Hydrological hazards: Mass movement wet (subsidences, rockfalls, avalanches and landslides)

 Definition and characteristics

Subsidence is the motion of the Earth's surface as it shifts downward relative to a datum (e.g. the sea level). Subsidence dry can be the result of: geological faulting, isostatic rebound, human impact (e.g. mining, extraction of natural gas) etc. Subsidence (wet) can be the result of: karst, changes in soil water saturation, permafrost degradation (thermokarst) etc.

Rockfall refers to quantities of rock or stone falling freely from a cliff face. It is caused by undercutting, weathering or permafrost degradation.

Avalanche describes a quantity of snow or ice that slides down a mountainside under the force of gravity. It occurs if the load on the upper snow layers exceeds the bonding forces of the entire mass of snow. It often gathers material that is underneath the snowpack like soil, rock etc (debris avalanche).

A landslide is the movement of soil or rock controlled by gravity and the speed of the movement usually ranges between slow and rapid, but not very slow. It can be superficial or deep, but the materials have to make up a mass that is a portion of the slope or the slope itself. The movement has to be downward and outward with a free face.

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Related Red Cross / Red Crescent operations:

  • Myanmar - Cyclone Nargis (2008).