Mudflow
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- Mudslide redirects here, which is also the name of a cocktail.
A mudflow or mudslide 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.
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[edit] 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". The 2006 Sidoarjo mud flow may have been caused by rogue drilling.
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.
[edit] 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.
