Frequency Of Landsliding As Part Of Risk Assessment

Phil Flentje and Robin Chowdhury

An integral part of systematic risk management for landslides is the assessment of landslide frequency. Qualitative approaches have frequently been used for landslide risk assessment but there is now an increasing trend towards quantitative assessments. In Australia, this has been highlighted by the publication of a comprehensive paper by the Australian Geomechanics Society in March, 2000. This paper replaced an earlier 1985 publication which provided guidelines and recommendations for qualitative risk assessment based primarily on site inspection, previous experience and engineering judgment.

The assessment of landslide risk requires assessment of hazard, elements at risk and the consequences of landsliding on those elements. Attention must be given to the mobility of a landslide as well as to the vulnerability of elements at risk before the consequences can be assessed reliably. A hazard-consequence matrix approach is often a convenient framework for an integrated approach to risk assessment on a qualitative or quantitative basis (AS/NZS, 1999; Flentje et al 2000; Walker et al., 2000).

The first important stage of any investigation concerns the assessment of landslide hazard which is often influenced by a range of factors. It is important to consider the basic causes and mechanisms of slope instability as well as the triggering factors. For example, the most common natural triggering factor in Australia is rainfall. Therefore, frequency of landsliding is often closely related to the intensity and duration of different rainstorms.

This paper will discuss different aspects of a landslide risk assessment task with particular reference to quantitative assessment of the frequency based on historical and observational data. Reference will be made to the monitoring of surface and subsurface movements and piezometric levels as well as to the detailed historical and spatial analyses of rainfall records.

The procedures and methods proposed in this paper will be illustrated with reference to two case studies. Both of these case studies are defined as ‘slide’ category (Cruden and Varnes, 1996) landslides and it is important to note that this paper is directed at the assessment of this type of failure mechanism.