RAINFALL AND LANDSLIDES IN THE SELE VALLEY (SOUTHERN ITALY): THE SENERCHIA LANDSLIDES. FIRST RESULTS

Polemio, M. (CNR CERIST, c\o Istituto di Geologia Applicata e Geotecnica, Politecnico di Bari, Via Orabona 4, 70125 Bari, Italy)

This research refers to the project "Landslide evolution controlled by climatic factors in a seismic area. Prediction methods and warning criteria" of EEC Environment Programme (Unit: Climatology and Natural Hazards) with regard to the research topic "Climate change impacts". The objective of the research is to establish the effects of climatic events on the mechanical behaviour and the stability of soil slopes under static and dynamic conditions in the upper Sele and Ofanto valleys. At present, the area is investigated to identify the spatial trend of environmental variables related to landslides; a site with an active landslide best suited for this research project has been selected. Heavy rains are very often responsible for extensive mass movements. It is the aim of this work to highlight the existence and intensity of the influence exerted by such rains in triggering selected landslides in the upper Sele valley, Southern Italy. The intention is to evaluate the threshold levels beyond which the slide mobilization becomes significant. 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 upper Sele valley.

Cumulative rainfalls over periods of from one to 180 days will be studied, applying regionalisation techniques to determine probability distribution functions. Depending on the set of data collected from calculation of cumulative rainfall it is intended to apply either three-coefficient statistical models (General Extreme Volume - Distribution) or - if the samples available for the variables under study happen to show the occurrence of separation condition - four-coefficient, two-component models (Two Component Extreme Volume - Distribution). 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. In locality Serra dell'Acquara, near Senerchia, an huge landslide was remobilized following the 1980 earthquake: the mudslide measures about 2500 m in length and the slip surface is at a maximum ascertained depth of 33 m. It is known the landslide was quiescent at least for 40 years and no appreciable remobilization occourred over the last forty years even following periods of very heavy rainfall.

The specific landslide, represents the enlargement of a subsidiary slide situated in the middle-upper portion of the left flank of the Serra dell'Acquara landslide. The reactivation of a lateral portion of the main Serra dell'Acquara mudslide was observed on December 29, 1993, with a retrogressive evolution. The detailed landslides study utilizes also direct data of a meteorological gauge which records rainfall, temperature, snowfall and evaporation.