Evaluating sediment (dis)connectivity in a study Alpine catchment

The construction of artificial reservoirs for hydropower production strongly alters sediment connectivity, which often produces significant impacts on the river reaches downstream morphology. Assessing sediment connectivity and transport variations is therefore crucial for predicting possible fluvial morphological trajectories and to define scientifically-based management practices in terms of water and sediment releases from Alpine reservoirs. For this reason APRIE, the agency responsible for hydropower regulation in the Trento Province (Italy), is carrying out a project to assess impacts caused by existing hydropower plants, in collaboration with the University of Trento.

We focused on the study case of the Travignolo River, a tributary of the Avisio River in the Dolomites. The valley longitudinal connectivity has been completely interrupted by the Forte Buso dam construction in 1953 and by a series of smaller derivations from the main tributaries. We aim at understanding to what extent the presence of the dam affects the overall sediment connectivity, by assessing the relative contribution of sediment sources that currently drain into the lake with respect to the sources that are still connected to the Travignolo River, and by evaluating to what extent the disconnectivity has compromised the morphological equilibrium of the river.

To this aim, structural and functional sediment connectivity are analysed through a three step integrated approach, considering connectivity at different spatial scales. First, fluvial morphological trajectories have been studied by investigating a dataset of historical images, which allowed us to identify both morphological changes and vegetation growth. Second, sediment connectivity has been modelled at the hillslope scale through the hydrological index of connectivity calculated by applying the SedInConnect model (Crema, S. & Cavalli, M., 2018, Computational Geosciences) on the basis of terrain elevation data and information on Quaternary deposits. The model allowed us to determine the potential sediment yield contribution from the different subbasins, as well as the position of sediment sources depending on their characteristic grain size. Finally, a quantitative analysis of sediment longitudinal connectivity has been carried on by applying the CASCADE Toolbox model (Tangi, M. et al., 2019, Environmental Modelling & Software) to the main river network of the Travignolo basin. Information on surface and subsurface grain size distribution have been obtained by collecting several samples along the main course of the Travignolo River and along their main tributaries, while channel width was estimated by analysing the high-resolution digital elevation model. To calibrate CASCADE model we have compared the predicted grain size distribution cascades with the measured subsurface composition. Furthermore, we have performed several simulations considering different methods of data spatialization and different choices of the main parameters, to obtain a general assessment of the model uncertainties.

Results highlight the potential sediment contributions of different subbasins to the fluvial system, depending on their geological characteristics, slope and distance from the permanent drainage network. Moreover, the analysis of multiple scenarios reveals how sediment transport processes are strongly affected by the dam presence and how they may change depending on water delivery strategies.