At global level, the population is increasingly concentrating in the cities. In Europe, around 75% of the population lives in urban areas and, according to the European Environmental Agency (2010), urban population is foreseen to increase up to 80 % by 2020. At the same time, the quality of life in the cities is declining and urban pollution keeps increasing in terms of carbon dioxide (CO2) emissions, waste, noise, and lack of greenery. Many of European cities struggle to cope with social, economic and environmental problems resulting from pressures such as overcrowding or decline, social inequity, health problems related to food security and pollution. Nowadays local authorities try to solve these problems related to the environmental sustainability through various urban logistics measures, which directly and indirectly affect the urban food supply system, thus an integrated approach including freight transport and food provisioning policies issues is needed. This research centres on the urban food transport system and its impact on the city environmental sustainability. The main question that drives the research analysis is "How the urban food distribution system affects the ecological sustainability in modern cities?" The research analyses the city logistics project for food transport implemented in Parma, Italy, by the wholesale produce market. The case study investigates the renewed role of the wholesale market in the urban food supply chain as commercial and logistic operator, referring to the concept of food hub. Then, a preliminary analysis on the urban food transport for the city of Bologna is presented. The research aims at suggesting a methodological framework to estimate the urban food demand, the urban food supply and to assess the urban food transport performance, in order to identify external costs indicators that help policymakers in evaluating the environmental sustainability of different logistics measures

Soils are a dominant source of nitrous oxide (N2O), a potent greenhouse gas. The complexity of drivers of N2O production and emissions has hindered our ability to predict the magnitude and spatial dynamics of N2O fluxes. Soil moisture can be considered a key driver because it influences oxygen supply, which feeds back on N2O sources (nitrification versus denitrification) and sinks (reduction to dinitrogen). Soil volumetric water content is directly linked to dissolved oxygen and to redox potential, which regulate microbial metabolism and chemical transformations in the environment. The relationship between soil moisture and N2O is usually based on incubations of soil at different soil moisture levels. Few studies have focused on the interaction between soil moisture and nitrogen dynamics in the vadose zone. In this thesis soil column and chamber experiments were performed in order to investigate the relationship of soil moisture dynamics to redox sensitive nitrogen dynamics in the organic matter layer of a pasture peatland in Sacramento, Bay Delta area, California. Field data has been analyzed and statistics has been used to evaluate the influence of irrigation practices on spatial pattern of measurements. Data indicate that organic peatland might be an important source of nitrous oxide emissions. The comparison of rainfall, saturation and deposition shown that trace gases emissions, dissolved nitrate and ammonium changed considerably along the soil column profile as a response of the microbial community to the high variability in redox, soil moisture, oxygen experienced by the soil at different depth. Water movement favored the formation of zones at different redox condition, redistributed the nutrient along the soil profile, and considerably changed mineralization,nitrification and dissimilatory reduction to nitrate (DNRA) rates. It was observed an asymmetrical behavior between nitrogen and ammonium profiles. Experiments shown that this assimetry is a function of the degree of saturation (as well as its duration). Also the fraction of the total N2O that is actually emitted to the atmosphere depends heavily on the structure and wetness of the soil. The nitrous oxide dynamic is therefore a function of the antecedent wetness condition, the nutrient content of the peat-land, the physical characteristics of the peat-land and the vertical stratification of layers at different redox and oxygen condition, which may affect the annual N budget. In addition, the combined use of soil column and chamber experiments suggest a negative correlation between soil moisture and N2O in dynamic condition and a functional dependence of N2O emissions from the oxygen concentration. We found that the time scale of water dynamic was faster than the biological scale of trace gas emissions. Finally, the relationship of nitrous oxide versus water content was reproduced by using a lumped model which include oxygen dynamic. Preliminary results suggest that by accounting for oxygen dynamic, it is possible to reproduce the functional behavior observed in the experiment and that the latter is depending on the physical and biological properties of the soil. Keywords: water dynamics, nitrous oxide emissions, nitrate ammonification,feammox, denitrification, soil heterogeneity, oxygen, redox.

