Human-induced hazardous debris flows in Carrara marble basins (Tuscany, Italy)

Abstract

In Carrara marble basins, the long and intensive quarrying activities (which began in the first century BC) have produced extensive dump deposits, locally known as ravaneti. Ravaneti are of such large dimensions and diffusion as to make them a widespread landform of the Apuane Alps (Tuscany). In recent years these quarry dump deposits have been affected by frequent debris flows, more than 50 in 1996/97. This phenomenon is the most significant currently active geomorphological process in this landscape. The evolution of quarrying techniques produced a variety of sedimentological compositions of ravaneti. The debris flows analysed involve only the surface layers where debris is mixed with fine material with a lower permeability (active ravaneti) than the coarser underlying debris (older ravaneti). The presence of different permeability layers causes a wetting front to move downwards. Source area observations indicate a soil slip movement in the initial phases of the failure. The transformation of landslides into debris flow occurs by means of both soil contractive failure and an increase of granular temperature. The morphological and sedimentological analyses of depositional lobes resulted in a classification of three types of lobe, based on fabric–morphometry relationships allowing the identification of different flow dynamics: (1) type A lobe (platy form), matrix-supported and well developed fabric with general tendency of ab clast plane strikes to lie generally parallel to flow direction as a consequence of laminar flow; (2) type B lobe (elongated form), clast-supported and random fabric as a consequence of both turbulent flow and coarser composition of starting material; (3) type C lobe, intermediate type A–B morphometry, tendency for ab clast plane to lie in a semi-circle around the main flow direction determined by the presence of secondary flow lines divergent from it in the stopping phase. In Carrara marble basins, the anomalous frequency with which debris flows tend to be triggered by medium-intensity rainstorms (about 30 mm h−1 rainfall) is due to the recent increases in silt dump produced by modern quarrying techniques. This represents a significant example of debris flows as an environmental problem in major anthromorphized landscapes. Copyright © 2000 John Wiley & Sons, Ltd.