The role of rainfall in the instability of plio-pleistocene sediments in southwestern Umbria

This study gives the results of an investigation about the landslide movements occurred in the last eighty years in Umbria on Plio-Pleistocene fluvial-lacustrine soils, located in the southwestern branch of the Tiber River basin. The sequence is made up of four lithostratigraphic units. “Lower grey clay unit” (Upper Pliocene): this is made up of grey-blue, laminated, silty clay deposited in an offshore lake system. The maximum thickness of the outcropping unit is about 175 m. “Clayey sandy silt unit” (Lower Pleistocene): grey, clayey and sandy, it alternates with conglomeratic lenses. This unit, whose maximum thickness reaches up to 50 m, lies on the previous unit or on the pre-Pliocene bedrock. The depositional system consists of an alluvial plain dominated by sandy gravel facies of the channel flow and by a clayey, sandy silt facies that originated from the spreading out over the interchannel areas. “Upper sandy gravel unit” (Lower-Medium Pleistocene): clast-supported, unsorted, sandy gravel makes up this facies. The unit thickness is about 150 m and outcrops with several trough cross-stratifications, where the sandy component is higher. “Old travertine unit”: partly heterophic with the clayey, sandy silt unit, this unit is made up of partially cemented, calcareous silt. Current, recent deposits (talus, fans and travertine deposits) lie on these old deposits, at different elevations and in different environments. These sediments, prevalently silty-sandy-clayey, are largely involved in more or less superficial earth slides and earth slump, of various dimensions. 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 favor 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 plowed 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 enviroment in order to analyse the correlation between the two sets of values. The rainfall stations are imported as a vectorial point. Buffering analisys allow to cover the entire study area with multiple rings. Each ring points out the values according to the results of rainfall analysis. On the contrary, landslide events are set in as polygons. 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.