Evaluation of sediment budgets in catchments prone to flash flood-related debris flows: A case study from the southern Apennines (Italy)

The evaluation of sediment budgets of flash-flood prone torrential catchments is important for the assessment of flood susceptibility and hazard in piedmont areas. Here, 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 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. The study basins are small size, steep, narrow and the stream network has a low order (maximum 3rd order). We estimated the volumes of loose sediment stored in the bottoms of the mainstem valleys of 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 field surveys. The results were mutually consistent so supporting our approach. The approach is fit to the analysis of torrential catchments with geomorphological features comparable to those of our study basins, provided (i) the stream is confined by hard rock slopes not affected by channel lateral erosion, (ii) all stored sediment is eroded and transported downstream during the flood, and (iii) no landslide occurs along the hillslopes. For these types of ungauged catchments, our method can be applied extensively, even in the absence of field data and measurements, for flash flood hazard assessment and planning of mitigation strategies.