DEBRIS FLOWS ON LOGGED HILLSLOPES: FIELD OBSERVATIONS AND THE IMPLICATIONS FOR NATURAL HAZARDS RISK ASSESSMENT

The identification of steep ground that will be subject to debris flows is important to resource management in Western Canada and the Pacific Northwestern United States. Debris flows constitute a hazard initiated by natural processes, by construction activities, or forest harvesting operations.

The physical characteristics and behaviour of debris flows from logged hillslopes are described with reference to a large database of events surveyed on the Queen Charlotte Islands, British Columbia (Fannin and Rollerson, 1993). This, and studies by others, led to the development of a framework for assessing debris flow travel distance downslope from the point initiation. The framework is described in the Mapping and Assessing Terrain Stability Guidebook of the Forest Practices Code of British Columbia (1999). A review of the framework, and its underlying assumptions, is provided to illustrate its advantages and limitations. The review comments are based on an interpretation of variability in the Queen Charlottes Islands database, together with additional insights to debris flow behaviour from field surveys of two other large events in British Columbia.

Summary remarks address the potential for development of models to predict the travel distance of debris flows, such as that of Fannin and Wise (2001). More specifically, they examine the utility of relatively simple empirical approaches in support of decisions regarding natural hazards risk assessment.

References:

Fannin, R.J. and Rollerson, T.P. (1993). Debris flows: some physical characteristics and behaviour. Canadian Geotechnical Journal, Vol.30, No.1, pp.71-81.

Fannin, R.J. and Wise, M.P. (2001). An empirical-statistical model for debris flow travel distance. Canadian Geotechnical Journal, Vol.38, No.5, pp.982-994.

Mapping and Assessing Terrain Stability Guidebook, 2nd edition (1999). Forest Practices Code of British Columbia (http://www.for.gov.bc.ca/tasb/legsregs/fpc/fpcguide/terrain/)