Debris flow is a process in which water-saturated masses of material ranging from sand grains to boulders move across low slopes. These flows range from gently flowing sand and water slurries to violently surging bouldery masses, and include events described as debris slides, debris torrents, mudflows, mudslides, earthflows, and lahars.

Observations have shown that debris flows often move in waves or surges, each wave consisting of a coarse-grained snout followed by a finer grained and more fluid tail. The consistency of flowing debris has been described as being similar to wet concrete, although water accounts for less than half of the debris flow volume. Debris flows typically have bulk densities almost identical to the water-saturated regolith or sediment from which they are derived. Clay- and silt-sized grains are generally very minor constituents.

Most debris flows begin as landslides or slumps. In order for a landslide to...