Debris flows and El Niño in southern California: What are they?

Understanding debris-flows and the damage they cause

Debris flows, sometimes called "mudslides", are mixtures of water with mineral particles and rock fragments. The particles range in size from clay and fine silt to large boulders, and include varying amounts of plant material, front ground litter to tree trunks. The blending of these materials produces thick slurries that are thoroughly mixed. The flowing slurries look like wet concrete. Like wet concrete, the slurry is more fluid with increasing water and decreasing solid particles. Many debris flows start as small (10 to 15 feet wide) shallow landslides, commonly termed soil slips, located high on steep slopes, where small movements can cause the structure of water-soaked soil to collapse, transforming a discrete soil slab into a slurry, which then flows downslope in existing swales, ravines and canyons. Debris flows that reach the base of a slope, the mouth of a ravine, or the fan at the mouth of a canyon, can be greatly enlarged from their initial volume by added water-charged sediment scoured from the channels along the way.

The speed of an individual flow depends on the fluidity and depth of the slurry and the steepness of the channel. At the foot of a long steep slope a flow may move at avalanche speed, 40 feet per second (which is 27 miles per hour, or faster than an Olympic runner in the 100-yard dash) and is capable of crushing buildings. In gently inclined channels, some debris flows move at speeds as slow as 1 ft/s. The slurry may slow and stop if water separates and drains from the slurry or the channel widens and the slurry spreads out and thins. As a result, large volumes of debris can stop abruptly in gently inclined channels. On alluvial fans, abruptly stopped debris can block a channel, causing later floods and flows to shift to alternative routes across a fan. As a result, a building far from a channel on an alluvial fan can be damaged by repeated debris flows if channels aren't cleared of debris.

Photo ­ Scar of soil slip and debris-flow channel above house in the Santa Monica Mountains, which occurred on January 25, 1969. Photo courtesy of the Department of the County Engineer, Los Angeles County.

More examples of debris flows: photographs taken over the last couple decades.

With knowledge of where debris flows occur on the landscape, and how they are triggered by rainfall, we can reduce the hazards they pose.

Debris flow hazards and landscape: what locations are dangerous?

Debris flow hazards and rainfall: what does heavy rainfall do?

How you can prepare for debris flows

Geologic mapping in southern California

For further information, contact David Miller

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