Mass movements

Mass movement is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples and falls, each with their own characteristic features, and which take place over timescales from seconds to years.

Landslide

A landslide is a geological phenomenon that incorporates a wide range of ground movement including rock falls, deep failure of slopes and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors affecting the original slope stability:

- erosion by rivers, glaciers, or ocean waves create oversteepened slopes

- rock and soil slopes are weakened through saturation by snowmelt or heavy rains

- earthquakes create stresses that make weak slopes fail

- volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.

- vibrations from machinery, traffic, blasting and even thunder may trigger failure of weak slopes

- excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure

- groundwater pressure

- in shallow soils, the removal of deep-rooted vegetation that binds the colluvium to bedrock.

Mudslide

A mudslide (sometimes called a mudflow) is the most rapid (up to 80 km/h) and fluid type of downhill mass movement comprising of at least 50 per cent silt and clay-sized materials and up to 30 per cent water.

Mudslides 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 vegetation. The speed can increase either by additional water, by steeper inclination, or if the flow becomes thinner, as its heavier debris is deposited during flow.

Snow Avalanche

An avalanche is a very large slide of snow down a mountainside, caused by the release of accumulated snow down a slope, and is one of the major dangers faced in the mountains in winter. The potential for a snow slope to fail depends on a range of factors including the terrain (steepness, direction, profile and surface of a slope), structure of the snowpack (compacted snow is less likely to move than powdery snow), solar radiation, temperature, wind and snowfall amount.