Bifurcation is one of the fundamental building blocks of a braided network; it is the process that determines the distribution of flow and sediments along the downstream branches. Braiding is a complex and highly dynamical system, whose evolution is at present predictable only on a short time scale; in this context bifurcations are the crucial process that control the adjustment of braiding intensity, being one of the main causes of the system continuous evolution. A complete description of river bifurcations is still lacking in the literature, though their importance for the onset of braiding is clearly recognized. Moreover, the physical quantitative description of river bifurcation appears as one of the main limitation of the most effective predictive models available at present, i.e. the branches or object-based models. In the first part of the work the attention has been focused on the quantitative description of the evolution of a single laterally unconstrained channel until the occurrence of the first bifurcation. The analyses has been carried out performing four different sets of experimental runs with both uniform and graded sediments. An objective criterion for the occurrence of the bifurcation has been established, using the data provided by the Fourier analysis of the evolving bank profiles; the procedure enabled to characterise the morphodynamic sequence leading to flow and channel bifurcation and to point out the importance of the mutual interactions between the bed deformation and the planimetric configuration of the channel. Along with the characterisation of the onset of bifurcations, it is crucial to investigate their further evolution, that has been pursued starting from the theoretical findings of Bolla Pittaluga et al. (2003), concerning their possible equilibrium configurations. Two sets of experiments has been carried out on a “Y-shaped†symmetrical configuration, in which the upstream channel diverge into two branches. The experimental results show the existence of an unbalanced configuration, when the Shields stress reaches relatively low values and the width to depth ratio is large enough. This asymmetrical configuration is characterised by different values of water and sediment discharges in the downstream branches and by a different bed elevation at their inlet, the channel carrying the lowest discharge showing a higher elevation. Experimental runs characterised by the presence of migrating alternate bars displayed an oscillating behaviour, generally leading to a more unbalanced configuration and, in some cases, to the abandonment of one of the branches. Experimental findings can be interpreted in the light of the morphodynamic influence theory (Zolezzi & Seminara, 2001): the distance of the flow from the resonant value of the aspect ratio seems to be a good parameter to represent such phenomenon. The dynamics of river bifurcation were also analysed in the field. Two field campaign were performed on the Ridanna Creek, Italy and on the Sunwapta River, Canada, joining an international research group. The detailed and repeated measurements allowed to point out the common features showed by the bifurcations, namely the unbalanced water distribution, the difference in bed elevation and the lateral shift of the main flow toward the external bank of the main downstream channel. The monitoring activity on the Ridanna Creek provided also the description of the planimetric and altimetric configurations of the study reach, employing both traditional survey techniques and digital photogrammetry together with the complete characterisation of morphological and hydraulic patterns. Moreover, the analysis of the long term evolution of the network pointed out the existence of three regions in the braided reach, with different morphological features and highlighted the crucial role of bifurcations in controlling braiding evolution. Theoretical analysis, laboratory and field investigations have allowed a much deeper insight in the bifurcation process, giving a quantitative detailed description of the phenomenon. The investigation now provides a suitable description of the bifurcation process that can readily be implemented in predictive models for braiding evolution, for which the adoption of physically based nodal point conditions would be highly desirable and represent the main sought outcome of the present analysis.

