Real-time assessment of debris-flow hazard is fundamental for developingwarning systems that can mitigate risk. A convenient method to assess thepossible occurrence of a debris flow is to compare measured and forecastedrainfalls to critical rainfall threshold (CRT) curves. Empirical derivationof the CRT from the analysis of past events' rainfall characteristics is notpossible when the database of observed debris flows is poor or when theenvironment changes with time. For debris flows and mud flows triggered byshallow landslides or debris avalanches, the above limitations may beovercome through the methodology presented. In this work the CRT curves arederived from mathematical and numerical simulations, based on theinfinite-slope stability model in which slope instability is governed by theincrease in groundwater pressure due to rainfall. The effect of rainfallinfiltration on landside occurrence is modelled through a reduced form of theRichards equation. The range of rainfall durations for which the method canbe correctly employed is investigated and an equation is derived for thelower limit of the range. A large number of calculations are performedcombining different values of rainfall characteristics (intensity andduration of event rainfall and intensity of antecedent rainfall). For eachcombination of rainfall characteristics, the percentage of the basin that isunstable is computed. The obtained database is opportunely elaborated toderive CRT curves. The methodology is implemented and tested in a small basinof the Amalfi Coast (South Italy). The comparison among the obtained CRTcurves and the observed rainfall amounts, in a playback period, gives a goodagreement. Simulations are performed with different degree of detail in thesoil parameters characterization. The comparison shows that the lack ofknowledge about the spatial variability of the parameters may greatly affectthe results. This problem is partially mitigated by the use of a Monte Carloapproach.
展开▼
