Debris flows are geological phenomena in which water-laden masses of
soil and fragmented
rock rush down mountainsides, funnel into
stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally have
bulk densities comparable to those of
rock avalanches and other types of
landslides (roughly 2000 kilograms per cubic meter), but owing to widespread sediment
liquefaction caused by high
pore-fluid pressures, they can flow almost as fluidly as water. Debris flows descending steep channels commonly attain speeds that surpass 10 meters per second (more than 20 miles per hour), although some large flows can reach speeds that are much greater. Debris flows with volumes ranging up to about 100,000 cubic meters occur frequently in mountainous regions worldwide. The largest prehistoric flows have had volumes exceeding 1 billion cubic meters (i.e., 1 cubic kilometer). As a result of their high sediment concentrations and mobility, debris flows can be very destructive.