High-concentration flows are complex phenomena typical of Alpine mountain areas. Essentially, they are free-surface flows with intense sediment transport, often caused by intense rainfall events and involving large volumes of solid material. Because of the amount of sediments moved, the intense erosion and deposition processes typically observed and the quite unexpected character, these phenomena represent a serious hazard in populated mountain areas, where reliable and effective hazard-management and -protection strategies are required. In mountain-hazard management, high-concentration flows modelling represents a key factor, since it allows to evaluate impacts of possible hazard scenarios and the effectiveness of possible protection and mitigation measures. However, the intrinsic phenomenon complexity makes high-concentration flow modelling and hazard assessment quite challenging. In this thesis, some of the effects of high-concentration flow complexity on modelling are experienced directly and suitable solutions are proposed, to make the phenomenon description more reliable and straightforward. Among very different modelling approaches present in the literature, this work embraced the quasi-two phase, mobile-bed approach proposed in Armanini et al. (2009b) and in Rosatti and Begnudelli (2013a), which is implemented in the TRENT2D model. TRENT2D is a quite sophisticated model that solves a system of Partial Differential Equations over a Cartesian mesh by means of a finite-volume method with Godunov-type fluxes. By means of TRENT2D, the back-analysis of a couple of real debris-flow events occurred in Italy was first performed. These applications revealed clearly some troublesome "complexity issues", i.e. modelling issues generated by phenomenon complexity that may affect hazard assessment. Because of the public importance of the subject, four of the "complexity issues" identified were then faced directly. According to the purpose of this thesis, possible solutions to the issues were proposed, to ensure a proper description of the flow behaviour and possibly limit intricacy in the model use. The first complexity issue is "operational" and regards the use of the TRENT2D model and, more in general, the amount of work necessary to perform a complete hazard-assessment job about high-concentration flows. Because of the phenomenon complexity and the sophisticated character of the model, the operational chain necessary to assess hazard by means of TRENT2D appears quite demanding. The large efforts required in terms of handwork, computational charge and resources may divert the user attention from the physical meaning of the hazard-assessment process, possibly leading to inaccurate results. To overcome this issue, a possible solution is proposed, based on the use of a loosely-coupled Service Oriented Architecture approach. The aim is to develop a unique, user-friendly working environment able to support high-quality, cost-effective hazard assessment and, in perspective, the possible development of a Decision Support System for mountain hazard. The second complexity issue is "geometrical" and "numerical" and concerns morphology representation. Because of the strong interaction between high-concentration flows and bed morphology, these phenomena require bed morphology to be described with the right level of detail, especially where heterogeneity is outstanding. This is typically the case of urbanised mountain areas, with their characteristic terrain shapes, buildings, infrastructures, embankments and mitigation structures. A believable representation of these geometrical constraints may be fulfilled acting on the computational mesh used to solve model equations, preferably avoiding regular Cartesian meshes. In this work, a new version of the TRENT2D model is developed, based on the use of Delaunay, triangular unstructured meshes. To reach second order accuracy, a MUSCL-Hancock approach is considered, with gradient computation performed by means of the multidimensional method proposed in Barth and Jespersen (1989) for Euler equations. The effects of different gradient limiters are also evaluated, aiming at a proper description of the flow dynamics in heterogeneous morphology contexts. The third complexity issue is both "geometrical" and "mathematical". It concerns the effects of artificial structures, i.e. artificial geometrical constraints, on the flow dynamics. Among different structures aimed expressly at controlling the high-concentration flow behaviour, attention was paid to sluice gates, which can be used in channels and hydropower reservoirs to control sediment routing. In the literature, the effects of sluice gates have been studied especially with reference to clear water flows over fixed beds, while knowledge about the influence on high-concentration flows over mobile beds is still limited. Here, a rough, bread new mathematical description is proposed, in order to take into account the 3D morphodynamics effects caused by sluice gates in high-concentration flow modelling. The last complexity issue is pretty "numerical" and arises from the challenge of numerical models to comply with the phenomenon complexity. Generally speaking, reliable numerical models are expected to catch the main characteristics of the physical processes at both a general and a local spatial scale, although with a certain level of approximation, depending on the numerical scheme. Sometimes it may be hard to close the gap between the local phenomenon complexity and its numerical representation, leading to non-physical numerical results that could affect hazard assessment. In this work, a particular numerical issue is investigated, which was identified through a thorough analysis of TRENT2D model results. In particular, it was observed that the direction of the numerical mixture-mass flux is occasionally opposite to the direction of numerical solid-mass flux, despite the isokinetic approach which the model is based on. This incoherence was studied with a rigorous method, trying to fix the source of the problem. However, the question turned out to be quite tricky, due to the sophisticated character of the model. These four, deliberately heterogeneous, "complexity issues" allow to perceive clearly the size of complexity effects on high-concentration modelling. Furthermore, they give the measure of how much diffcult is reaching the right level of detail in describing and modelling high-concentration flows. The research of solutions that are accurate and as much simple as possible was not straightforward and required a quite large effort. Nonetheless, possible solutions were found in the end for three of the four "complexity issues", therefore the goal of the thesis can be considered as achieved.

