Forecasting the Hydrogeological Hazard in the Anomalous Basin-Fan System of Sernio (Northern Italy)

Abstract

The paper deals with the analysis of the hydrogeological hazard in the anomalous basin-fan system of Sernio (Northern Italy). Previous studies pointed out that its genesis can be related to the collapse of a sector of a deep-seated gravitational slope deformation. The collapse developed as a rock avalanche, and created a fan shaped deposit, because of a flow confinement during the runout. In present days, the main hydrogeological hazard in this anomalous basin-fan system is related to the occurrence of debris flows, favoured by the following conditions: the large amount of debris, the high slope both in the basin and in the fan area, the lack of a well-developed hydraulic network. In order to forecast the hydrogeological hazard of the area, the 3D model RAMMS DEBRIS FLOW was used. At this aim, the source areas were identified based on the in situ surveys: they are generally small and mainly located in the head zone of the basin. As no monitoring data were available for site-specific calibration, the model parameters were defined as probability distributions based on the results of previous studies on debris flows. Afterwards, the simulations were carried out considering the entrainment phenomena typical of these anomalous systems and with a stochastic approach, therefore obtaining a probability distribution of the runout susceptibility, as well as the flow height. Numerical results pointed out the influence of entrainment phenomena on the hydrogeological hazard of the study area, especially with reference to the runout distance and the extension of the depositional area, which reaches the urban area on the fan.