debris flows

debris flows A debris flow is a gravity-induced, rapidly moving body of sediment particles, water, and/or air. Such flows are an intermediate stage between landsliding and water flooding, but debris flows originate when poorly sorted rock and soil debris are mobilized from hillslopes and channels by the addition of moisture. These conditions exist in a variety of settings, including mountainous areas in arid and semi-arid, arctic, and humid regions. The exact mechanism by which landslides change into debris flows is uncertain, but the transformation from a solid, rigid soil mass to a viscous fluid may occur as a landslide changes from a close-packed and dense soil structure to an open-packed structure, accompanied by an increase in pore volume. Incorporation of moisture then transforms the sliding mass into a flowing, viscous fluid.

Debris flow resemble wet concrete, and when it flows usually follows pre-existing drainage ways, but debris flows can also move down hillslopes and across unobstructed alluvial fans in almost any direction. Flows may appear as a series of waves or surges with periods ranging from a few seconds to several hours. Flow velocities can be very fast, from 1 to 20 m per second, and are controlled by the characteristics of the sediment in the flow (size, shape, sorting) and by the topography (channel slope, width, shape, and sinuosity).

Physical properties of debris flows vary widely. The density of clear water is 1.0 g ml⁻¹, but during floods, when large amounts of sediment are being moved, the density of stream flow is typically 1.01 to 1.3 g ml⁻¹. Measured densities of debris flows range from about 1.40 to 2.53 g cm³, and of common rocks from about 2.7 to 3.0 gm/cm³. Large and small pieces of rock can thus nearly float in debris flows because they are so similar in density. The high density of debris flows imparts an internal strength to the material that must be overcome before the material will begin to flow. This is very different from sediment-free water, which has no internal strength. Debris flows have been known to transport boulders weighing 30–40 tonnes for tens of kilometres, and during the passage of flows, witnesses have reported ground shaking and loud roaring and rumbling noises. Debris flows are capable of exerting enormous impact forces on objects in their path. Buildings have been destroyed, and large trees snapped off.

Debris flows can flow for many kilometres beyond their source areas, and tend to stop upon reaching areas with relatively low gradients or areas of decreased confinement, such as alluvial fans at the mouths of small watersheds or canyons. At Mount St. Helens in 1980, when glacial ice within a huge debris avalanche that accompanied the major eruption melted it mobilized over 150 tonnes of sediment into an extraordinary debris flow that moved over 60 kilometres from the volcano. This flow deposited so much sediment in the Columbia River that shipping lanes were closed until dredging eventually cleared the channels. Debris-flow deposits from side tributaries are the origin of most of the large rapids in deep canyons such as the Grand Canyon in the United States. The exact mechanism by which debris flows stop flowing is uncertain. Lateral spreading might result in the thickness or depth of a flow to decrease below the minimum required for flow movement to continue; the escape of pore fluids such as water, clay, and fine silt might result in an increase in internal friction. Because debris flows have a finite strength, their deposits have unique characteristics. At the distal and marginal edges of flows, lobes and levées with steep fronts and surface concentrations of large boulders commonly occur. These landforms are characteristic of debris flows and can be preserved for many years. In cross-section, the deposits consist of unsorted pebbles, cobbles, and boulders in a matrix of fine-grained debris. Bedding, characteristic of river-laid sediments, is absent in debris-flow deposits.

Damage and loss of life from debris flows can be mitigated by four general kinds of remedial measures: (1) identification and avoidance; (2) control of grading, clearing, and drainage; (3) protective structures; and (4) warnings and evacuations. Because of their elevation above floodplains, alluvial fans have long been favoured sites for development. Unfortunately, mitigating procedures and identification of risk areas for debris flows are poorly developed in comparison with those for water floods. Dangerous areas cannot be identified systematically and consistently, nor can reliable data be obtained on frequency of inundation. As a general rule, the bottoms and mouths of small, steep ravines that originate in steep, hilly or mountainous terrain (especially volcanic areas), or in areas of historic and prehistoric debris flows, should be considered as potential debris flow areas and avoided.

It is generally believed that erosion by debris flows can be reduced by strict controls of land use, grading, and drainage. On artificial slopes, this could include limiting the height of slopes, properly compacting fills, and ensuring that drainage is channeled away from potential source areas. Devegetation by wildfires or overgrazing in source areas generally increases the likelihood of debris flows.

The construction of protective barriers to stop, slow, or divert debris flows may be necessary if avoidance of hazardous areas is not possible. Channelling of debris flows is usually ineffective because channels can quickly become choked with sediment, allowing subsequent surges to overflow the channel and flow in different directions. Closely spaced trees can be quite effective in stopping boulders and other large debris. Structural fences of steel and reinforced concrete, steel cable nets, debris fences, and sediment barriers can be effective in stopping or separating large boulders from debris flows. Large reservoirs to trap and store debris upstream of developments have been successful in many locations. Because debris flows frequently originate from sudden landslides in remote locations and travel at high speeds, it is difficult to provide direct warnings. Ground shaking and loud noises may provide a short warning, and sensors and trip-wires installed in upstream locations can detect the passage of debris flows and enable alerts to be issued. Longer-term warnings may consist of identification of minimum precipitation thresholds for slope failures in debris-flow prone areas. Despite the expenditure of large sums of money on protective and warning devices, debris flows will probably continue to reap a large toll in property and lives throughout the world.

John E. Costa

Bibliography

Johnson, A. M. with contributions by J. R. Rodine (1984) Debris flow. In Brunsden, D. and Prior, D. B. (eds) Slope instability pp. 257–361. John Wiley and Sons, Chichester.
Pierson, T. C. and and Costa, J. E. (1987) A rheologic classification of subaerial sediment–water flows. In Costa, J. E. and Wieczorek, G. F. (eds) Debris flows/avalanches: process, recognition, and mitigation, pp. 1–12. Geological Society of America, Reviews in Engineering Geology, vol. VII.