Heavy Rains and Flash Floods at Rocky Coast. The Costiera Amalfitana (Southern Italy)

Bedrock rivers are commonly affected by debris torrents during periods of heavy rain. This is particularly true in steep rocky coastal areas characterized by headwater catchments and alluvial fan at stream mouths. In these settings 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 ranging from debris flood to coarse-grained flow with significant catastrophic implications. At Amalfi Coast alluvial fan flooding has been the most frequent and destructive geologic hazard since historical times. The steep coastal slopes are deeply dissected by a complex fluvial system with small catchments that are very high relative to the base sea level. These rivers show a distinct seasonality and torrential behavior with main delivery areas into the adjacent marine shelf. The reported rainfall events last from about 1 h to few hours with maximum rainfall intensity ranging from 100 to 200 mm h⁻¹, and are strongly conditioned by the orographic features and the thermic anomalies of the coastal waters. The analysis of historical data indicate that more than 100 events have occurred in the last five centuries on the Amalfi coast, confirming the severe impact produced by flash floods in this area.