Landslides are of interest to geographers for three main reasons. First, by eroding, transporting and depositing soil and rock, they represent one of the important geomorphic processes involved in shaping the surface of the Earth. In unstable areas, they may displace up to 2000 m3/km2/year (Crozier 1989), severely depleting the soil resource and threatening the sustainability of primary production (Sidle et al. 1985). Although they are particularly common in tectonically active mountainous areas, and along river banks and coasts, they may also occur in other areas that have weak material or a susceptible geological structure.
The second reason for geographical interest is that landslides are sensitive indicators of environmental change. As a geomorphic process, a landslide represents a short-term adjustment to disturbance of the natural system. As they take place, they rapidly convert unstable slopes to a more stable condition, allowing other slow-acting processes to assume the role of denudation. In terms of landform evolution, this means that most slopes are stable for most of the time. Thus when landslides occur they are generally responding to some significant change within the natural system. Initiating factors may include tectonic activity, climate change, and natural or human-induced disturbance to the vegetation cover, slope hydrology or slope form. Knowledge of both past and present landslide activity can therefore provide useful information on environmental change. Indeed, there has been a major international research effort aimed at reconstructing past climates and climatic change in Europe, based on landslide evidence preserved in the landscape (Crozier 1997).
The third reason landslides are often studied by geographers is that landslides can present a serious natural hazard (Varnes 1984; Crozier 1996). A full appreciation of hazard requires knowledge not only of the physical process but also of the nature of the threatened society. In a sense, hazards are an aspect of human ecology. They involve interrelationships between physical, social and economic systems; as such, they constitute a field of study in which geographers are able to make a valuable contribution.
This chapter focuses on the principles of landslide hazard and risk assessment. It briefly introduces the physical process and then goes on to discuss different approaches to landslide hazard assessment.
In dealing with landslides, it is important to use a classification that distinguishes between characteristics that are relevant to the intended end-use of the study. The classification should use clearly defined and internationally understood terms. The working party on the World Landslide Inventory (1990) has made an attempt to standardise terminology and defines a landslide simply as: ‘the movement of a mass of rock, earth, or debris down a slope.’ A more comprehensive definition, which helps to distinguish landslides