Strutture idrogeologiche critiche per la stabilita' delle ripe dei corsi d'acqua: Il caso del torrente pioverna (Valsassina, Lecco, Italia)

Background knowledges about inundation phenomena in Lombardy (Italy) demonstrates that the behavior of rivers during the floods is characterized by high erosion capacity, especially in Alpine and Prealpine Valleys. The purpose of this study is to verify if riverbank instability can be enhanced by hydrogeological causes in addition to erosion due to the high stream speed. These events happen during flash floods, which are characterized by quick high increases in water surface variations, followed by rapid decreases to the previous conditions. In this paper also are presented studies about landslides, which are caused, during floods, not only by flow velocity, but also by increasing of hydraulic gradients of the aquifer connected to the river. In these conditions it is possible to have both piping, characterized by removal of fine sediments, and, sometimes, terrain liquefaction. Another purpose of this study is to find criteria to identify the geological settings most subjected to instability, in order to facilitate prevention of riverbank landslide