Interactive impacts of climate change and human activities (e.g. hydropower production) have posed urgency in examining the patterns of hydrological and thermal response in riverine ecosystems, and the potential ecological implications manifested. Hydro-geomorphic conditions are the major factors in shaping water qualities in river networks, especially under the extreme climatic events. However, when the power of nature is encountered with human regulations, represented by hydropower production, it would be well worth discussing how the pictures of riverine hydro- and thermal regimes would change over the certain range of time and space. Moreover, the possible utility of hydropower regulation as mitigation of extreme climate changes is still open question to be verified. Above-mentioned questions are answered in three aspects specifically: • Governing factors and spatial distribution model for water residence time in river networks across Germany. Based on the machine learning technique of boosted regression trees (BRT), spatial distribution of water residence time is estimated for the long-term annual average hydrological conditions and extreme cases of flood and drought. • Impacts of hydropower over temporal and spatial range are investigated by analyzing the mechanisms of hydropeaking propagation. Hydrologic and geomorphic contribution framework is proposed and applied for the upper Rhone River basin in Switzerland, a typical hydropower exploited river basin in the mountainous area. • River water temperature response as an indication for ecological status is investigated for the alpine rivers across Switzerland, excellent representatives of sensitivity and vulnerability to climate change while under highly exploitation of hydropower activities. Extreme climate change case of heatwaves in 2003 and 2006 are selected and analysed especially. Results of the three research components in correspondents to listed research questions showed that river hydrological regimes have more directly/important influence on the variation of flow availability in comparison with the geomorphologic settings. Nevertheless, geomorphologic and topologic conditions (e.g. river width, slope, and roughness coefficient) that largely control the hydraulic waves diffusion processes in a hydropower-dominated river basin determine the spatial range of hydropeaking impacts. A hierarchy framework of geophysical obstructions, hydrology, and hydraulic waves diffusion process is proposed for analyzing the spatial range of hydropeaking propagation. When the effects of hydropeaking and thermopeaking that induced by hydropower production activities are dominated in the river reach, hydropower regulation offers as great potential to mitigate extreme climate events (i.e. heatwaves). By looking into specific perspective of river hydro- and thermal regimes, hydropower regulation, and climate extremes via different scales, we investigated the interactive effects between riverine ecosystem and human-climatic impacts. We expanded the approach of water residence time estimation into the field of machine learning with spatial predictions. Impacts of hydropower regulation are first elaborated with a framework of hydropeaking propagation mechanisms. Hydropower regulation has been identified to have great potential to mitigate extreme heatwaves through altering thermal regimes in rivers. Results of the study not only contribute to river hydrology and ecology studies, but also to the river management and climate change mitigation practices.

This thesis develops and tests a classification of ‘near-natural’ European single-thread rivers, which are free to adjust to fluvial processes. The research involves subdividing rivers along a continuum of geomorphological characteristics to assign river reaches to geomorphologically-meaningful classes according to their channel dimensions and forms, and floodplain characteristics. The classification was developed and tested through three research components. First, a preliminary classification was developed using information entirely derived from a new information system containing remotely-sensed imagery and digital terrain data: Google Earth. This research stage required the development of rules for identifying, extracting and standardising information from this source for a large sample of river reaches. 221 single-thread river reaches distributed across 75 European rivers were investigated. Analysis of the derived information resulted in the development of a classification comprising six classes of European single thread river. Second, the robustness of the classification was explored including assessments of (i) the degree to which the classes were interpretable in relation to the geomorphic features they displayed; (ii) the degree to which sub-divisions of the six classes could be identified and justified; (iii) the accuracy of some specific types of information extracted from Google Earth; and (iv) the degree to which the six classes corresponded to expected gradients in two controlling variables: stream power and bed sediment calibre. Thirdly, bar theory was applied to a sample of rivers representative of the six classes. Since bars are an important contributor to river channel form and dynamics, the correspondence of the bars in the six river classes to their expected distribution as indicated by bar theory, provided further confirmation of the robustness of the classification. The outputs of the research are (i) a fully-tested classification of European single-thread rivers; and (ii) a demonstration of how Google Earth can provide valuable information for research in fluvial geomorphology. Some additional future research stages are proposed that could turn the classification into an operational tool in the context of river assessment and management.

