Flash flood occurrence and magnitude assessment in an alluvial fan context: the October 2011 event in the Southern Apennines

This study presents the analysis of flash floods triggered by an extreme rainfall event that occurred on 7 October, 2011, over the Marzano carbonate massif (Southern Apennines). The rainfall event reactivated alluvial fans built up at the outlet of two mountain basins. Detailed geological surveys carried out immediately after the event allowed the reconstruction of the main erosion and depositional processes that occurred both in the drainage basin and in the fan areas. The volume of materials eroded in the basin and deposited in the fan was evaluated by means of accurate topographic surveying and GPS measurements. Morphological and morphometric properties of the basin/fan system as well as the presence of human interventions and structures along the main channel and in the fan area influenced flow propagation. The transported materials came mainly from debris and gravels previously accumulated along the stream beds and mobilised by the flow during the event. No significant evidence of landslide contribution to transported bed load was detected. Extensive damage was done to buildings, river bank structures and agricultural crops. Despite the existence of hundreds of similar alluvial/fan systems in the Southern Apennines, few studies have been conducted to support adequate risk mitigation action in these areas. Indeed, to our knowledge, this is the first study focusing on assessing the magnitude of alluvial fan flooding in the context of the Southern Apennines. Studies like the present one may help determine the volumes involved during flash floods whilst providing support for detailed flood hazard zoning and for risk mitigation planning.