Multi-temporal analysis of the role of check dams in a debris-flow channel: Linking structural and functional connectivity

Check dams represent one of themost common hydraulic engineering solution to manage debris-flow risk.However, the design of these structures only rarely considers their impact on sediment connectivity and sediment fluxes, both at the channel reach and catchment scale. In the last two decades, High-Resolution Topography (HRT) provided fundamental data to analyse the morphology evolution of landscapes through the realization of sequential DTMs (Digital Terrain Models) and DTMs of Difference (DoDs). In the context of mountain catchments, HRT offers essential information on the evolution of erosion and deposition patterns and allows the application of multi-temporal geomorphometric indexes that can enrich the DoDs analysis with important data as the assessment of sediment connectivity. In this research, multi-temporal LiDAR-derived DTMs were used to derive DoDs and to perform a multi-temporal analysis of an index of sediment connectivity (IC) in three reaches of a debris-flow channel (Moscardo torrent, eastern Italian Alps) where several check dams have been built over time. The research aims at outlining the effects of torrent controlworks on sediment dynamics in steep channels affected by frequent debris flows. The integration of the difference of IC with DoD maps allowed quantifying the impact of newly built check dams on changes in sediment pathways and related erosion processes, highlighting a close relationship between structural and functional connectivity: in areas where erosive processes prevailed, therewere increases in connectivity,while deposition areas showed a decrease of IC. The geomorphometric analyses show that the new check dams have affected the sediment transfer along the main channel but also the hillslope-to-channel connectivity by altering sediment pathways. In the Moscardo torrent, however, the strong impact of these structures on sediment transfer is not matched by long-termeffectiveness in debris-flow control.