Derivation of critical rainfall thresholds for debris flow warnings through mathematical and numerical modeling

Abstract

The aim of the work is to develop a system capable of providing debris flow warnings in areas where historical events data are not available as well as in the case of changing environments and climate. For these reasons, critical rainfall threshold curves are derived from mathematical and numerical simulations rather than the classical derivation from empirical rainfall data. The possible formation of debris flow is simulated through infinite-slope stability analysis. Land instability is governed by the increases of groundwater pressures due to rainfall. The simulations are performed in a virtual basin, representative of the one studied, taking into account the uncertainties linked with the definition of the characteristics of the soil. A large number of calculations are performed which take into account the entire range of the governing input dynamic variables (rainfall characteristics) and different combinations between them. The dynamic variables considered are the antecedent rainfall, the intensity of the triggering rainfall and its duration. The multiple combinations of the input dynamic variables giving failure is therefore obtained. For each failure, the corresponding debris flow volume is estimated. The resulting database is elaborated in order to obtain rainfall threshold curves. These curves may be used for the real time evaluation of possible debris ow events on the basis of observed and forecasted rainfalls.