Failure and flow development of a complex slide: the 1993 Sesa landslide

Large landslides are often characterised by complex activity resulting from their ability to suddenly change behaviour. In fact, these landslides can pass from a slump to a flow characterised by relatively high mobility. This characteristic is a cause of extremely high severity and hazard in areas affected by such phenomena. An interesting case of a large slide in glacial deposits, which occurred in 1993 in the Valcamonica of the Italian Alps, is presented. Different approaches both to characterise the coarse frictional materials involved in the slide and to model the slide initiation and its evolution are adopted. The initial slide mass had a volume of almost 2 Mm³ and after its initial sliding motion along the slope, flowed for about 7 km down the main valley stream. Voight's method has been used to analyse measured slope displacements recorded for a few years before the slope failure. This analysis provided evidence that different times of failure could have been predicted beforehand by rigorous examination of the available data. Laboratory tests and image/processing techniques have been performed to characterise the landslide material as resembling a “bimrock” (block in matrix material). Slope stability analyses have been performed and a dynamic modelling method has been tested on the flow by comparing its results to those obtained by analysis of eyewitness statements and field observations on the mudline left by the flow. Finally, existing empirical approaches for hazard assessment on an alluvial fan have been applied and modified to produce a hazard zonation map for the alluvial fan area by considering maximum expected discharge, local geometry of the channel and the possibility of channel occlusion and subsequent avulsion.