Understanding and predicting the effects of width variability and the controls on width adjustment in rivers has a key role in developing management approaches able to account for the physical, ecological and socio-economical dimensions of a river system. Width adaptation in a river occurs due to erosion and accretion of banks, within various geomorphic, environmental and anthropogenic contexts, which set the most relevant factors controlling the morphological dynamics of the river corridor. In turn, changes in channel width imply alterations of the river channel morphodynamics at a variety of space and time scales, implying, for instance, modifications of important controlling parameters, like the width-to-depth ratio, which is closely related to the planform morphology of alluvial rivers. Width adaptation bears crucial implications for river management: on one hand, channel widening may result in loss of valuable land and in the increase of the damage risk of infrastructures in surrounding areas, which are often subjected to increasing pressures related to human settlements and economic activities. On the other hand, several approaches to river restoration are based on the concept of “giving more room to the river”, and thus allow the banks to erode and widen, to increase morphological and physical habitat diversity. In view of these implications, the prediction of width adaptation, understanding of its main causes and controlling factors, and quantification of the riverbed morphodynamic response to width variability is of crucial importance to support effective river management. The practical and engineering interest on stable cross-sections of alluvial channels has attracted a considerable amount of scientific research since late 19th century. Much of the research has focused in developing width prediction tools mostly based on empirical approaches and methods based on extremal hypothesis and to lesser extent on mechanistic methods. In the past two decades, research has advanced in developing numerical models including geotechnical as well as fluvial processes to simulate bank failure mechanism more accurately. Despite significant development on the width predictors, research in controls on width evolution of river channels cannot still be considered a fully settled issue. The study of the morphodynamic response of the riverbed to width variability in space and time is somehow more recent, and has focussed on the dynamics of large-scale bedforms (river bars) that produce a variety of riverbed configurations and planform morphologies. The effect of spatial width variability on river bars has mainly been based on assessing the role of such planform forcing effects to the bed topography, both in case of straight and meandering river channels. The amplitude of width variability has been related to fundamental questions as those behind the transition between single- and multi-thread river morphologies, and most studies consider regular spatial variations of the channel width. Research on the response of channel bed to spatial width variability has mostly consisted of modelling and theoretical approaches, which point out the limit cases of a purely “free” system response, associated with morphodynamic instability, an of purely “forced” bedform pattern by spatial planform non-homogeneity. The large spectrum of mixed configurations between those two theoretical limits has been so far seldom investigated, despite its strong relevance for real river systems. The limits of what can actually be considered a “planform forcing” effect, or has instead a too small variability have never been clarified, a well as its role on the resulting channel morphodynamics. For instance, the effects of small amplitude width variations on straight channels, which may be due to imperfect bank lines or protrusion due to vegetations, on morphodynamics of river bed has been neglected so far. This study has two main scientific goals. The first goal is to quantitatively investigate the role of potentially controlling factors on the width evolution of bedload-dominated straight river channels, including the initial channel width, the flow regime and the sediment supply regime. The major question driving the research is whether a river would attain the same width independently of the initial conditions and whether this would be true for all types of discharge regimes of water and sediment supply. The study is carried out using both laboratory experiments (Chapter 3), analytical model (Chapter 4) and numerical model (Chapter 5) tested with reference to real river data. Integrating the results of the experiments with those of analytical and numerical models allows deriving a more robust and complete understanding of the processes involved, including transient width evolution, time scales to morphodynamic equilibrium, equilibrium conditions and role of each controlling factor. In Chapter 3 a set of controlled laboratory experiments have been performed to study channel adjustments in a movable-bed, erodible-bank channel under different flow and sediment regimes and different initial widths. The long-term width evolution is observed to be independent of initial channel width under uniform formative discharge without upstream sediment supply. Width evolution rate is observed to depend on the initial channel width when the sediment is supplied from upstream with the narrowest initial channel evolving at the highest widening rate and resulting into the widest channel. A physics based analytical model of channel adjustment (Chapter 4) has been applied to some of the experiments described in Chapter 3. Furthermore, in Chapter 5 a field scale numerical model was setup using the flow and topographic data of gravel bed reach of Upper Severn River near Abermule (UK). The trend of width evolution computed by analytical model is also qualitatively in agreement with the observations in the experiments. The results of numerical modeling have