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A mudslide is the most rapid (up to 80 km/h, or 50 mph) and fluid type of downhill mass wasting.
It is a rapid movement of a large mass of mud formed from loose dirt and water. Similar phenomena include mudflow, mud stream, debris flow (e.g. in high mountains), jökulhlaup, and lahar (from volcanoes). These terms cover a broad variety in water content (from loamy mud, to almost liquid, and even steam); material (wet soil, sandy sediments and/or silt, dirt, rock, debris, volcanic ash, small plants, and even trees); length; total mass; and velocity.
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; others begin very quickly 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. On December 14, 1999 in Vargas, Venezuela, a mudslide known as The Vargas tragedy, which significantly altered more than 60 kilometers (37 mi) of its coastline, was triggered due to heavy rainfall and caused estimated damages of US$1.79 to US$3.5 billion, a death toll considered to be between 10,000 and 30,000, 85,000 people evacuated and led to the complete collapse of the state's infrastructure.
Landslide is a general term referring to all types of surface movement particularly avalanches involving the mass movement downhill of soil, rock or snow under the action of gravity. Landslides and avalanches can be devastating in terms of human life and general destruction.
Mudslides are caused by unusually heavy rain or a sudden thaw. They consist mainly of mud and water plus fragments of rock and other debris, this causes them to behave like a flood. They can move houses off their foundations or bury a place within minutes, this is due to their incredibly strong currents.
When a mudslide occurs it is given four named areas, the 'main scarp', in bigger mudslides the 'upper and lower shelves' and the 'toe'. The main scarp will be the original area of incidence, the toe is the last affected area(s). The upper and lower shelves are located wherever a large dip (due to mountain or natural drop) in the mudslides path. Due to this a mudslide can have many shelves.
The world's largest historic landslide (in terms of volume) occurred during the 1980 eruption of Mount St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA. The volume of material was 2.8 km³. Directly in the path of the huge mudslide was Spirit Lake. Normally a chilly 5°C, the mudslide instantly raised the temperature to near 38°C. Today the bottom of Spirit Lake is 100 feet above the original surface. The lake now has two and a half times more surface area than it did before the eruption.
However, the world's largest prehistoric landslide, discovered to date, took place in south-western Iran, and is named the Saidmarreh Landslide. The landslide was located on the Kabir Kuh anticline in Southwest Iran at 33 degrees N, 47.65 degrees E. The landslide had a volume of about 20 cubic kilometers, a depth of 300 m, a travel distance of 14 km and a width of 5 km. This means that about 50 billion tons of rock moved in this single event.
The largest prehistoric landslide including submarine ones was an enormous submarine landslide that disintegrated 60,000 years ago produced the longest flow of sand and mud yet documented on Earth. The massive submarine flow travelled 1,500 kilometres – the distance from London to Rome – before depositing its load.
The area most generally recognized as being at risk of a dangerous mudslide are: