Topographic predictors of susceptibility to alluvial fan flooding, Southern Apennines.

The flooding susceptibility of alluvial fans in the Southern Apennines has long been neglected. To partly address this oversight, we focus on the region of Campania which contains highly urbanised piedmont areas particularly vulnerable to flooding. Our findings are based on stratigraphic analysis of the fans and morphometric analysis of the basin-fan systems. Using geomorphological analysis we recognised active alluvial fans while stratigraphic analysis together with statistical analysis of the morphometric variables was used to classify the fans in terms of the transport process involved. The results indicate that in the examined geological context, the best discrimination between debris flow (Df) and water flood (Wf) processes is achieved by means of two related variables, one for the basin (feeder channel inclination, Cg) and one for the fan (fan length, Fl). The probability that an unclassified fan belongs to group Wf is computed by applying a logistic function in which a p value exceeding 0.5 indicates that a basin/fan system belongs to group Wf. This important result led to the classification of the entire basin/fan system data. As regards process intensity, debris flow-dominated fans are susceptible to the occurrence of flows with high viscosity and hence subject to more severe events than water flood-dominated fans. Bearing this in mind, the data gathered in this study allow us to detect where alluvial fan flooding might occur and give information on the different degrees of susceptibility at a regional scale. Regrettably, urban development in recent decades has failed to take the presence of such alluvial fans into account due to the long recurrence time (50-100 years) between floods. This paper outlines the distribution of such susceptibility scenarios throughout the region, thereby constituting an initial step to implementing alluvial fan flooding control and mitigation.