The relationship between rainfall and mass movements on plio-pleistocene sediments in south-western Umbria (central Italy).

The Umbria region (central Italy) is one of the most affected in Italy by mass movements and related natural hazard. Due to its geological structure, many of he most important historical towns are built on hills where Plio- Pleistocene sediments outcrop. This lithological complex has a very high leaning to mass movements, in particular where clays percentage is significant. This study gives the results of an investigation about the landslide movements occurred in the last eighty years in the southwestern branch of the Tiber River basin. These sediments, prevalently silty-sandy-clayey, are largely involved in more or less superficial earth slides and earth slumps. In particular, an attempt was made in order to correlate these landslides with the pluviometric characteristics of the period before the landslides occurrence. Rainfalls that are not particularly heavy but are protracted over time are those which most favour the increases in interstitial pressure in these soils which have low-medium permeability, resulting in the collapsing of the slope. The heavy but brief rainfalls that took place in the period of observation caused mostly superficial landslides, which generally involved the ploughed and cultivated part of the soil. The analysis of the rains accumulated for several days before the landslide made it possible to find out the minimum amounts of rainfall necessary for generating critical conditions in the subsoil. These conditions are variable, according to the month in which the landslides took place. The highest values are observed in the periods in which the aquifer levels are lowest, and the lowest values in the spring months. The hydrological analysis was done using not only total rainfall but also useful rain, given by the difference between rainfall and potential evapotranspiration. Data sets concerning rainfall values and landslide location are introduced in a GIS environment in order to analyse the correlation between the two sets of values. A table associated to the landslide layer shows the morphologic and morphometric characteristics of the mass movements bodies. Furthermore, the dates of the landslide events are an input information in the table of attributes. This kind of information allows to select the events to the rainfall date. The project allows to achieve a statistical analysis in the GIS environment and to compare, for each rainfall station considered, a correlation between the values of maximum rainfall accumulated in several consecutive days for different return times and the area distribution of landslides.