What hydraulic conditions drive debris floods in mountain rivers? Evidence from the Cordevole catchment (Italian Alps)

The hydraulic conditions that drive debris flood occurrence are relatively understudied, despite posing a substantial hazard. Advances in computational hydraulic modelling present new opportunities to investigate the local conditions that govern when and how debris floods occur. This study investigates debris floods and their associated hydraulic conditions in the Cordevole River (Italian Alps). Building on previous work that collected post-event evidence of debris floods in the region, this study developed a spatially explicit hydraulic model. The model was used to reconstruct historical events and investigate the local hydraulic conditions at sites with observed debris flooding and at sites characterised by water flow only. The analysis was then extended to the application of probabilistic design storms, enabling assessment of debris flood susceptibility across a range of flood magnitudes and frequencies. The results support previous suggestions that hydraulic conditions corresponding to τ*/τc*(D90) = 2 are associated with the onset of observed debris flood occurrence. They also provide novel insights into the upstream channel length most relevant for assessing their occurrence. Simulations based on probabilistic design storms further suggest that debris floods may occur in some locations during events with return periods as low as 1 in 5 years. The same sites were consistently identified as experiencing debris flooding across a wide range of simulated flood events, indicating relatively low sensitivity to event magnitude. This finding suggests that the frequency of debris floods at sites with conducive physical characteristics may be higher than previously assumed and could support flood hazard assessment, infrastructure planning, and prioritisation of field investigations.