Mudflow

This article is geological phenomenon; a mudslide is also an alcoholic drink.

A mudflow (German Mure) is the most rapid (up to 80 km/h) and fluid type of downhill mass wasting. Similar terms are debris flow (e.g. in high mountains), mudslide (not very liquid) and mud stream (e.g. from volcanoes; see also lahar).

However, these terms show a broad variety

in water content (from loamy mud up to almost liquid),
in material (wet soil, sandy sediments and/or silt, dirt, rock debris, volcanic ash, small plants and even trees)
and in length, total mass and velocity.

Regions with frequent mudflows

The flow velocity mainly depends on the water percentage and the material; further on the mountain altitude, the mudflow length and its mass. Altogether, the dangerous phenomena occur in different regions:

in alpine countries, especially after heavy rainfalls or when the soil is wet. For instance, after the thaw in spring the soil can easily be detached from the subsurface. Most mudflows begin at steep slopes where the protection by forest may be disturbed, or at the source of a torrent. In the Alps many hundred incidents per year are recorded (all over the world it may be 100 times more) - with an increasing number and damage in the last 20 years.
in arid and semiarid climates when torrential rainfall fails to drain properly and thus causes the nearby rock and dirt to become unstable and to "slide" down a hill or mountain.
at volcanoes, often released by an eruption with simultaneous rainfall. Some disasters of the last decades show: not the size of the volcano or the eruption is the most important parameter, but sometimes rather the coincidence of local slope inclination, special ash sorts, and rainy or thunderstorming weather.

Triggering of mud flows

Heavy rainfall, snowmelt, or high levels of ground water flowing through cracked bedrock may trigger a movement of soil or sediments. Floods, debris- and mud flows may also occur when strong rains on hill or mountain slopes cause extensive erosion and/or what is known as "channel scour".

Some broad mudflows are rather viscous and therefore slow (some meters/sec). Others begin very quick and continue like an avalanche. If "large enough" they can devastate villages and countrysides. They are composed of at least 50% silt and clay-sized materials and up to 30% water. Mudflows are common even in the hills around Los Angeles where they have destroyed many homes built on hillsides without sufficient support (?).

The point where a muddy material begins to flow, depends on its grain and the water content. Fine grainy material or soil has a smaller friction angle than a coarse sediment or a debris flow, but falling rock pieces can trigger a material flow, too.

Movement and deposits

Mudflows at steep slopes may be generated when hillside colluvium and other material becomes rapidly saturated with water. Flowing down in a torrent or a slope channel it can increase in volume very quickly and pick up rock fragments and even trees. The speed can be increased either by additional water, by steeper inclination (e.g. at valley shoulders) or if the flow becomes thinner, as its heavier debris are deposed during the flow

Some debris or sediment flows tend to move in pulses as friction or other barriers are overcome on the way. Sometimes earlier pulses or previous flows form levees that channel the flow until they are breached. The presence of older levees indicates the type and recurrence of frequent flows in a particular area. This knowledge may be essential for developing land on alluvial fans, and for regional policy. Alluvial sediment fans are often deposited on valley floors or plains at the exit of a side valley or a canyon.

Some villages in narrow valleys can be destroyed even if they are build at "safe" places - far away from frequent torrent routes - because obstacles (trees, blocks) can change the mudflow direction.

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Regions with frequent mudflows

Triggering of mud flows

Movement and deposits

See also