Linking high-resolution hydrology and geomorphometry for flash flood analysis

Stefano Crema1,4, Francesco Marra2, Marco Cavalli4, Lorenzo Marchi4, Francesco Comiti3, Marco Borga1, 2018, Linking high-resolution hydrology and geomorphometry for flash flood analysis, GIT - Geosciences and Information Technologies 2018, Sarzana, Italy, 11-13 giugno 2018,

Flash flood represent a major natural hazard and their investigation is usually challenging due to the rapid temporal evolution and severe but localized spatial dynamics. Post-event monitoring of flash floods represents a key step in order to improve the assessment of both hazard and vulnerability. In this study, we present a methodology for an improved characterization of flash flood dynamics and related instabilities phenomena, focusing on a flash flood occurred on Aug 5-6 2017 over a 100 km2 alpine catchment of northeastern Italy. The analyzed flash flood caused important damages to infrastructures and posed a particular threat to human lives due to the simultaneous evolution of slope instability phenomena such as debris flows. The examined dataset encompasses calibrated radar rainfall, the flood hydrograph recorded near the outlet of the basin, peak discharge from post-flood survey of ungauged cross sections, LiDAR terrain data and satellite/aerial imagery. The methodological framework of the hydrological analysis embraces the calibration of a rainfall-runoff model, its downscaling to ungauged catchments and its cross-validation with field surveys for an estimation of peak discharge and related geomorphic effects. The application of a morphometric index of sediment connectivity has allowed investigating the topographic potential for sediment to be routed downstream and to analyze spatial connectivity patterns at high resolution for all the sub catchments. Preliminary results portray the inner dynamics of the event as characterized by severe spatial and temporal variability and show consistency between simulated discharge and post-flood field surveys. The flood response of analyzed sub catchments shows as well important correlation with the spatial patterns of the triggering thunderstorm. Sediment connectivity pattern has proved to be an important factor controlling the severity of the response at the analyzed catchments. In particular, the integration of geomorphic analysis with input forcing severity has allowed for a more precise recognition of the most critical conditions in terms of sediment availability, sediment connectivity and storm severity. The present work paves the way for more detailed analysis (e.g., Geomorphic Change Detection) that will permit to investigate the relation between the hydrological processes leading to flash flood occurrence and related instability and sediment transfer processes.

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