further supported the observations in the experiments which reinforce the findings in agreement with laws of physics. The second goal of the present PhD research is to analyze the morphodynamic response of the riverbed to small-scale spatial variability of the channel width, focusing on alternate bars. The main question driving the investigation (Chapter 6) is to which extent small-amplitude, irregular width variations in space affect the morphodynamics of river bars, the fundamental riverbed patterns at the scale of the channel width. The key theoretical question behind this investigation is to which extent “small amplitude” width variations can be considered as a planform forcing, for the channel bed morphodynamic response, and whether it is possible to establish a threshold amplitude below which they may act as a near bank-roughness element. The study is based on hydraulic conditions typical of bedload-dominated piedmont streams, often having flows with Froude numbers around 1 or higher at bar-forming or channel-forming conditions. The study is developed through a numerical modeling approach. Because of the considered hydraulic conditions (close to critical-Froude number) first, a comparison is made between one semi-coupled numerical morphodynamic model, expected to be most suitable for sub critical flows, and one fully-coupled numerical morphodynamic model which can handle Froude-critical flows to assess the potential shortcomings of applying a semi-coupled model under close-to-critial Froude conditions. Such test, (Appendix B) supports the use of both models, and the semi-coupled model is eventually preferred for the advantages in computational speed. Such model is used for the numerical investigations performed in Chapter 6 and to some extent also in Chapter 5. The comparison is based on the reproduction of alternate bars morphodynamics observed in existing sets of flume experiments with fixed banks and super-critical flow conditions. The results of numerical modeling have shown that the small width variations have accelerated the development of the steady bars suppressing the free bar instability. Further investigations reveal that the effects of small width variations to a certain extent can be captured by parameterizing them in the form of increased roughness close to the banks or as small obstructions along the banks.

Looking over a tidal wetland, the tidal network characterised by its intricate system of bifurcating, blind-ended tidal courses clearly stands out from the overall landscape. This tidal landform exerts a fundamental control on the morphology and ecology within the tidal environment. With today’s recognition of the ecological, economical and societal values provided by tidal wetlands, which has been notably reflected in the development of restoration management strategies across Europe and USA, there is a need to fully understand the nature and development of tidal networks as well as their relationships with associated landforms and biotic components (e.g. vegetation), to eventually guarantee the success of current and future restoration practices. Accordingly, this research aims to bring further insights into the bio-physical controls on the geomorphology of tidal networks. To this end, a combination of remote sensing, modelling and field activities was employed. A geo-spatial analysis was performed at Queen Mary, University of London (UK), to address the variability of tidal network patterns. A series of network scale morphometric variables was extracted using airborne LiDAR data among selected tidal networks across the UK depicting different planview morphologies, and supplemented with the collection of corresponding marsh scale environmental variables from published sources. Multivariate statistics were then performed to characterise the variability of tidal network patterns and identify the inherent environmental controls. The analysis has revealed that every network type can be characterised based upon measures of network size and complexity, with each network pattern depicting proper morphometric aspects. Particularly, the stream Strahler order and the median depth of the network main channel have the highest discriminating weight on the patterns investigated. High correlation between the latter variable and network main channel width has revealed that linear, linear-dendritic and dendritic networks followed a transitional gradient in their aspect ratio approximated by a power law and thus are seen to depict similar erosional processes. To the contrary, meandering networks clearly depart from this relationship, and show particular segregation in their aspect ratios with respect to dendritic networks. Globally, differentiation on network morphometric properties has been linked to environmental conditions specific to the marsh physiographic setting within which a tidal network develops. Conceptually, tidal networks seem to adapt to marsh environmental conditions by adopting suitable morphologies to drain their tidal basin effectively.An eco-geomorphic modelling framework was developed at University of Trento (Italy), to address tidal network morphological development. In line with current theories as well as modelling advances and challenges in the field of tidal network ontogeny, emphasis was thus placed on the investigation of tidal channel formation and evolution in progressive marsh accretional context. Under these environmental conditions, tidal network development can be ascribed to the combination of two channel-forming processes: channel initiation results from bottom incisions in regions where topographic depressions occur; channel elaboration results from differential deposition, contributing to the deepening of the tidal channels relative to the adjacent marsh platform. Further evolutionary stages including channel reduction proceed from the horizontal progradation of the marsh platform which may lead eventually to channel infilling. Moreover, both qualitative