The Hill of Todi (Umbria, Central Italy)

The historical town of Todi, in central Italy, lies on the top of a hill made up of clastic sediments in continental facies, typical of a filling stratigraphic series of a Plio-Pleistocene lacustrine basin (Ancient Lake Tiberino). Its particular geologic-lithologic conformation, with basal clays underlying sands and gravels, is responsible for extensive landslides which have threatened the town’s stability since historical times. The paper describes the geological, geomorphological and hydrogeological characteristics of the Hill of Todi, the physical-mechanical properties of the basal soils (clays, the main cause of the hydrogeological risk detected) and the types of landslides occurring over time on the hill. The landslide areas are subject to intensive monitoring; here, thanks to the administration of the Region of Umbria, the morphological and hydrological systematization works carried out have made it possible to obtain substantial stability in these areas.