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This is where land movement at a range of speeds results in destruction of property and/or loss of life. It is often triggered by human activity. In its broadest sense it is the movement down slope of any weathered material (regolith) under the influence of gravity. The type of movement is influenced by:
- Angle of slope (steeper is faster)
- Nature of regolith
- Amount and type of vegetation
- Type and structure of rock
- Human activity
Unlikely to be hazardous.
Soil Creep: Speed is below 1cm per year. Common in humid climates, and can be nearly continuous.
Solifluction: Very slow, and of limited importance. Occurs in highland of Scotland at a speed of 5 to 10cm per year often due to a thawed top layer moving over a frozen lower layer.
Involve both mud and earth flows: type is dependent on amount of water involved.
Earth flow: Occurs on slopes between 5 and 15 degrees, often after the regolith has become saturated, and flow then results. Vegetation can be destroyed and speeds range from 1 to 15km per year.
Mudflow:The best-known example is that of Aberfan. A small mining community was virtually destroyed as coal spoil heaps were placed on slopes over 25 degrees 200 metres above the village. The heaps became unstable as they were saturated from springs. A massive mudflow resulted on Oct 21st 1966 in which 147 people were killed, 116 being children.
Slides: A movement of material 'en-masse' which remains together until impacting the bottom of a slope.
Rockfalls: Slopes must be extremely steep. They are a result of a variety of causes, for example, extreme weathering such as freeze-thaw action may loosen material.
Slumps: Usually found on weaker rocks (Clay), that becomes saturated and heavy. Undercutting of cliffs by wave action can also be a cause, as can human activity increasing pressure on rocks, as shown in Scarborough in 1993, where the Holbeck Hall Hotel slumped into the sea.
Avalanches: Very very fast movements down slope of snow, ice, rock and earth. The average speed of descent is 40km/h.