Debris flows consists in fast gravitational particulate flows involving multi-phase media in which water-saturated masses of material are ranging from grains to boulders. These flows range from gently flowing sand and water slurries to violently surging bouldery masses, and include events described as debris slides, debris torrents and mudflows. Observations during real events have shown that debris flows move in waves or surges, each wave consisting of a coarse-grained snout followed by a finer 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. Unlike rock avalanches and sediment ladden water floods, both solid and fluid forces affect the motion in debris flow.
- Debris flows are generally depicted in three parts: the debris flow front, the debris flow body and the debris flow tail.
Spatially, the debris flow source area, the debris flow track and the debris flow accumulation zones have to be analysed.
- Debris flow track
Two types of debris flows are described.
Muddy debris flows are very watery slurries with a big silt rate and a water content higher than 50%. Muddy debris flows are characterized by high front velocity (of about 6 to 18 m.s-1) and rather low density of about 1800 kg.m-3.
Granular debris flows have lower water content. The granular flow spread out slowly with a front velocity of about 1 to 5 m.s-1. The density of a granular debris flow is in a range of about 2000 up to 2300 kg.m-3. The release of debris flows is often intense rainfall but there are also other triggers like snowmelt in spring or a dam break failure.
- Example of debris-flows. Upper left: debris flow track in the South French Alps. Upper right: Granular debris-flow deposit in Venezuela. Bottom: Muddy debris-flow deposit in the South French Alps.
Origin of a debris flow
Debris flows initiate in a variety of geomorphic settings: they can be triggered from shallow landslides originating on steep slopes, from landslides in topographic swales or hollows, from the entrainment of materials within stream channels, from diffuse erosion, from rock glacier bursts.
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.
The acting forces caused by debris flows are enormous high so the debris flow is a hazard to buildings, infrastructure and people. To define protective measures against debris flows, it is necessary to study parameters such as potential debris flow volume, mean velocity, peak discharge and runout distance. In general, debris flows cannot be predicted in nature because there is little knowledge about the initiating conditions.