and quantitative results allude to an acceleration of the morphological development of the synthetic tidal networks with increasing sediment supply. These different observations thus emphasise the prevalence of depositional processes in shaping tidal channels. In a second stage, the investigation was extended to the role of the initial tidal flat morphology as an inherent control on tidal network development, by considering different scenarios of topographic perturbations, which has revealed its legacy on tidal network morphological features. Modelling experiments have also acknowledged salt marsh macrophytes as a potential control on network evolution depending on their biomass distribution within the tidal frame. However, tidal channel morphodynamcis appears to be sensitive to the way biomass growth is mathematically parameterised in the model. In view of the current challenges in transcribing mathematically such a dynamic process and the relevance of bio-physical interactions in driving salt marsh and tidal network evolution, a field survey was conducted in a temperate salt marsh in the Netherlands, as part of the mobility to UNESCO-IHE (Netherlands) in partnership with University of Antwerp (Belgium), to assess vegetation distribution and productivity in the tidal frame. Particularly, emphasis was placed on extending investigations on the possible presence of relationships involving vegetation properties in different climatic and ecological conditions from those characterising these previously documented relationships. Regression analysis has revealed that biomass growth can be expressed as a linear function of marsh relative elevation, providing therefore direct empirical validation for corresponding assumptions reported in the literature and used in the present modelling framework; surprisingly, that increase did not correlate with an increase in species richness and diversity. Analysis of likely associations between vegetation morphometrics and total standing biomass yielded only a single linear relationship linking the latter variable to stem height. In truth, these observations may bear reconsiderations on the global validity of the assumptions used in the formulation of some eco-geomorphic processes which are applied in the study and prediction of wetland resiliency facing climate change.

This thesis is about the development and testing of a novel two-dimensional numerical model (the GIAMT2D model) able to address the hydro-morphodynamic evolution of gravel-bed rivers. The model solves the two-dimensional hyperbolic system of partial differential equations (PDEs) arising from the shallow water-Exner model, describing free surface shallow flows over erodible bed, with suitable closure relations for bedload transport. A coupled formulation of the mathematical problem, which is needed in order to correctly handle sediment transport in Froude trans-critical flow conditions, is implemented, resulting in a non-conservative hyperbolic problem, which requires the adoption of a path-conservative scheme. A drawback of the fully-coupled shallow water-Exner model is that in general the solution of the Riemann problem is not easily available, at least if complex empirical sediment transport formulae are applied, which makes the upwind approach inadequate for designing numerical approximations to the solutions. Adoption of the more general, Riemann solver-free centred approach is thus required, the drawback being that centred schemes are significantly less accurate than upwind schemes in some specific cases, namely for intermediate waves and computations at low CFL number. In GIAMT2D an original centred upwind-biased scheme (UPRICE2-C delta) is applied, recovering accuracy typical of upwind methods, still being able to include any bedload transport formula. The proposed scheme results from original studies in applied mathematics, presented in the first part of the thesis, concerning the development of upwind-biased variations of the centred FORCE scheme for the solution of hyperbolic systems of PDEs, in conservative and non-conservative form. The performance of these schemes is thoroughly assessed in a suite of tests for the shallow water equations. The GIAMT2D model embeds the UPRICE2-Cd scheme extended to second-order accuracy in the ADER framework, inserted in a robust second-order preserving splitting technique for the treatment of frictional source terms, and includes an original wetting-and-drying procedure. The model performance is checked in well-established classical test cases with fixed and movable bed. These applications highlight the capability of the model in correctly and accurately solving the equations in various cases, e.g. in computations at low local CFL number, in the solution of wet-dry fronts with fixed and movable bed and in the prediction of sediment transport in Froude trans-critical conditions. The concept of "morphodynamic benchmark" is introduced for the purpose of assessing the model performance in reproducing basic river morphodynamic processes for which established theoretical and experimental knowledge is available. Unit processes with utmost importance for gravel-bed river morphodynamics, like free and forced bar instability and the stability of channel bifurcations, are chosen for this aim. In this novel approach for assessing the model capabilities, the numerical solutions satisfactorily compare with approximate analytical morphodynamic solution and laboratory data. Having proved that the model is able to reproduce the salient features of these classical morphodynamic solutions, an original morphodynamic study is finally carried out, concerning the non-linear interaction of free and forced bars in straight channels, for which a mature analytical theory is not available at present. The numerical runs of GIAMT2D are used to validate the research hypotheses developed on the basis of existing analytical theories and satisfactorily compare with field observations.

