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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, mobilebed 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:(i) 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. (ii) 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. (iii) 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. (iv) 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. (v) 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).

Thesis (M.S.) in Physics--University of Maine, 2004. / Includes vita. Includes bibliographical references (leaves 45-46).

Thesis (M.S.) in Marine Biology--University of Maine, 2001. / Includes vita. Includes bibliographical references (leaves 91-98).

A l-D tidally resolving hydrodynamic model of the Tweed Estuary has been encoded using the Estuarine Contaminant Simulator, ECoS. The model results of axial and time series variations in water elevation, salinity and turbidity are compared to field data recorded during the LOIS survey programme. The organic and inorganic controls on estuarine pH have also been investigated by encoding a new template to predict the effect of changing salinity and temperature on the pH of estuarine water. The template has been coupled to the hydrodynamic model to predict pH variations in the estuary. The model results have shown conservative behaviour of the inorganic carbon system through out the majority of the estuary, but also identified an area of potentially high photosynthetic activity near the limit of saline intrusion during periods of low summer flows. Low (< 2 ^g 1"') concentrations of chlorophyll a in the water column and increases in pH correlated with tidal inundation of the river estuary banks imply that benthic photosynthetic process are important water chemistry in the upper estuary. The effects of salinity, turbidity and pH on Kd(Cd) and K<i(Zn) has been investigated using radiotracer incubation experiments and analytically determined measurements. The results show a reduction in IQ with increased salinity, but that the K<iS determined analytically are an order of magnitude higher than those measured when radiotracers are used. Analytically determined K<iS are reduced with increasing SPM concentration and increased at higher pH. Although no photosynthetically mediated control of radiotracer uptake was identified partitioning was significantly reduced (> 90 %) when a metabolic inhibitor was added. The Kd has been encoded as an exchange transfer and used to predict axial distributions and the flux of these metals from the riverine catchment to the sea. Model results indicate that the partition coefficients determined fi-om the radiotracer studies can not fully account for the analytically determined distribution between phases. It is hypothesised that colloids and Fe-Mn oxides precipitates play a significant role in trace metal transpon in low turbidity, high pH conditions. The results have implications for the measurement and modelling of chemical fluxes in low turbidity systems.

Laboratory studies show that turbulence controls the size of flocs by disrupting those flocs which exceed a critical diameter. Estuarine floc sizes have been shown to vary with the spring/neap cycle and turbulence has been suggested as the mechanism. A survey of the tidal variations of cohesive sediment floc size distributions and turbulence parameters has been undertaken in the Tamar estuary in south-west Britain. In-situ particle size distributions have been obtained using a 'marinised' version of the 'Malvern' laser diffraction sizing system. Turbulent current speeds were obtained using 10 cm diameter annular electromagnetic current meters. Velocity data is analysed using the inertial dissipation method to provide turbulent dissipation rates. Turbulence and size data, along with profiles of current, salinity, temperature and suspended solids concentration, record the passage of turbidity maximum and salt intrusion over four complete tidal cycles. Time series of observed particle size distributions vary smoothly over timescales of about one hour and these variations are linked to the flow conditions. Eight subsections of the tidal cycle were selected over which size distributions and flow conditions were slowly varying and the size distributions were time averaged over these subsections, and the resulting distributions compared. Size distributions in the turbidity maximum are strongly influenced by the mean current speed and this is found to be due to the different resuspension characteristics of newly formed aggregates and consolidated primary particles. Distributions are less dependent on tidal range at other stages during the tidal cycle. The size dependence of settling velocities strongly influences the size distribution of particles reaching the bed during the final stages of erosion of the salt intrusion, when the salt/fresh interface descends at a rate less than the settling velocity of large flocs but greater than that of small particles. This tends to sharpen the downstream edge of the turbidity maximum and preferentially retain floc aggregates in the upper reaches of the estuary.