Polemio, M. Sdao, F., 1996, Critical rainfall and landslides in Basilicata region (Southern Italy), 30th International Geological Congress, pp. 354, Beijng, Cina, 1996,
URL: http://www.cnr.it/prodotto/i/233729

In Basilicata in the last decades there has been periodically a widespread and severe susceptibility to landslides, often involving urban areas with consequent serious damage, caused and/or intensified by even unexceptional rainfall events. A times this susceptibility to landslides has been so widespread and visible as to create a form of geomorphological crisis on the slopes. In particular, reference is made to that which occurred following the rainfall events of 24th-25th November 1959, from 24th to 28th March 1973, on 21st November 1976, from 18th to 23rd April 1985 and in the period 24th-27th December 1990. Studies have shown that the many landslides produced following these rainfall events were for the most part in the form of rototranslational or translational slides, often evolving into earth slips. Although there were first generation landslides, the major part of the mass movements can be attributed to the total or partial mobilization of already existing slide bodies. While some large mass movements were also produced, the greater part of the landslides were in the form of superficial slides, often with a depth of only a few metres. This susceptibility to landslides, created after various rainfall events, occurred almost to the same extent both in the mountainous areas of Lucania, formed by structurally complex formations which are to varying degrees clayey, and also on the hillside slopes formed by sandy conglomerate and clayey plio-pleistocenic deposits. The intensity of the landslides which followed the different meteorological events in question was in general the same over the whole region. Despite this it is possible to distinguish, for each rainfall event, the areas in which the landslides produced are more visible and better known. The intention in this work is to evaluate the threshold levels beyond which the slide mobilization becomes significant. In order to do this the connections will be examined between each seperate meteorological event listed above and the most important landslides produced by it. The selection of landslides to analyze depended essentially on certain factors, including the movement mechanism, the location and the geological environment in which they were formed, the possession of data on their principle geomorphological, geometric and evolutive characteristics. In order to give a regional valence to these correlations care was taken to refer to each rainfall event a series of landslides present in the different geological and geomorphological environments of Basilicata. The complex of rainfall-landslide events in the course of this century in Basilicata is studied by means of a hydrological-statistical method which employs as primary data the totals of the daily rainfall by time periods of between 1 and 180 days. These hydrological variables are associated with each rainfall-landslide event and compared statistically with the almost 100 year historical series of rainfall data which is available. In this way the critical rainfall period is determined for a specific landslide and the return time is quantified of the hydrological event associated with the landslide.The calculation of suitable statistical coefficients allows the comparison between the different rainfall-landslide events, showing how specific conditions and quantities of rain are significant in relation to the geometric and kinematic characteristics of each landslide. On this basis useful criteria are formulated for the assessement of the risk of new phenomena of the instability of the slopes.

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