Debris-flow channel evolution at the triggering and transport zone: learning from a very active case study in the Dolomites

Understanding morphology changes and sediment spreading along a debris-flow channel is a key step in hazard mitigation planning. This research analyses a 10 years evolution of erosion/deposition patterns in an active debris-flow upper channel located on the Dolomites (rio Soial, Val di Fassa, Trento, Italy). The morphologic evolution of the channel has been analysed performing a Difference of DEM (DoD) (Cavalli et al., 2015; Wheaton et al., 2010). DEM differencing enables quantitative and spatially-distributed representation of erosion and deposition within the analysed time window, at both channel reach and the catchment scale. In this study, the analysis were performed using two high-resolution Digital Terrain Models (DTMs). The 2008 LiDAR-derived DTM of the Autonomous Province of Trento with a DTM created from a UAV-based point cloud from July 2018 were compared. This data set was also used to determine the changes in the sediment Connectivity Index (CI), which explains the existing degree of linkage between sediment sources and channel network (Cavalli et al., 2013). During the period 2008-2018 five debris flow events have occurred. Each associated rainstorm was analysed in order to assess the evolution of the threshold rain intensities in relation to the evolution of the channel-valley morphology. The results on the CI analysis show a general decrease in CI values, meaning an increased disconnection between the head basin areas and the outlet at the end of the transport reach. Also, the rain thresholds show a slight increase after the lasts event, indicating a gradual stabilization of the basin and a possible reduction of the expected frequency of debris flow events.