‘Land slide’ is a term used to denote movement of a mass of rock or soil down a slope. The term is used to describe a variety of phenomena, from rock falls to the gradual downhill flow of soil. Landslides occur when the force of gravity acting on the materials within a slope overcomes the material's resistance to shearing. Among the processes that can lead to a landslide are the steepening of a slope by natural erosion or excavation, the overloading of the slope by an inflow of water, and the motion caused by an earthquake.
When a road is cut into a slope, the water flowing downhill seeps out of the side of the cut and across the road surface, then continues on its path to the bottom of the hill. If lots of rain falls in a very short period of time, the soil above the road gets supersaturated with water and becomes a big muddy mass. If the mass is not held to the slope by vegetation, it may start to move downhill, picking up loose rocks and vegetation. Eventually, the flow reaches the road cut and dumps everything onto the surface. If the debris flow has enough speed and force, it can move across the road and continue its path downhill.
A landslide (or landslip) is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Landslides are caused when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:
Human causes include:
Landslides that occur undersea, or have impact into water, can generate tsunamis. Massive landslides can also generate mega tsunamis, which are usually hundreds of metres high.
Soil erosion is when the soil is blown away by the wind or washed away by the rain. Soil erosion is common in areas with steep slopes, where trees have been cut down, in droughts when crops and other vegetation grow poorly and in rural areas which are overpopulated. Nepal, in the Himalayan Mountains, has severe problems caused by increased population density and steep slopes.
The ill-effects of land slips can be reduced by building terraces on hillsides, irrigation schemes to overcome droughts, planting more trees to bind the soil together and make wind breaks, and using fertilisers in overpopulated areas to make the soil more fertile. It is very important that the farming techniques used do not damage the structure of the soil, as this makes it easily eroded. Dams, bridges etc. are to be constructed only after doing the environment Impact Assessment. Good farming techniques include contour ploughing, crop rotation and keeping the soil rich in humus. |
