Debris - flow susceptibility assessment along the railway network: methodological framework and testing in alpine area

Almost 40% of the Italian territory is mountainous and precisely in such fragile environment, as a result of intense rainfall and depending on geolitologichal and morphological conditions geo-hydrological instabilities, such as landslides and debris flows are likely to occur. These instabilities frequently represent a major hazard for urbanized areas and transport infrastructure. The present contribution aims at describing a methodology to assess debris-flow susceptibility along the railway network. The methodology has been developed by CNR IRPI in the frame of a project with the Italian railway infrastructure manager (Rete Ferroviaria Italiana). The methodology is structured into two levels. The first level is based on the use of conceptual models, namely MSF (Modified Single Flow) and r.randomwalk, in order to identify the railway sections potentially more exposed to debris flows. The application of such conceptual models requires few inputs that can be derived starting from a careful identification of debris-flow sediment sources areas. The identification of triggering areas is based on the integration of photo interpretation and geomorphometric approaches along with a critical exploitation of available dataset on historical events and hazard, and risk mapping. The second level includes field campaigns and the application of physically-based models to achieve detailed assessment of inundation areas, flow depth and velocity; this level of analysis is applied in the basins that highlight major problems along the stretches of the railway, identified in the first analysis level. A section of the railway network in the autonomous Province of Bolzano has been chosen as study area to test and validate the methodology. The study area, located between Fortezza and Mezzaselva localities and covering an area of about 78 km2, has been interested by several important debris-flow events. The present work reports the preliminary results of the first-level modelling and the critical analysis of the collected information of the study area. The methodological application for the presented study area highlighted the importance of identifying sediment source areas and creating inventories of debris flows for a better application of conceptual models and a more reliable assessment of susceptibility at small scales.