Dry granular flows are an important paradigm for a large number of problems, from industrial applications to geophysical events. Most of the research published has studied inclined granular flows over bumpy and at rigid base, while only few of them have been focused on fully developed steady flows over an erodible beds. This kind of flows are important to understand the dynamics of complex phenomena such as dense snow-avalanches and landslides, whose rheology is still ambiguous and not well defined. In this thesis, we focus on the following three aspects: (1) dynamics of uniform flows over loose bed, where the condition of uniformity and steadiness has accurately checked, (2) the influence of collisional parameters on the behavior of the flow, (3) the side-wall effect. Experimental investigations were carried out in laboratory through imaging techniques and direct methods to analyze the stresses exerted by the flow. The thesis contributes with accurate measurements of the mean velocity, solid concentration, and granular temperature pro files obtained along the depth and the free surface of the flow. Considerations have been derived in comparison to the existing data, pointing out the importance of collisional parameters and the conditions for uniform and steady flows. Experimental evaluation of the stresses was carried out at the side-walls of the channelized granular flows by the means of new device developed in the laboratory and tested for different con figurations of the flow.

Braided rivers are complex, fascinating fluvial pattern, which represent the natural state of many gravel and sand bed rivers. Both natural and human causes may force a change in the boundary conditions, and consequently impact the river functionality. Detailed knowledge on the consequent morphological response is important in order to define management strategies which combine different needs, from protection of human activities and infrastructures to preservation of the ecological and biological richness. During the last decades, research has made significant advance to the description of this complex system, thanks to flume investigations, development of new survey techniques and, to a lesser extent, numerical and analytical solutions of mathematical models (e.g. Ashmore_2013). Despite that, many relevant questions, concerning the braided morphodynamics at different spatial and temporal scales (from the unit process scale, to the reach scale, and eventually to the catchment scale) remain unanswered. For example, quantitative analysis of the morphological response to varying external controls still requires investigation and needs the definition of suitable, stage-independent braiding indicators. In addition, the morphodynamics of the fundamental processes, such as bifurcations, also needs further analysis of the driving mechanisms. General aim of the present study is to develop new methods to exploit, in an integrated way, the potential of the new possibilities offered by advanced monitoring techniques, laboratory models, numerical schemes and analytical solutions. The final goal is to fill some gaps in the present knowledge, which could ultimately provide scientific support to river management policies. We adopted analytical perturbation approaches to solve the two-dimensional shallow water model; we performed laboratory simulations on a large, mobile-bed flume; we analysed existing topographic measurements from LiDAR and Terrestrial Laser scanning Devices; and we simulated numerically the river hydrodynamics. Within each of the six, independent, research chapters, we interconnected results from the different approaches and methodologies, in order to take advantage of their potential. Summarising, the more relevant and novel outcomes of the present work can be listed as follows: 1) We explored the morphological changes during a sequence of flood events in a natural braided river (Rees River, NZ)and we proposed a morphological method to assess the sediment transport rate. In particular we propose a semi-automatic method for estimating the particles path-length (Ashmore and Church, 1998) on the basis of the size of the deposition patches, which can be identified on the basis of DEM of differences. Comparison with results of numerical simulation confirmed that such an approach can reproduce the response of the bedload rate to floods of different duration and magnitude. 2) We developed a new indicator of the reach-scale morphology and, on the basis of existing laboratory experiments, we explored its dependence, under regime conditions, to the controlling factors: slope, discharge, confinement width, grain size. In spite of its synthetic nature, this simple indicator embeds the information needed to estimate the variability of the Shield stress throughout the braided network, and consequently enables to assess the transport-rate and its variation with the driving discharge. 3) We investigated, through flume experiments, the effect of the flow unsteadiness on the sediment transport in a braided river. This is possible only by following a statistical approach based on multiple repetitions of the same flow hydrograph. Results revealed that for confined network an hysteresis of the bedload response occurs, which leads to higher sediment transport during increasing flow, whereas relatively unconfined networks always show quasi-equilibrium transport rates. 4) A second set of laboratory experiments provided information on the morphodynamics of a braided network subject to variations of the sediment supply. We proposed a simple diffusive model to quantify the evolution of the one-dimensional bed elevation profile. Such simple approach, albeit having a limited range of practical applications, represents the first attempt to quantify this process and enables to study the relevant temporal and spatial scales of the phenomenon. 5) We solved analytically the two-dimensional morphodynamic model for a gravel-bed river bifurcation. This furnishes a rigorous proof to the idea proposed by Bertoldi and Tubino (2007) to interpret the morphological response of bifurcation in light of the theory of the morphodynamic influence. The analytical approach enables to investigate the fundamental mechanics which leads to balance, and unbalance, configurations and, from a more practical point of view, allows for a better prediction of the instability point than the existing 1D models (e.g. Bolla Pittaluga et al., 2003).

