A form of rapid mass movement in which loose soil, rock and sometimes organic matter combine with water to form slurry that flows downslope. They have been informally and inappropriately called mudslides due to tha large quantity of fine material that they may be present in the flow.
Debris flow can be deadly as they can be extremely rapid and may occur without any warning.
Sketch of a debris flow
Debris flows occur around the world and are prevalent in steep gullies and canyons; they can be intensified when occurring on slopes or in gullies that have been denuded of vegetation due to wildfires or forest logging. They are common in volcanic areas with weak soil.
These types of flows can be thin and watery or thick with sediment and debris and are usually confined to the dimensions of the steep gullies that facilitate their downward movement. Generally the movement is relatively shallow and the runout is both long and narrow, sometimes extending for kilometers in steep terrain. The debris and mud usually terminate at the base of the slopes and create fanlike, triangular deposits called debris fans,which may also be unstable.
Velocity of travel
Can be rapid to extremely rapid (35 miles per hour or 56 km per hour) depending on consistency and slope angle.
Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. Debris flows also commonly mobilize from other types of landslides that occur on steep slopes, are nearly saturated, and consist of a large proportion of silt- and sand-sized material.
Debris flows can be lethal because of their rapid onset, high speed of movement, and the fact that they can incorporate large boulders and other pieces of debris. They can move objects as large as houses in their downslope flow or can fill structures with a rapid accumulation of sediment and organic matter. They can affect the quality of water by depositing large amounts of silt and debris.
Flows usually cannot be prevented; thus, homes should not be built in steep-walled gullies that have a history of debris flows or are otherwise susceptible due to wildfires, soil type, or other related factors. New flows can be directed away from structures by means of deflection, debris-flow basins can be built to contain flow, and warning systems can be put in place in areas where it is known at what rainfall thresholds debris flows are triggered. Evacuation, avoidance, and (or) relocation are the best methods to prevent injury and life loss.
Maps of potential debris-flow hazards exist for some areas. Debris flows can be frequent in any area of steep slopes and heavy rainfall, either seasonally or intermittenly, and especially in areas that hhave recently burned or vegetation removed by other means.
September 23, 2019