ORIGINAL PAPER Flash flood occurrence and magnitude assessment in an alluvial fan context: the October 2011 event in the Southern Apennines Antonio Santo 1 • Nicoletta Santangelo 2 • Giuseppe Di Crescenzo 1 • Vittoria Scorpio 3 • Melania De Falco 2 • Giovanni Battista Chirico 4 Received: 8 March 2014 / Accepted: 20 March 2015 Ó Springer Science+Business Media Dordrecht 2015 Abstract 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 al- lowed 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 de- tected. 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. & Nicoletta Santangelo nicsanta@unina.it 1 Dipartimento di Ingegneria Civile, Edile ed Ambientale (DICEA), Universita ` di Napoli Federico II, Piazzale Tecchio, 80125 Naples, Italy 2 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Universita ` di Napoli Federico II, Largo San Marcellino 10, 80138 Naples, Italy 3 Dipartimento di Bioscienze e Territorio, Universita ` del Molise, Contrada Fonte Lappone, 86090 Pesche, Isernia, Italy 4 Dipartimento di Agraria, Universita ` di Napoli Federico II, Via Universita ` 100, 80055 Portici, Napoli, Italy 123 Nat Hazards DOI 10.1007/s11069-015-1728-4