Assessing the effect of torrent control structures on sediment continuity and connectivity

The study investigated the effect of torrent control structures on the sediment cascade in the Vegliato mountain basin (Italy) related to an intense rainfall event of 50 years return interval. The Index of Connectivity (IC) was exploited to analyze the interaction of structures with longitudinal sediment (dis)connectivity. Moreover, the Sediment Continuity Ratio (SCR) was used to assess the effect on sediment (dis)continuity. The SCR is a novel parameter considering for each torrent control structure, the net balance of sediment deposition and erosion, and the cumulative proportion of the sediment cascade arriving from upstream. The SCR emphasises which structure was more prone to continuity or discontinuity during an event and to what extent compared to other structures. Moreover, a multi-perspective framework was carried out to help the interpretation of the SCR results within the context of the study area. The results of the (dis)connectivity assessment showed that the torrent control structures impacted sediment dynamics by influencing the slope and flow confinement, which in turn affected the IC. The (dis)continuity assessment showed structures prone to continuity mainly located in the upstream part of the catchment, where most of the over 60000 m3 of sediment was generated. In contrast, structures prone to discontinuity were located in the downstream part, where deposition processes were favoured during the analyzed period. A total of 65 % of the structures similarly affected both (dis)continuity and (dis)connectivity. Data on individual structure functions and maintenance conditions were also included, emphasizing the importance of these factors in planning mountain basins management interventions. The study proposed a new metric that, despite being based on a single large event and a single basin, still provides a useful approach to investigate the interaction between sediments and individual structures, the entire sediment cascade and the channel control system.