The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance.

A recent storm (27th-30th October 2018), named Vaia, hit most part of the Northeast of Italy affecting the geomorphic aspect of almost all mountain catchments of the area. The event triggered new instabilities such as windthrows, landslides and debris flows. At present, few studies dealt with the analysis of the impact of a Large Infrequent Disturbance at large catchment scale. This work provides a focus on the Tegnas Torrent Basin (Belluno Province) and aims at detecting how, where, and how much this storm affected the basin. Moreover, it integrates two different approaches considering both the dynamic and static aspects of the sediment, via DEM of Difference (DoD) and Index of Connectivity (IC), respectively. The Tegnas sub-basins responded contrastingly: the Bordina (volcanic origin and covered by pastures and spruce forests) was mainly affected by windthrows (7% of the sub-basin area) and landslides (0.5%), while the Angheraz (outcropping dolomite rocks), was stricken only by debris flows (1.0%). Morphological changes were clear along the entire channel network, with predominant erosion in the steepest upstream parts (over 2 m of the channel elevation), and deposition in the lower main valley floor (over 3 m of the channel elevation). The IC analysis along the instabilities highlighted that the windthrows occurred mainly in areas of high connectivity, which may be important for future management strategies. Moreover, the proposed integrated approach, based on the combination IC-DoD, permitted a detailed identification of sediment routing and a contemporary estimation of erosion and deposition volumes generated by a high magnitude low-frequency event. Based on these results, cascading processes are expected and further analysis are required to fully consider the impact of a Large Infrequent Disturbance.