flows. Compared to vegetated hills, debris flows in burned areas start earlier -- no prior rainfall is necessary -- and after less intense, briefer storms. A hydrophobic layer in the soil can be created by intense fires; this layer repels water and increases runoff from later storms, increasing the likelihood of debris flows.

Debris flow hazards and landscape: what locations are dangerous?

Debris flow hazards and wildfires

How to prepare for debris flows

Geologic mapping in southern California

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/rainfall.html, 29 December 1997, Contact: El Niño Web Team