ELEMENTS OF RISK MAPPING FOR RAPID LANDSLIDES

Oldrich Hungr

Department of Earth and Ocean Sciences, UBC, Vancouver, Canada, E-mail: ohungr@eos.ubc.ca

Rapid landslides can move over considerable distances and pose danger to areas remote from the location of instability. In such cases, the risk assessment must include the determination of the following:

1. Magnitudes and probabilities of the full range of possible initial detachments.
2. A spectrum of velocities, reach distances and damage corridor widths. These are obtained by applying suitable runout models to the events identified under 1. (This step may be refered to as intensity determination.)
3. Vulnerabilities of elements at risk to the intensity distributions obtained in 2.
4. Estimates of specific risks and their accumulation to obtain the total risk.

This analytical chain is very complex and difficult to connect, particularly as many of the tools required to accomplish the individual tasks are poorly developed. The analysis can be approached from two directions. The potential of the source areas to generate landslide events can be studied first ("stability analysis") and the risk assessment is carried out in the order presented above. Alternatively, an inverse approach can be taken, where past impacts in the hazard area are studied and a magnitude-frequency relationship is derived backwards from such data.

Two examples will be given. The first is the risk assessment on a large fan subjected to volcanic debris flow hazards. The second is a calculation of risks arising from rock falls of different magnitudes and frequencies, reaching a busy highway.