Forest management involves dealing with conflicts between the protection of nature and the use of natural resources. Bad management practices have led to significant forest degradation worldwide. It is estimated that globally about 13 million hectares of forest are lost every year, leading to a massive loss of biodiversity and other forest-related ecosystem services, such as soil stabilisation and watershed protection. This is particularly dangerous in poor regions, where livelihoods are strongly based on locally available natural resources. In 2000, IUCN and WWF have introduced a new restoration approach called Forest Landscape Restoration (FLR) that aims to regain ecological integrity and enhance human well-being in deforested or degraded forest landscapes. FLR goes well beyond planting trees: it is about re-designing the landscape mosaic in a way that both nature and people are benefited. To this purpose, different actions should be taken at different locations across the landscape. From a planning perspective, this calls for proper methods and tools that help identifying where to act and what to do. The present research aimed to contribute to this problem by developing and testing spatial decision tools to support the design of landscape mosaics. More specifically, the study had three main objectives. The first objective was the identification of criteria and indicators (C&I) for the prioritisation of forest restoration interventions. Knowing which areas are ecologically more suitable to host a restoration intervention is a prerequisite of any FLR-based plan. There can be areas where restoration is more urgent, areas where it is more likely to succeed and areas where it is expected to bring the highest ecological benefits. Unfortunately, a widely accepted framework for the prioritisation of forest restoration areas is lacking. This problem was addressed by conducting an expert survey to define a set of readily applicable C&I. This was based on a two round Delphi involving 37 people, aimed at defining the key criteria and a broad set of indicators, and a final face-to-face meeting with a smaller group of experts, aimed at refining the list of indicators and making them operational. Finally, 8 criteria and 22 indicators were obtained, whose main advantage is their spatial character, which makes them suitable for spatial analysis and mapping. The second objective was the development of a GIS-based multicriteria methodology to identify reforestation priorities, to design a number of landscape-scale reforestation options and to assess them according to their socio-ecological performance. The prioritisation was based on two main non-compensatory factors: the need for biodiversity conservation and the ecological feasibility of reforestation. Suitability maps were generated for both factors through spatial multicriteria analysis and threshold pairs used to extract priority areas. The minimum suitability levels and the total area to be reforested were used as input parameters to generate a finite number of resulting reforestation options. These were assessed for their ability to conserve biodiversity and improve living conditions of local communities by introducing additional ecological and socioeconomic indicators. The methodology was tested in an area of Chiapas (Mexico), where forest degradation is significant and poverty widespread. The tool proved to be effective in shaping compact reforestation areas and easy to use. Nevertheless, it does not allow the user to a priori define targets on both conservation and livelihood standards. Also, the forest-poverty link was little explored and the issue of access to forest resources totally neglected. This leads to the third objective of the thesis: the definition of a spatial optimization model to re-design the landscape mosaic through reforestation in a way that nature protection is enhanced, the provision of ecosystem services is ensured and livelihoods are sustained. Either one of two possible uses was assigned to forest: protection, if forest is primarily devoted to biodiversity conservation, and harvest, if forest is available for the collection of timber. The model, which is an Integer Programming-based one, identifies land to be reforested and assigns this to the two uses such that all environmental classes over the landscape are adequately covered by protected forest, each village has a sufficient amount of harvestable forest at short distance and a given amount of erosion-prone land is reforested. The model also accounts for opportunity costs, by limiting the amount of economically strategic lands (e.g. agriculture) to be converted to forest. The model is the first of its kind to account for local peopleâ€TMs livelihoods by ensuring the accessibility to natural resources. The application to a case study in central Chiapas (Mexico) showed that increasing the demand for the provision of an ecosystem service does not significantly affect the ecological benefits up to a given threshold. Although some assumptions had to be made, the model provided a demonstration that the principles of the FLR can be put in practice and ad hoc planning tools can be designed to support decision-makers in their activity. Most of all, the model provided a solution to the problem of conserving biodiversity in poor regions where maintaining the access to local natural resources is vital to people. Redesigning forest landscapes for nature conservation and livelihood improvement is a difficult task. But one of dramatic importance as well. This study provided tools that can be of practical help to decision-makers and planners willing to undertake the challenge. Nevertheless, the problem is complex and intrinsically affected by uncertainty: further research effort is needed to test indicators, include the time dimension into the model and involve stakeholders in the decision process.

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.

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).