In the past decades, hydrodynamic cavitation (HC) process was the subject of study by many researchers worldwide. This phenomenon was widely studied in order to understand the reason of its negative effects on hydraulic machinery such as pumps,turbines, valves, etc. Many efforts were made in order to better understand mechanisms of HC process with the main aim of preventing its generation and trying to avoid severe physical damage such as erosions, vibrations and noises. In recent years, in order to cope with a decrease in available water resources worldwide, an increasing demand of water by population in developing/developed countries and more restrictive environmental legislations on water quality, HC was increasingly used as a novel energy-efficient technique in the field of wastewaters treatment. The main purpose of this thesis is to investigate on the effectiveness of a modified swirling-jet device called Ecowirl reactor, patented by Econovation GmbH, Germany and produced and commercialized by Officine Parisi s.r.l., Italy. Experimental studies were carried out in order to evaluate the effects of different operative conditions and parameters such as reactor geometry, flow rate, flow velocity, pressure, medium pH, medium concentration and medium temperature on (i) the degradation of a toxic and carcinogenic pollutant dye (Rhodamine B, RhB) in waste dye aqueous solutions and on (ii) the improvement of activated sludge solubilisation and aerobic sludge biodegradability in the field of biological wastewater treatments. In order to better understand the fluid dynamics into Ecowirl reactor, it was modelled. The model based on previous experimental data was implemented in a Computational Fluid Dynamics software (ANSYS, 16.2).

Driftwood is widely recognized as a relevant component of riverine systems due to its complex interactions with flow, sediment transport and vegetation dynamics. In-channel large wood has a relevant geomorphic and ecological role as it enhances morphological diversity and creates a variety of physical habitats that sustain high biodiversity. Its presence can also increase flood risk and therefore wood is often removed from streams especially in densely populated areas. Recent river restoration policies aim to maximise the environmental benefits of driftwood and minimise risks. The study of wood dynamics can provide useful information to define guidelines for sustainable wood management. Multi-thread systems represent a particularly interesting and challenging context for the investigation of wood dynamics because of their complex geometry, the presence of vegetated islands and the frequent, intense changes in channel pattern observed even for moderate discharge fluctuations. However, comparatively few studies focus on driftwood in large braided rivers and limited quantitative information is available on wood transport, deposition and remobilisation in these systems. The goals of the present work are: a) characterising the spatial organisation of wood deposits and identifying typical retention sites and styles; b) analysing the influence of flow regime, channel morphology, wood supply and log properties (size and shape) on dispersal patterns; c) investigating wood remobilisation induced by discharge fluctuations and bed reworking; and d) analysing long-term wood storage volume and budget. A combination of field-scale direct observations, remote sensing techniques and physical modelling was used to investigate wood and channel dynamics. Field-scale monitoring carried out on the Tagliamento River (Italy) allowed the observation of complex interactions and feedbacks between channel, vegetation and wood dynamics. Laboratory simulations – carried out in two large flumes at the University of Trento (Italy) and at the University of Hull (UK) – were employed to investigate individual wood dispersal mechanisms under controlled conditions and to explore the role of governing parameters. In large rivers, floods are the primary driver of wood recruitment through the erosion of vegetated banks and islands; field-scale observations showed that these localised wood inputs control wood storage at sub-reach scale because a large proportion of eroded trees is retained close to the input point in sparse, small jams. Physical modelling highlighted a complex relationships between flow stage and the longitudinal and vertical distribution of wood; high discharge increases the ability of the system to transfer wood, but at the same time generates complex inundation patterns where a larger number of sites are available for wood retention. No clear link between flow stage and the vertical distribution of wood is observed, probably because water surface elevation exhibits small changes with discharge in flat braided river cross-sections. Driftwood element properties also influence deposition patterns; log diameter controls travel distance as it governs flotation and therefore the likeliness of deposition. High element length and complex piece shape