Meteorology triggering factors analysis for rainfall induced hydrogeological events in alpine region

Abstract. This paper presents an extended back analysis of the major hydrogeological events that occurred in the last 70 years in the alpine area of the Lombardy region, Italy. This work is focused on the description andthe interpretation of the major meteorological triggering factors that have caused these mass movements. The triggering factors for each hydrogeological event were analysed into twofold approaches, with the final intent of ranking their magnitude in terms of consequent damages. Firstly, an analysis of precipitation was conducted using local rain-gauge data, comparing them against rainfall-threshold curves proposed by several authors. Moreover, the return time of precipitation and the information about thespatial extension of the triggering factors were considered for the assessment of an empirical magnitude index of the hydrogeological event. Secondly, considering the currently available meteorological reanalysis database, provided globally by National Centres of Environmental Prediction (NCEP), additional information on the dynamics, the nature and intensity of meteorological triggers were taken into account. The two approaches were compared throughout two indexes that tried to assessthe strength of rainfall phenomena: the first one is empirical while the second one is physical. The results obtained from the application of the two methodologies have been discussed. The rainfall method permits to highlight which are the critical hydrogeological events, not giving any quantitative information about their magnitude. The second approach analyses better the characteristic and the dynamic of meteorological triggers, suggesting, through a physical index, a quantitative ranking of their intensities that has revealed to be a good predictor for the magnitude of hydrogeological rainfall-induced events.