Hydrometeorological Characterization of a Flash Flood Associated with Major Geomorphic Effects: Assessment of Peak Discharge Uncertainties and Analysis of the Runoff Response

Postflood indirect peak flow estimates provide key information to advance understanding of flash flood hydrometeorological processes, particularly when peak observations are combined with flood simulations from a hydrological model. However, indirect peak flow estimates are affected by significant uncertainties, which are magnified when floods are associated with important geomorphic processes. The main objective of this work is to advance the integrated use of indirect peak flood estimates and hydrological model simulations by developing and testing a procedure for the assessment of the geomorphic impacts-related uncertainties. The methodology is applied to the analysis of an extreme flash flood that occurred on the Magra River system in Italy on 25 October 2011. The event produced major geomorphic effects and peak discharges close to the maxima observed for high-magnitude rainstorm events in Europe at basin scales ranging from 30 to 1000 km2. Results show that the intensity of geomorphic impacts has a significant effect on the accuracy of postflood peak discharge estimation andmodel-based flood response analysis. It is shown that the comparison between rainfall-runoff model simulations and indirect peak flow estimates, accounting for uncertainties, may be used to identify erroneous field-derived estimates and isolate consistent hydrological simulations. Comparison with peak discharges obtained for other Mediterranean flash floods allows the scale-dependent flood response of the Magra River system to be placed within a broader hydroclimatological context. Model analyses of the hydrologic response illustrate the role of storm structure and evolution for scale-dependent flood response.