Debris-flow hazard assessment and methods applied in engineering practice

Abstract

Debris flows constitute a major natural hazard in mountainous regions. The main elements required for a practical hazard assessment include the following steps: (i) estimation of potential initiation zones and sediment sources, (ii) establishment of a relation between the magnitude and frequency of expected future debris-flow events, and (iii) assessment of the flow behavior and delineation of areas potentially endangered by flowing debris. A general overview is presented of the main triggering conditions and initiation mechanisms for debris-flow formation. A brief summary is given of methods to establish a magnitude-frequency relation and to estimate the total volume of sediments transported to the fan during so-called “design” events. To assess the runout distance of debris flows and potentially affected areas, either simple empirical approaches or more physically based numerical simulation models may be used. An example application for a Swiss debris fan illustrates the variability of the results when using three different debris-flow simulation models, even though all three models were first calibrated based on the observed deposition areas of a past event.