Validation and potential forecast use of a debris-flow rainfall threshold calibrated with the Backward Dynamical Approach

The aim of this work is to propose a way of validating a rainfall threshold for stony debris flow calibrated with the Backward Dynamical Approach (BDA), a physical-based approach that combines the debris flows' forcing and dynamics. Given the assumptions underlying the BDA, two different validation approaches are developed and then applied to a suitable study area for which a calibrated BDA threshold and a dataset related to a calibration-independent reference period are available. The first approach assesses the reliability of the threshold by applying the BDA to compute the rainfall conditions related to the debris flows that occurred. Instead, the second approach, thanks to the introduction of the concept of “potential debris flows”, considers all rainfall events, regardless of whether or not they are associated with a debris flow, providing a complete overview of the threshold reliability. In both approaches, the threshold's performances are quantified by employing the contingency table and skill scores. The results of the validations applied to the study area threshold highlight its satisfactory reliability. Moreover, the introduction of the potential debris flows concept has cleared the way to apply a BDA-based threshold in a predictive framework and to roughly estimate the solid volumes we may expect if a debris flow occurs.