The simple idea that a landslide could occur is frightening people in every area prone to such phenomena. That is because the effects of landslides can be devastating, leaving thousands of people without a home, threatening their lives and those of their loved ones.
Landslides are usually caused by several factors, but three of them are preeminently important:
- Slope saturation with water;
- Seismic activity in the area;
- Volcanic activity.
Water and Landslides
Water is a primary cause for landslides, as, when present in large quantities, it saturates the superficial slope soil layers and causes them to slide down. Intense rainfall or melting snow, changes occurred in ground-water levels or changes of water levels along coastlines, lake banks, earth dams, canals, reservoirs, and rivers – all these may cause mass movements leading to landslides.
There is always a relationship between flooding and landslides, because both are related to water saturation of the ground. Moreover, mud flows and debris flows occurring in steep stream channels can be mistaken for floods, but it is a fact that these two phenomena often occur at the same time and in the same area.
Landslides may cause flooding when they form landslide dams that eventually block stream channels and valleys and allow large masses of water to back up. Backwater starts flooding and, when the dam fails, it will cause subsequent downstream flooding. Solid debris can add both volume and density to a normal stream flow and can cause channel blockages. Another effect of landslides is overtopping reservoirs and reducing their capacity to store water.
Seismic Activity and Landslides
Certain mountainous areas are vulnerable to landslides but have also been affected by earthquakes, at least at moderate rates. When an earthquake occurs in such areas, the risks for a landslide to occur as well grow tremendously. Ground shaking allows water to infiltrate rapidly between ground layers and then make these layers slide on one another. Ground shaking also causes widespread rock falls.
Volcanic Activity and Landslides
Volcanic activity is the factor that provokes the most devastating type of landslides. Lava melts snow at high speed and can cause a deluge of soil, rock, ash, and water which rushes down on the steep slopes of the volcano. This deluge devastates everything in its path. Volcanic debris flows at great distances and can damage the structures in areas surrounding the volcano. There were numerous cases when the eruption of a volcano triggered a massive landslide.