sustain the formation of large jams. The presence of a root bole is also associated to short travel distance and low relative elevation. At reach-scale, the spatial distribution of wood is the product of local inputs during major floods and reorganisation of deposits induced by minor events. Wood pattern exhibits a threshold behaviour with supply. High input rates determine very high spatial density and the formation of large, stable jams. Two processes govern wood reorganisation over different time scales, namely network inundation – inducing rapid changes in flow field – and bed reworking. In the first case, the persistence of deposits depends on the magnitude of discharge fluctuations as wood dispersed by small floods is easily removed by larger events. High relative elevation and large jam size enhance wood stability, while the presence of a root wad has a dual effect as it determines large accumulations at low elevation. Channel pattern reworking determines intense turnover of driftwood deposits regardless of supply rate, piece properties and jam size, save for very large accumulations. As a consequence, wood deposition occurs mostly on empty braidplain areas as opposed to pre-existing sites. These results suggests that wood (alone) has little direct effect on reach-scale bed geometry in a large braided river; however, deposited wood significantly influences local hydraulics and morphology, enhancing physical habitat diversity. Moreover, deposited wood favours the accumulation of fine sediment, nutrients and seeds and often exhibits vegetative regeneration. These processes lead to the transformation of instable driftwood pieces into vegetated islands, which in turn can trap more wood. Therefore, wood has a relevant, indirect effect on braided river morphodynamics through the establishment of vegetation, whose presence influences network complexity and evolution.

Rivers provide to society many important goods and benefits. Some of these ecosystem services depend on the river flow regime, which has been deeply modified by human structures and activities. These alterations have a direct influence on biodiversity, natural habitat and on the supply of river ecosystem services. The release of water from storage hydropower plants generates rapid flow and stage fluctuations (hydropeaking) in the receiving water bodies at a variety of sub-daily time-scales. In this thesis, we describe an approach to quantify such variations, which is easy to apply, requires stream flow data at a readily available resolution, and allows for the comparison of hydropeaking flow alteration amongst several gauged stations. Hydropeaking flow alteration is quantified by adopting a rigorous statistical approach and using two indicators related to flow magnitude and rate of change. We utilised a comprehensive stream-flow dataset of 105 gauging stations from Italy, Switzerland and Norway to develop and test our method. Next, we introduce a modelling approach to evaluate the spatial and temporal variations of a discharge-related ecosystem service, the rafting. The application of hydraulic and habitat models allowed to define spatially thresholds of suitability in each river reach and the application of an hydrological model allowed to assess temporally the suitability for the rafting navigability in different discharge conditions. We applied the method to the Noce River, an Alpine River in Northern Italy affected by hydropeaking. Our analysis showed that in this river, the water releases are fundamental to maintain high flow conditions required for rafting, which can be granted only by hydropower production especially in summer months. Together with present discharge conditions, our approach allows to analyse also the effects of an additional withdrawal which locally has a negative impact on river suitability. Finally, the application of the methodology was extended to include in the analysis the fish habitat and the small hydropower production, along with the rafting. The effects of hydropeaking on these ecosystem services were assessed in space and time. Hydropeaking has a strong influence on rafting navigability and less obvious consequences on the other services. Different management scenarios of the water releases from the hydropower plants were produce, with the aim to evaluate spatially the reciprocal effects of optimizing each ecosystem services. Only the scenario of rafting optimization will significantly increase rafting navigability, while the effects of other scenarios are less evident. Moreover, two additional increasing withdrawals have been simulated to evaluate their impacts on the services. The small hydropower withdrawals will have a negative impact on rafting and fish habitat, while the preservation of requirements for rafting will greatly affect the small hydropower production. This ecosystems-services based approach can be integrated in the decision-making process to evaluate river management alternatives.

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.