Interactions between fluvial processes and vegetation along the natural channel margins have been shown to be fundamental in determining meandering rivers development. By colonizing exposed sediments, riparian trees increase erosion resistance and stabilize fluvial sediment transport through their root systems, while during a flood event the above-ground biomass interacts with the water flow inducing sediment deposition and altering scour patterns. In turn river dynamics and hydrology influence vegetative biomass growth, affecting the spatial distribution of vegetation. These bio-morphological dynamics have been observed to direct control accretion and degradation rates of the meander bend. In particular, vegetation encroachments within the point bar (i.e. colonizing species and strand wood), initiate pioneeristic landforms that, when evolving, determine the lateral shifting of the margin that separates active channel from river floodplain and thus inner bank aggradation (bar push). This diminishes the portion of the morphologically active channel cross-section, influencing the erosion of the cutting bank and promoting channel widen- ing (bank pull ). As a result of the cyclical occurrence of these erosional and depositional processes, meandering rivers floodplain show a typical ridge and swale pattern characterized by the presence of complex morphological structures, namely, benches, scrolls and chutes within the new-created floodplain. Moreover, difference in migration rate between the two banks have been observed to induce local temporal variations in channel width that affect river channel morphodynamics and its overall planform through their influence on the local flow field and channel bed morphology. Despite enormous advances in field and laboratory techniques and modelling development of the last decades, little is known about the relation between floodplain patterns and their controlling bio-morphological interactions that determine the bank accretion process. This knowledge gap has so far limited the development of physically-based models for the evolution of meandering rivers able to describe the lateral migration of banklines separately. Most existing meander migration models are indeed based on the hypothesis of constant channel width. Starting from this knowledge gap, the present doctoral research has aimed to provide more insight in the mutual interactions among flow, sediment transport and riparian vegetation dynamics in advancing banks of meandering rivers. In order to achieve its aims, the research has been designed as an integration of remote sensing and in-situ field observations with a mathematical modelling approach to i) provide a quantitative description of vegetation and floodplain channel topography patterns in advancing meanders bend and to ii) explore the key control factors and their role in generating the observed patterns. The structure of the present PhD work is based on four main elements. First, two types of airborne historical data (air photographs and Lidar survey) have been investigated, in order to quantify the effects of spatial-temporal evolution of vegetation pattern on meander morphology and to provide evidence for the influence of vegetation within the topography of the present floodplain. Such remote sensing analysis has highlighted a strong correspondence between riparian canopy structure and geomorphological patterns within the floodplain area: this has clearly shown the need to interpret the final river morphology as the result of a two-way interaction between riparian vegetation dynamics and river processes. Second, field measurments have been conducted on a dynamic meander bend of the lower reach of the Tagliamento River, Italy, with the initial aim of checking the outcomes of the remote sensing analysis through ground data. The outcomes of the field measurements have further supported the results, providing ground evidence on the relations between vegetation and topographic patterns within the transition zone that is intermediate between the active channel bed and the vegetated portion of the accreting floodplain. The influence of vegetation on inner bank morphology has also been interpreted in the light of the expected time scales of inundation and geomorphic dynamics that characterize the advancing process of the inner bank. The combined analysis of both remotely sensed data and field measurements associated with the historical hydrological dataset have allowed to quantitatively characterize the biophysical characteristics of the buffer zone, close to the river edge, where the accretion processes take place. The third research element has foreseen the development of a biophysically-based, simplified bio-morphodynamic model for the lateral migration of a meander bend that took advantage of the empirical knowledge gained in the analysis of field data. The model links a minimalist approach that includes biophysically-based relationships to describe the interaction between riparian vegetation and river hydromorphodynamic processes, and employs a non linear mathematical model to describe the morphodynamics of meander channel bed. Model application has allowed to reproduce the spatial oscillations of vegetation biomass density and ground morphology observed in the previous analyses. Overall, the model allows to understand the role of the main controlling factors for the ground and vegetation patterns that characterize the advancing river bank and to investigate the temporal dynamics of the morphologically active channel width, providing insights into the bank pull and bar push phenomena. The fourth and concluding element of the present PhD research is a analytical investigation of the fundamental role of unsteadiness on the morphodynamic response of the river channel. Results obtained in the previous elements have clearly showed the tendency of a meander bend to develop temporal oscillations of the