Debris flow

Debris flow channel with deposits left after 2010 storms in Ladakh, NW Indian Himalaya. Note coarse bouldery levees on both sides of the channel, and poorly sorted rocks on the channel floor. Click for more detail.

Scars formed by debris flow in greater Los Angeles during the winter of 1968-1969. The photograph was taken within several months of the debris flows occurring.

Debris flows are fast moving, liquefied landslides of mixed and unconsolidated water and debris that look like flowing concrete. They are defined by their non-newtonian flow dynamics, and behave as Bingham plastics. This characteristic can lead to the formation of levees at the margins of unconstrained debris flows as the margins of the flow freeze. They are differentiated from mudflows by their coarser and more poorly sorted sediment load. Flows can carry material ranging in size from clay to boulders, and may contain a large amount of woody debris such as logs and tree stumps. Flows can be triggered by intense rainfall, glacial melt, or a combination of the two. Speed of debris flows can vary from 5 km/h to up to 80 km/h in extreme cases.^{[citation needed]} Volumes of material delivered by single events vary from less than 100 to more than 100,000 cubic metres.^{[citation needed]} Variables considered important in debris flow initiation include slope angle, available loose sediment, and degree of land disturbance by activities such as forest harvesting. Debris flows are often more frequent following forest and brush fires, as experience in southern California clearly demonstrates. Debris flows are extremely destructive to life and property, and claim thousands of lives world-wide in any given year. They are a particular problem in steep mountainous areas subjected to intense rainstorms, and have received particular attention from researchers in Japan, Western USA, Western Canada, New Zealand, the European Alps, and Kazakhstan.

True debris flows are very sediment dense, typically > 60 % sediment by volume. Less concentrated flows would be hyperconcentrated or fluvial.

Features and behavior

Ancient debris flow deposit, Resting Springs Pass, California.

Debris flows generally form when unconsolidated material becomes saturated and unstable, either on a hillslope or in a stream channel. Flows are accelerated downhill by gravity and tend to follow steep mountain channels. The front, or 'head' of debris flow often contains a great deal of coarser material such as boulders and logs. Trailing behind this frontal lobe is the less viscous, main part of the flow that contains sand, silt and clay. Debris flows eventually become 'thinner', or less viscous, muddy flood waters as they deposit their coarser components in areas of reduced gradient. Debris flows tend to move in pulses, or discrete surges, as friction or other barriers are overcome during the flow. Sometimes earlier pulses or previous debris flow deposits form levees which confine the flow until they are breached by later, larger flows. The presence of older levees (a.k.a. lateral deposits) provides some idea of the magnitudes of previous debris flows in a particular area, and through dating of trees growing on such deposits, may indicate the approximate frequency of destructive debris flows. This is important information for land development in areas where debris flows deposit material, known as a debris cone or colluvial fan. The big debris flow or landslide is called yamatsunami (山津波), literally mountain tsunami in Japan.

Another debris flow in Ladakh, triggered by storms in 2010. Note poor sorting and levees. Steep source catchment is visible in background.

Types

Other geological flows can also be described as debris flows, though are typically given more specific names. These include:

Lahar

Main article: lahar

A lahar is a debris flow related in some way to volcanic activity, either directly as a result of an eruption, or indirectly by the collapse of loose material from the flanks of a volcano. A variety of factors may trigger a lahar, including melting of glacial ice due to volcanic activity or climate change, intense rainfall on loose pyroclastic material, or the out bursting of a lake that was previously dammed by pyroclastic or glacial material. The word lahar is of Indonesian origin, but is now routinely used by geologists world-wide to describe volcanogenic debris flows.

Jökulhlaup

Main article: Jökulhlaup

A jökulhlaup is a debris flow that originates from a glacial outburst flood. Jökulhlaup is an Icelandic word which refers specifically to floods having a glacial trigger. In the case of Iceland, many such floods are triggered by sub-glacial volcanic eruptions, since Iceland sits atop the Mid-Atlantic Ridge. Elsewhere, a more common cause of jökulhlaups is the breaching of ice-dammed or moraine-dammed lakes. Such breaching events are often caused by the sudden calving of glacier ice into a lake, which then causes a displacement wave to breach a moraine or ice dam. Downvalley of the breach point, a jökulhlaup may increase greatly in size through entrainment of loose sediment and water from the valley through which it travels. Travel distances may exceed 100 km.

Theories and models of debris flows

Debris flow is an example of granular convection which occurs in many different environments and scales. Numerous different approaches have been used to model their properties and kinematics, some of which are listed here.

Debris flows as mud flows

• Rheologically based models that apply to mud flows that are treated as a homogeneous liquid (Examples include: Bingham, viscoplastic, Bagnold-type dilatant fluid, thixotropic, etc.)
• The mixture theory of Iverson
• Dam break wave, e.g. Hunt, Chanson et al.
• Roll wave, e.g., Takahashi, Davies

Unsaturated "rocky" or "stony" debris flows

• Progressive wave
• A type of translating rock dam

Damage prevention

Almaty, Kazakhstan, after the catastrophic debris flow of 1921. A number of facilities, including the Medeu Dam, have been built since to prevent flows of this kind from reaching the city.

In order to prevent debris flows reaching property and people, a debris basin may be constructed. Debris basins are designed to protect soil and water resources or to prevent downstream damage. Such constructions are considered to be a last resort because they are expensive to construct and require commitment to annual maintenance.

See also

• Colluvium
• Illhorn below which lies Illgraben a popular debris flow tourist spot
• Landslide classification
• Rheology

References

Further reading

• McPhee, John. The Control of Nature. New York: Noonday Press (Farrar, Straus & Giroux, 1989 ISBN 0-374-12890-1)

External links

• Dongchuan Debris Flow Observation and Research Station
• Mass Movements. Section of the information platform "Natural hazards in Switzerland"
• Washington State information about Debris flows, and related material