A new geomorphological method for the evaluation of debris flow magnitude: a case study from the southern Apennines (Italy)

Flash floods occur quickly and instantly with respect to triggering rainfall and are particularly frequent in river basins of limited extent. In the Mediterranean area, flash floods are acknowledged as the most widespread geo-hydrological hazard. They may have a particularly catastrophic character due to combination of intense precipitation and strong orographic forcing (cumulative rainfall values in the order of hundreds of mm in a few hours) with the widespread presence of steep, small river basins, referred to as Small Mediterranean Catchments. The natural susceptibility associated with Small Mediterranean Catchments eventually results in high hydrogeological risk notably for densely-urbanized and populated alluvial fans at catchment outlets. Therefore, the evaluation of sediment budgets of flash-flood prone, torrential catchments is important for the assessment of flood susceptibility and hazard in piedmont areas.Our study addresses the evaluation of the sediment volume that has the potential to be entrained and delivered downstream during flash floods in torrential catchments. With the final aim of developing an approach useful to the definition of the order of magnitude of future flash flood events in terms of debris loads, our goal is to obtain reliable quantification of sediment storage through the geomorphological analysis of high-resolution topographic data.We developed and tested a predictive, quantitative geomorphological technique for estimating the magnitude of debris flows related to flash floods triggered by extreme rainfall events. We applied our method to steep torrential catchments that (i) are underlain by resistant rocky bedrock, therefore not prone to large landslides, and (ii) allowed testing of our technique through quantification of the effects of recently-occurred flash floods. The three study catchments are located in southern Italy and experienced flash flood-related debris flows triggered by a rainstorm with a recurrence interval > 200 yrs. We estimated the sediment volumes stored in the investigated mountainous catchments prior to the flash flood and compared our results with the sediment volumes entrained by the debris flows. The latter volumes were estimated by post-event data collected by means of both field surveys and remote sensing (drone surveys). The results were mutually consistent so supporting our approach. The approach can be applied extensively, even in the absence of field data and measurements, to ungauged catchments similar to those we analysed for flash flood hazard assessment and planning of mitigation strategies.