A landslide is the movement of rock, soil or debris down a sloped part of the land caused by rain, earthquakes, volcanoes or other factors that cause slope instability.

There are three main causes that lead to landslides: geology, morphology and human activity. Geology refers to the characteristics of the material itself. The strength of the soil or rock becomes weak or broken, or the soil layers have different strengths. Morphology refers to the structure of the soil. For example, slopes lose their vegetation due to logging, fire or drought thus becoming more prone to landslides. Plants keep the soil in place and without the root systems of trees and shrubs, the soil is more likely to slide. The cause of classic landslide morphology is the erosion or weakening of the soil or rock surface due to water. Human activities, such as agriculture and construction can increase the risk of landslides. Irrigation, deforestation, excavation and water leakage are some of the common activities that cause instability and weaken the slopes.

There are several ways to describe how landslides move. These include falling, translational sliding, lateral spreading and flow. In falls and tumbles, heavy blocks of material fall after detaching from a very steep slope or cliff. In a translational slide, the surface material is separated from the more stable base layer of the slope. Earthquakes may shake topsoil loose from the earth's surface. Lateral spread or flow is the movement of material sideways or laterally. This happens when a strong shock, such as an earthquake, causes the ground to move quickly, like a liquid.

Landslides may contain rock, soil, vegetation, water or a combination of these materials. Landslides caused by volcanoes can also contain hot volcanic ash and lava from eruptions. Landslides in the mountains may have snow. Volcanic landslides, also called lava, are among the most devastating types of landslides. Another important factor in describing landslides is the speed of movement. Some landslides move at a speed of metres per second, while others creep along by a centimetre or two per year. Some landslides, called mudslides, contain high amounts of water and move very quickly.

Direct and indirect costs of landslide loss

Landslide losses consist of direct and indirect costs. Direct costs are damage directly associated with landslides. Indirect costs include travel diversions, economic restrictions and environmental impact. Indirect costs often equal or exceed direct costs.

Activities that prevent or reduce the adverse effects of landslides are called mitigation. Mitigation includes structural and geotechnical measures, as well as political, legal and administrative measures to protect endangered populations. Reducing the global impact of landslides on vulnerable populations and critical infrastructure is an economic, social and environmental imperative. Loss data show that although losses are increasing everywhere, the consequences are much higher in developing countries than in developed countries.

The relevant authorities should identify the vulnerable areas and action should be taken in this regard.

Proposed action in vulnerable areas

Among the proposed actions are as follows:

• Establish an early warning system and a high-risk slope monitoring system.

• Danger slope mapping can be done to identify areas that are at high risk of being exposed to landslides.

• Restrictions on construction in areas at risk of landslides and protecting the existing population.

• Carrying out afforestation and grass planting programmes in hilly areas.

• Relevant authorities should prepare standards for the construction of buildings and other purposes in risky slope areas and these must be complied with.

• Provide adequate and proper drainage system in slope areas and establish effective cleaning and maintenance routines.

• Insurance facilities should be taken by the people to deal with losses.

• Relevant response teams should take immediate and proper landslide mitigation if they occur, otherwise there will be risk of larger landslides.

Landslide prone areas are fairly predictable but the timing is difficult to predict. One of the main ways to predict landslides is to look at areas where landslides have occurred before. Areas that have experienced landslides are likely to face landslides in the future. Steep slope areas are also at risk of collapse, especially during the rainy season.

-- BERNAMA

Ir. Ts. Dr. Diana Che Lat is a Senior Lecturer in the Civil Engineering School of Universiti Teknologi MARA (UiTM) Pasir Gudang, Johor.