From deterministic hazard modelling to risk and loss estimation

Several steps need to be accomplished for a quantitative landslide risk assessment. First, an analysis of the hazardous process and intensity has to be performed. Afterwards, the physical consequences inflicted by the hazard need to be quantified, preferentially in monetary values. For that purpose, a hazard deterministic modelling has to be coupled with information about the value of the elements at risk and its vulnerability to the hazard intensity. Dynamic run-out models for debris flows are able to calculate physical outputs (extension, depths, velocities, impact pressures) and to determine the zones where the elements at risk could suffer an impact. These results can then be applied for vulnerability and risk calculations. The presented risk assessment has been conducted in the area of Tresenda in the Valtellina Valley (Lombardy Region, Northern Italy). Three quantitative hazard scenarios for different return periods were prepared using available rainfall and geotechnical data. The FLO-2D program was applied for the simulation of the debris flow propagation. FLO-2D uses an Eularian formulation with a finite difference numerical scheme that requires the specification of a discharge hydrograph as an input and the basal shear stresses are calculated using a quadratic model. The modelled hazard scenarios were consequently overlaid with the elements at risk. The prospective damage to the houses was estimated using proposed vulnerability functions (flow height and impact pressure). Direct economic losses to the buildings were estimated, reaching € 610,088 to 6,453,366, depending on the hazard scenario and vulnerability curve used