Debris-flows dominated fan delta system at Amalfi Coast, Southern Italy

The study area is part of the Sorrento Peninsula, a major Quaternary morpho-structural unit of the western flank of the Southern Apennines consisting of a steep and elevated mountain range (up to 1444 m) that separates two major embayments of the eastern Tyrrhenian margin, namely the Naples and Salerno bays. It is mostly formed by a pile of Mesozoic carbonate rocks, covered by Tertiary to Quaternary siliciclastic and pyroclastic units, and is deeply cut by a complex pattern of bedrock rivers and channels characterized by small catchment areas that are very high relative to the base level. These rivers show a distinct seasonality and torrential behaviour (ESPOSITO et alii, 2004a,b; VIOLANTE, 2009), with main delivery areas into the adjacent marine shelf. The Costa d'Amalfi is located about 20 km south of the Somma-Vesuvius and has been repeatedly mantled during the last millennia by the pyroclastic products of the volcano. The most recent explosive eruptions of Vesuvius, particularly the AD 79 Plinian event, have accumulated loose pyroclastic material over large areas of the Campania region, thus creating favourable conditions for volcaniclastic debris to generate debris flows and flash floods in concomitance with rainy periods. In particular, during the Plinian eruption that destroyed the Roman cities of Pompei, Stabiae and Herculaneum in AD 79, the study area was covered by up to 2 m of pyroclastic air-fall tephra (SIGURDSSON et alii, 1985; CIONI et alii, 1999) now occurring as weathered levels up to a few metres thick or as deeply incised streamflow deposits (locally called Durece) up to 30 m thick along the stream valleys (CINQUE & ROBUSTELLI, 2009) and mostly redeposited as alluvial fans and coarse fan deltas at mouth of main streams. Natural disasters resulting from debris flows and flash floods are an intimate part of the study area as testified by maritime Roman villas buried by flow deposits. In more recent times, heavy damage was produced by a number of catastrophic and less catastrophic floods, documented both in the historical and environmental records. Geological and hydrological data point to elevated fluvial bed load transport strictly associated with sediment delivery from slope to streams in conjunction with rainstorm events. The slides involve a water saturated mass of materials rapidly flowing all the way down to the coast. Erosion and transport of material causes major physical changes and exposes coastal communities and human activity to hazard with potential damage to property and infrastructure, and loss of life