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 be mobilized into a debris flow, two conditions (in addition to initial landsliding) must be met. First, the debris mass must contain enough water to flow when agitated during sliding. Second, the gravitational potential energy possessed by the debris must be converted into enough internal kinetic energy to change the mode of movement from rigid block sliding to fluid flow throughout the debris mass. Landslides that mobilize into debris flows often occur along topographic concavities or hollows, which concentrate groundwater flow and contain thicker accumulations of regolith than surrounding ridges. Concentrated groundwater flow increases the wetness of regolith in hollows, making it particularly susceptible to destabilizing groundwater pressure increases during and immediately after rainstorms. Debris stops flowing when the internal kinetic energy drops below the level necessary to maintain fluid flow, commonly because the channel through which the debris flows flattens or widens.
Debris flows are also common after intense wildfires. Fires that spawn debris flows create a layer of water repellent (hydrophobic) soil a few millimeters below the ground surface. Hydrophobic soil impedes the infiltration of rainwater, mobilizing the overlying soil into small debris flows and forming a drainage network throughout the burned area . The small debris flows contribute sediment to nearby stream channels, which can then be mobilized into larger debris flows during heavy rainstorms. Debris flows can also begin when hot volcanic ash flows melt snow and ice or when floods incorporate large amounts of sediment, but these are rare occurrences relative to debris flows mobilized from landslides.
See also Catastrophic mass movements; Drainage basins and drainage patterns; Erosion; Mass movement; Mass wasting; Mud flow