active channel width during its evolution, but no predictive analytical tool was previously available to investigate the channel bed response to such non-stationary planform dynamics. A non linear model has therefore been proposed to investigate the effect of active channel width unsteadiness on channel bed morphology. The basic case of free bar instability in a straight channel has been used in this first investigation, which has shown the tendency of channel widening to increase river bed instability compared to the steady case, in qualitative agreement with experimental observations. Overall, the research conducted within the present Doctoral Thesis represents a step forward in understanding the bio-morphodynamics of meandering rivers that can help the development of a complete bio-morphodynamic model for meandering rivers evolution, able to provide support for sustainable river management.

Over the past 200 years, rivers in industrialized countries have been significantly altered by human interventions such as channelization, hydropower development, and sediment mining causing observable biogeomorphological changes. In the European Alpine region, many large rivers have been impounded and channelized, yet few studies have conducted in-depth research on the temporal patterns of the causes and trajectories of these biogeomorphological responses, in comparison to rivers that can adjust their planform. Moreover, it is well-known that within channelized rivers alternating bars may appear due to an instability of the riverbed, but the development and influence of vegetation on such bars, its feedbacks on the morphodynamics of the bars and the degree to which these mutual interaction processes responds to anthropic stressors related to alterations in the flow and sediment supply regimes has received little attention. The present research aims to disentangle the mechanisms that may determine dramatically diverging biogeomorphological trajectories in regulated Alpine rivers. It further intends to identify the underlying relations of the triad that connects vegetation – sediment – flow regime and its feedbacks in regulated, channelized, rivers with vegetated bars. The methodology comprises an interdisciplinary approach which combines field and historical investigations with theoretical predictions, and integrates a variety of spatial and temporal scales and different levels of detail in characterising processes. Two case studies in the Alpine region (the Isère river in southeast France and the Noce river in northeast Italy) were selected for a quantitative, historical analysis of the bio-morphological trajectories using remotely sensed data to investigate the apparent responses to human-induced modifications of natural processes. Both rivers have been heavily impacted, with a notable increase of human stressors since the mid-20th century which can be associated with the transition of both systems from an initial, stable dynamic state characterized by bars having only sparse colonizing vegetation with a frequent turnover to a new, apparently stable state characterised by reduced morphodynamics and an increased vegetation cover in recent decades. The Isère river, which underwent a shift from unvegetated, migrating bars to vegetated, stable bars, was further explored with a hydromorphodynamic modelling approach to investigate historical changes in riparian vegetation recruitment and survival related to changes in the flow regime. The Windows of Opportunity model was successful at revealing temporal changes in recruitment conditions in response to flow regime alterations. Further results indicated a reduction in relevant high flow events that might be competent to induce large bar migration in the system. Alterations of the flow regime are assumed to have played a major role in vegetation encroachment directly by affecting vegetation recruitment through reduced flow disturbances and indirectly inducing modifications of bar morphodynamics. Field observations of root development were also made on the Noce and Isère rivers, focusing on two species Salix alba and Phalaris arundinacea, with the aim of improving understanding of the role of roots on the presence and movement of vegetated bars. When comparing results from different sites, more predictable linear relationships between root properties and depth below the ground surface were associated with stronger flow regulation. Bar morphology (surface elevation or depth of sedimentation and sediment calibre) and flow regime were found to be the main drivers of root architecture. Furthermore, roots were found to have an important role in the stabilization of the bars with the ability to stabilise fine sediments trapped by the plant’s canopy during phases of bar aggradation. To understand the current state of channelized Alpine rivers, which often show diverging biogeomorphic features, it is necessary to understand the underlying interactions between flow, sediment, and vegetation dynamics. Only through investigating the historical biomorphological evolution of rivers and the main drivers of that evolution it is possible to design measures that can be effective in rehabilitating desired ecosystem functions that have been markedly modified by those state transitions. In summary, this study has provided novel, quantitative insights about the complexity of flow – vegetation – morphology interactions occurring in channelized river systems in relation to anthropogenic stressors causing alteration in their flow and sediment supply regimes. By integrating different approaches, this study has shown how these river systems can be highly sensitive to even small changes in the anthropogenic stressors, depending on the stage in their evolutionary trajectory, which is crucial to be detected to support the development of sustainable management strategies aimed at restoring or improving target riverine functions and processes.

