The effects of debris flow on structural sediment connectivity: case studies in the Italian Alps and in Southern Brazil

Debris flows are major geophysical processes which are able to modify the landscape. Structural sediment connectivity describes the physical coupling of landscape units, and it may be affected by the occurrence of single large-magnitude debris flows and/or by the cumulative changes determined by frequent, small-magnitude events. Understanding the coupling of hillslopes to the main channel during and after debris flows is essential for comprehending catchments sediment transfer at different timescales. Debris flows might provoke sudden changes in the landscape through processes such as bed and bank erosion, overbank deposition and natural dam formation. While debris flows may modify the landscape, their characteristics (e.g., path, runout) are strongly affected by the geomorphological settings. Indeed, there is an interplay between landscape morphology and debris flows, one conditioning the other and vice versa. In this regard, determining how much structural connectivity influences the coupling of debris flow with the channel network remains a challenge. An evaluation of the structural connectivity before and after storm events that triggered debris flow has been carried out on multi-temporal DTMs available for the Stolla basin (Autonomous Province of Bozen-Bolzano, Italian Alps) utilizing the Index of Connectivity (IC) and on a pre-event DTM for the Revolver basin (Santa Catarina state, Southern Brazil). To understand whether the morphological changes caused by the debris flows had an impact on flow routing during the event, some of the debris flow events were simulated by a physically based model. The topographic changes caused by the simulated scenarios have been used to compare pre- vs post-event sediment connectivity.