Evaluation of landslide dams hazard and risk: an application in Upper Tiber Valley (central Italy)

The note presents the results of a project, carried out by the Engineering Geology Group of Department of Physics and Geology of University of Perugia and funded by the Umbria Region, aimed at evaluating the hazard of occlusion of riverbeds by landslide (landslide dams) in the Upper Tiber Valley. The authors explain the used methodology that, starting from the data available in the catalogs of IFFI (Inventario dei Fenomeni Franosi in Italia, i.e. Italian Landslides Inventory) and PAI (Piano di Assetto Idrogeologico, i.e. Hydrogeological Assessment Plan), concerning the active and quiescent landslides, selects, through different steps (preliminary identification of landslides for which it is assumed the interference with riverbeds; photointerpretation; geomorphological survey; simulation of the total occlusion of riverbed; simulation of the collapse of the natural dam) the landslides for which is considered realistically possible the occlusion of riverbed due to a reactivation or persistence of the mass movement. The methodology also provides for a first evaluation of conditions of risk related to the complex phenomenon of interference between landslide and dynamics of riverbeds, for both areas located upstream of the dam, at risk of drowning due to the formation of the dam lake, both for those located downstream, where it is possible the transit of an abnormal flood wave, resulting from the eventual collapse of the natural dam and the rapid emptying of the lake.