During the past decades, the systematic investigation of the morphodynamics of meandering rivers mostly involved the theoretical-analytical methodology. The development of analytical models enabled the definition of equilibrium conditions, stability and evolution of river meanders and to investigate the interaction between planform and bedform processes and mechanisms. In recent years the new branch of remote sensing applied to river morphodynamics has been constantly developing simultaneously to the rapid increase of computational and satellite resources. The remote sensing analysis is nowadays employed in a wide range fields in geophysics; for this reason, the past years have seen the prolific development of numerous algorithms for remote sensing analysis. However, remote sensing of meandering river morphodynamics has not been consistently integrated with morphodynamic modelling so far. There is a lack of sophisticated algorithms for the extraction of extensive morphodynamic information from the available remotely sensed data; this gap prevented researchers from seeking systematic validation of analytical models to define their range of applicability, and to exploit their potential for improved insight on observations in real world meandering rivers. The evolutionary dynamics of the channel width, at local and bend scale, as well as the dynamics of bars in meandering rivers represent two major unsettled issues in our present understanding of river meandering dynamics. In this thesis I first provide a systematic methodology for the automated extraction of meandering river morphodynamic information from multitemporal, multispectral remotely sensed data, coded in the PyRIS software. Moreover, I develop an analytical model to investigate the long-term planform evolution of periodic sequences of meander bends incorporating spatio-temporal variations of channel curvature, width and slope. A first model component predicts the temporal evolution of the channel width and slope based on a novel treatment of the sediment continuity at the reach scale. A second model component is a fully analytical, evolutionary model of periodic meanders with spatially and temporally oscillating width accounting for nonlinear feedbacks in flow and sediment transport by means of a two-parameters perturbation approach. Application of the PyRIS software to several long reaches of free-flowing meandering rivers allows me to develop a consistent set of observations on the temporal and spatial evolution of channel width and curvature with unprecedented level of detail. Furthermore, model outcomes indicate that meander-averaged width and slope invariably decrease during meander development, and that the temporal adjustment of the hydraulic geometry is controlled by the ratio between the evolutionary timescales of planform and riverbed, quantified from the analyzed meandering rivers dataset. The nonlinear perturbation model indicates that width and curvature co-evolve according to a hysteretic behavior in time and predicts that the meander belt width dramatically decreases when the meander resonance threshold is crossed. The modelling approach predicts wider-at-bend meanders when the bank pull is dominant with respect to bar push, which in turn promotes meander bends that are wider at inflections. Analytical modeling and remote sensing analysis are mostly integrated through a statistical approach; bend-scale evolutionary analysis of planform descriptors such as channel width, width oscillations and curvature in large pristine meandering rivers exhibit good agreement with the outcomes of the proposed analytical models. Finally, the integration between analytical modeling and remote sensing analysis allows me to identify the key processes controlling the interaction between migrating sediment bars and planform-driven steady point bars. The conditions for the formation of migrating bars in meandering rivers are mostly related to the production of sediment supply by the basin, contrarily to the widespread idea that meandering rivers exhibiting migrating bars typically display lower values of the channel curvature.