Abstract
Abstract. In this paper we use a multi-hazard approach to analyse the 9 September 2010 flash flood in the Dragone basin, a 9 km2 catchment located along the Amalfi rocky coastal range, southern Italy. In this area, alluvial fan flooding has been the most frequent and destructive geologic hazard since Roman times. Sudden torrents of water (flash floods) are caused by high-intensity and very localized cloudbursts of short duration, inducing slope erosion and sediment delivery from slope to stream. The elevated bed load transport produces fast-moving hyperconcentrated flows with significant catastrophic implications for communities living at the stream mouth. The 9 September 2010 rainstorm event lasted 1 h with an intensity rainfall peak of nearly 120 mm h−1. High topographic relief of the Amalfi coastal range and positive anomalies of the coastal waters conditioned the character of the convective system. Based on geological data and post-event field evidence and surveys, as well as homemade videos and eyewitness accounts, it is reported that the flash flood mobilized some 25 000 m3 of materials with a total (water and sediment) peak flow of 80 m3 s−1. The estimated peak discharge of only clear water was about 65 m3 s−1. This leads to a sediment bulking factor of 1.2 that corresponds to a flow with velocities similar to those of water during a flood.
Highlights
The Amalfi Coast consists of a steep mountain front that rises abruptly from the Tyrrhenian Sea (Fig. 1)
Detailed field surveys and measurements, along with information from eyewitnesses and home-made videos, proved to be critical for reconstructing and modelling the flash flood that affected the village of Atrani on 9 September 2010
The collected data were combined with meteorological and historical analyses and marine geophysical surveys in order to reconstruct the physical features of the flood event and to evaluate the recurrence of flash floods in the study area
Summary
The Amalfi Coast consists of a steep mountain front (up to 1444 m a.s.l.) that rises abruptly from the Tyrrhenian Sea (Fig. 1). Geologic evidences for rapid slope erosion following the Pompeii pyroclastic fall include alluvial reworked volcaniclastic sequences (locally called Durece) occurring as residual outcrops along narrow stream valleys (Cinque and Robustelli, 2009) and coastal fan deltas fed by small alluvial fans at the mouth of the main streams (Sacchi et al, 2009; Violante et al, 2009). The latter are composed of wedge-shaped coarse-grained alluvial deposits that thicken towards the sea and represent the subaqueous counterpart of small fans at the river mouths (Fig. 2, inset). Reconstruction and recurrence of past events based on different historical sources and marine geophysical and geological data of the Dragone submerged delta have been taken into account
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