<p>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.</p><p>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.</p><p>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.</p><p>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.</p>
<p>Meandering is one of the most common morphological pattern through which rivers manifest themselves. Here, the attention is devoted to meandering streams carving their path through permafrost floodplains, which typically characterize cold environments such as the Arctic. Despite meandering rivers have been widely studied in the last fifty years, little is known about the dynamics of streams where banks are composed of perennially frozen material. It is inquired whether there is a morphological signature in the planform of permafrost streams potentially deriving from specific thermo-mechanical processes occurring in Arctic landscapes, like the formation of thermo-erosional niches and sediment slumps caused by thaw-weakened soil. To this aim, a bend scale analysis of the planform geometry of several Arctic streams by means of Landsat satellite imagery is employed. Morphodynamic features such as lateral migration rates, channel curvatures, and width variations, are extracted from multispectral remotely sensed data by combining Google Earth Engine (GEE) with an established process-based software (PyRIS).&#160; Following a methodology based on continuous wavelet transform, a set of metrics quantitatively defining the meander shape, which include fattening and skewing coefficients, are used to compare permafrost streams with a series of natural meandering rivers from tropical and temperate regions obtained from the literature. The present analysis opens the way to a systematic integration between remote sensing and physically-based morphodynamic models able to incorporate thermo-mechanical processes uniquely related to permafrost environments.</p>
<p>Climate change is already altering the hydrological regime of Arctic rivers. However, still little is known about fluvial morphological processes and trajectories in permafrost environments. In such iced floodplains, both hydrological and thermal regimes affect sediment transport and riverine morphological processes. Remote sensing represents a powerful approach to investigate fluvial systems in those isolated areas. Nevertheless, its application presents challenges linked to ice seasonality and the limited time window of the morphological activity, alongside the complex permafrost/river spatial patterns and related spectral signatures, which imply significant computational efforts. Addressing this, we propose an improved integration of existing tools for the spatio-temporal extraction of fluvial morphological indicators, combining in a unique working environment the cloud computing capability of Google Earth Engine (GEE) and a process-based tool for riverine multitemporal planform analysis (PyRIS). Fluvial morphological metrics have been extracted from a set of meandering rivers in the Arctic region, outlining the potential of anisotropic image filtering and image segmentation to enhance active channel detection in complex spatial-pattern areas. A 20-40% refinement in small object removal in river mask detection emerges. The synergy among existing instruments enhances the observation of natural river systems in permafrost environments, setting the basis for further studies on morphological processes and the evolution of such pristine and climatically-sensitive river systems.</p>
<p>In the last two decades several models have been proposed to analyse the evolutionary trajectories of meandering rivers (e.g., Seminara et al., 2001; Camporeale et al, 2007; Frascati et al., 2009; Frascati & Lanzoni, 2013; Eke et al., 2014; Bogoni et al., 2017; Monegaglia et al., 2019). These models are based on the assumption that channel migration, which is locally driven by the differential excess of flow speed at the banks, is globally governed by the average bankfull geometry. Previous studies suggest that bankfull parameters strongly affect meander development. More specifically, the planform shape depends on the width ratio falling below or above a resonant threshold: sub-resonant meanders are typically downstream skewed, while super-resonant meanders exhibit upstream skewed loops and are prone to evolving much faster. Therefore, the model adopted to define, at each time step, the variation of bankfull parameters fundamentally affects the morphodynamic regime of meanders. A common strategy to initialize the simulations and to set the reference values of bankfull parameters is the use of a quasi-straight configuration. This is a legitimate way to obtain a fully developed meandering planimetry; however, this initial configuration is often used in conjunction with bankfull parameters derived from field data, which implies that the values of the external independent variables, water discharge and sediment supply, that characterize the simulated configuration are similar between such initial state and the fully developed one. However, when combined with the widely adopted assumption that the channel slope must decrease proportionally to meander elongation, this leads to significant variations of bankfull parameters, with a dramatic drop of the transport capacity, as the channel length can increase by two-four times. Therefore, the values of bankfull parameters of the statistical steady-state that the system eventually achieves in long-term simulations (Camporeale et al., 2008; Bogoni et al., 2017) can be quite different from those selected as initial reference values, which may lead to simulating unrealistic evolutionary scenarios and shifts of the morphodynamical regime. However, such a strong variation of bankfull parameters must be viewed as a gimmick introduced by the initial quasi-straight configuration. Based on these considerations, results of long-term simulation need to be revisited taking the bankfull parameters of the statistical steady-state as reference values.</p><p>In our work we analyze planimetry features, such as the sinuosity, and their oscillations, when we vary these bankfull parameters. Moreover, we look into the dependency on the aspect ratio of the fully developed state to better understand how super or sub-resonant regime affects the planimetrical configuration.</p>
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