What is a landslide?
A landslide is a mass movement of material, such as rock, earth or debris, down a slope. They can happen suddenly or more slowly over long periods of time.
When the force of gravity acting on a slope exceeds the resisting forces of a slope, the slope will fail and a landslide occurs. External factors can lead to landslides happening, including:
- heavy rainfall leading to saturation of the ground
- erosion of the base of a slope
- changes to the material’s strength through weathering
Landslides are classified by their type of movement. The four main types of movement are:
Landslides can be classified as just one of these movements or, more commonly, can be a mixture of several.
Why do landslides happen?
A landslide may occur because the strength of the material is weakened. This reduces the power of the ‘glue’ that cements the rock or soil grains together. Located on a slope, the rock is then no longer strong enough to resist the forces of gravity acting upon it.
What can increase the chance of a landslide?
Several factors can increase a slope’s susceptibility to a landslide event.
- Water: adding water to the material on a slope makes a landslide more likely to happen. This is because water adds weight, lowers the strength of the material and reduces friction, making it easier for material to move downslope.
- Erosion processes: if the bottom of a slope is continually eroded, for example by the sea or a river, the slope will eventually become too steep to hold itself up.
- Slope angle (steepness of slope): the slope angle is a key factor as far as landslides are concerned. Any change to this that makes it steeper (such as coastal erosion) increases the likelihood of a landslide.
- Rock type: the type of rocks in the slope, and their combination, can increase the chance of a landslide.
- Grain shape: the shape of the grains that make up a rock can affect the risk of a landslide.
- Jointing and orientation of bedding planes.
- Arrangement of the rock layers.
- Weathering processes: for example freeze-thaw reduces the cohesion (‘stickiness’) between the rock grains.
- Vegetation: vegetation helps bind material together; removing vegetation increases the chance of a landslide.
- Volcanoes and earthquake activity nearby.
- Human activity: mining, traffic vibrations or urbanisation change surface water drainage patterns.