A debris flow is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. To be considered a debris flow the moving material must be loose and capable of "flow", and, at least 50% of the material must be sand-size particles or larger.
Some debris flows are very fast - these are the ones that attract attention. In areas of very steep slopes they can reach speeds of over 100 miles per hour (160 km/hour). However, many debris flows are very slow, creeping down slopes by slow internal movements at speeds of just one or two feet per year (30 to 60 centimeters per year). The videos at right illustrate debris flows and show how they move.
Debris-flow source areas are often associated with steep gullies, and debris-flow deposits are usually indicated by the presence of debris fans at the mouths of gullies. Image and caption by the United States Geological Survey.
The Debris Flow Hazard
The speed and volume of debris flows make them very dangerous. Every year, worldwide, many people are killed by debris flows. This hazard can be reduced by identifying areas that can potentially produce debris flows, educating people who live in those areas and govern them, limiting development in debris flow hazard areas, and developing a debris flow mitigation plan.
Conditions Required to Produce a Debris Flow
The source area of a debris flow must have: 1) a very steep slope, 2) an abundant supply of loose debris, 3) a source of abundant moisture, and 4) sparce vegetation. Identifying areas where debris flows have happened in the past or where these conditions are present is the first step towards developing a debris flow mitigation plan.
Is It a Debris Flow, a Mud Flow or a Landslide?
Debris flows differ from slides because they are made up of "loose" particles that move independently within the flow. A slide is a coherent block of material that "slides" over a failure surface.
A mud flow is composed of mud and water. Debris flows have larger particles - at least 50% of a debris flow is made-up of sand-size or larger particles.
What Causes Debris Flows?
Debris flows can be triggered by many different situations. Here are a few examples:
Addition of Moisture: A sudden flow of water from heavy rain, or rapid snowmelt can be channeled over a steep valley filled with debris that is loose enough to be mobilized. The water soaks down into the debris, lubricates the material, adds weight, and triggers a flow.
Removal of Support: Streams often erode materials along their banks. This erosion can cut into thick deposits of saturated materials stacked high up the valley walls. This erosion removes support from the base of the slope and can trigger a sudden flow of debris.
Failure of Ancient Landslide Deposits: Some debris flows originate from older landslides. These older landslides can be unstable masses perched up on a steep slope. A flow of water over the top of the old landslide can lubricate the slide material or erosion at the base can remove support. Either of these can trigger a debris flow.
Wildfires or Timbering: Some debris flows occur after wildfires have burned the vegetation from a steep slope or after logging operations have removed vegetation. Before the fire or logging the vegetation's roots anchored the soil on the slope and removed water from the soil. The loss of support and accumulation of moisture can result in a catastrophic failure. Rainfall that was previously absorbed by vegetation now runs off immediately. A moderate amount of rain on a burn scar can trigger a large debris flow.
Volcanic Eruptions: A volcanic eruption can flash melt large amounts of snow and ice on the flanks of a volcano. This sudden rush of water can pick up ash and pyroclastic debris as it flows down the steep volcano and carry them rapidly downstream for great distances. In the 1877 eruption of Cotopaxi Volcano in Ecuador, debris flows traveled over 300 kilometers down a valley at an average speed of about 27 kilometers per hour. Debris flows are one of the deadly "surprise attacks" of volcanoes.