Disaster Management - Landslides

Landslides

A landslide is the gravitational movement of a mass of rock, earth or debris down a slope. Landslides are usually classified on the basis of the material involved (rock, debris, earth, mud) and the type of movement (fall, topple, avalanche, slide, flow, spread). Thus, the generic term landslide also refers to mass movements such as rock falls, mudslides and debris flows. Volcanic mudflows and debris flows are also called lahars. 

Shallow landslides usually involve only the soil layer and upper regolith zone, while deep-seated landslides additionally involve bedrock at higher depth. Landslide volume can vary from some tens of cubic metres to several cubic kilometres for giant landslides, while landslide speed may range from a few centimetres per year for slow-moving landslides to tens of kilometres per hour for fast, highly destructive landslides. According to the state of activity or movement, existing landslides can be classified as active, dormant (potentially reactivated) or inactive (often relict or fossil).

Landslides are generally induced when the shear stress on the slope material exceeds the material’s shear strength. The occurrence and reactivation of landslides is conditioned by a number of terrain and geo-environmental factors related to bedrock and soil properties, weathering conditions, jointing and structure, slope morphology, land cover/use, surface and ground water flow, etc. 

Landslides can be triggered by natural physical processes such as heavy or prolonged rainfall, earthquakes, volcanic eruptions, rapid snow melt, slope undercutting by rivers or sea waves and permafrost thawing. They can also be triggered by man-made activities such as slope excavation and loading (e.g. road and buildings construction, open-pit mining and quarrying), land use changes (e.g. deforestation), rapid reservoir drawdown, irrigation, blasting vibrations, water leakage from utilities, etc, or by any combination of natural and/or man-induced processes.

Impacts of Landslides

Landslides are a major hazard in most mountainous and hilly regions as well as in steep river banks and coastlines. Their impact depends largely on their size and speed, the elements at risk in their path and the vulnerability of these elements. Every year landslides cause fatalities and result in large damage to infrastructure (roads, railways, pipelines, artificial reservoirs, etc.) and property (buildings, agricultural land, etc.).

Large landslides in mountainous areas can result in landslide dams blocking river courses. These natural dams cause valley inundation upstream and can be subsequently breached by lake water pressure, hence generating deadly flash floods or debris flows downstream. Submarine and large coastal cliff landslides can trigger tsunami, as can landslides in lake and reservoir shores.

Landslides can also affect mine waste tips and tailings dams and landfills, causing fatalities and contaminating soils and surface and ground water.

In areas affected by landslides, these are a major source of soil erosion and sediment yield to valleys and rivers.

However, most statistics on natural disasters underestimate the impacts from landslides as they often do not separate them from other triggering or concurrent natural hazards such as storms, floods or earthquakes.