In 1993 concerns were expressed by English Nature that Devon's three largest sand dune systems Braunton Burrows, Northam Burrows and Dawlish Warren were drying out to the detriment of the dune habitat flora and fauna. Research was therefore required to understand how these systems functioned hydrologically, to determine whether they were drying out and if so to recommend sustainable management options aimed at reinstating former water levels, or preventing any further lowering of the water tables. At Braunton water table elevations have been monitored on a monthly basis by the Nature Conservancy, the Nature Conservancy Council and English Nature since 1972. These data were invaluable in describing the spatial and temporal hydrological characteristics and functioning of the groundwater system. Braunton Burrows was the main study site of the research. At both Northam and Dawlish, at the start of the fieldwork programme a dipwell monitoring network was installed and water table elevations were recorded weekly. Hydrological characteristics of each dune system were related to temporal variability in effective precipitation, the tide, the underlying geology and sediment properties. The groundwater system at Braunton was mounded, with effective inputs accumulating over an impermeable basal layer close to mean sea level. The system was very sensitive to seasonal variability in effective precipitation. At the centre of the groundwater mound, during the winter months, the elevation of the water table was 10 in above OD. The groundwater mound was asymmetric, with the highest water table elevations occurring along the eastern margin of the system. The transitional zone from dune sands to marshland, of a lower permeability, was restricting the inland lateral drainage regime and was controlling both the shape and elevation of the water table. At Northam the groundwater system was also mounded and again the shape and elevation of the water table were dependent upon effective precipitation. Unfortunately the monitoring network at Dawlish proved insufficient to describe either the shape or elevation of the groundwater table. Within the smaller systems of Northam and Dawlish variable sediment properties lead to intra-site variability in annual cyclical water table fluctuations. A prominent trend in the long-term water table data for Braunton Burrows was the general overall decline in the elevation of the water table from 1983 to mid 1992. With precipitation as the primary source of groundwater recharge, consecutive years with below average effective precipitation (1983-1992) was undoubtedly the primary cause, but was exacerbated by the drainage improvement works carried out on West Boundary Drain in 1983. Scrub growth, artificial drainage of the golf course and marine erosion were also possibly influencing the groundwater drainage regime. At Northam and Dawlish, without historical data it was not possible to determine if the systems were drying out, however factors influencing annual cyclical water table elevations were identified. Again climate was the key variable controlling the long-term elevation of the water table and undoubtedly the dry spell between 1983 and 1992 would have had repercussions on the elevation of the water table within these two systems. At Northam the drainage ditch network and reduced tidal inundation were the other main factors influencing groundwater levels. At Dawlish the golf course pump drainage system and scrub encroachment were effectively reducing annual groundwater recharge. At Braunton a numerical groundwater flow model was used as a predictive management tool, to assist in the recommendation of sustainable water level management options. A range of commercial groundwater flow models were reviewed and Visual MODFLOW, incorporating the original United States Geological Survey's MODFLOW code, with a fully integrated pre and post processor, was selected as the most suitable model for the Braunton scenario. The modelling exercise had three objectives; to test whether a commercial model such as Visual MODFLOW could be applied successfully to simulate the hydrology of Braunton Burrows; to gain further detail on the hydrological functioning of the system and ultimately if the model was calibrated to test a set of management scenarios to predict the hydro-ecological consequences of introducing new management practices into the system. Having identified the most probable factors influencing water table elevations within each dune system, sustainable hydrological management options were recommended with the aim of raising water levels, or preventing any further decline in water table elevations. The management options afforded nature conservation the highest priority, but also took into consideration the long-term requirements of all the other land user groups.A t Braunton when formulating the managemenrt ecommendationst he modelling predictions were also taken into consideration. Potential areas for future research were also identified. Water level monitoring should continue at all three sites, so that the longer-term impact of any water level management strategies implemented as a result of this research can be evaluated. Also at both Northam and Dawlish a more detailed analysis of the geology and sediment properties would be invaluable in providing a more comprehensive hydrological description of the functioning of the groundwater systems. The Braunton groundwater model could be developed further, addressing and overcoming problems encountered in this study and evaluating a wider range of water level management scenarios. As a result of this research far more is understood about the hydrological functioning of Devon's three largest dune systems and the recommendation of sustainable remedial/restorative water level management options will help to ensure that these ecologically diverse habitats are conserved for future generations. This research has also provided both the applied and theoretical framework to address water resource management problems within small and large scale dune systems around the shores of Great Britain.

During the second half of the twentieth century, the Antarctic Peninsula exhibited a rapid increase in air temperatures. This was accompanied by a reduction in sea ice extent, increased precipitation and a dramatic retreat of glaciers associated with an increase in heat flux from deep ocean water masses. Isotopic tracers have been used previously to investigate the relative importance of the different freshwater sources to the adjacent Bellingshausen Sea, but the data coverage is strongly biased toward summer and unambiguous determination of the different meteoric water contributions remains challenging. Here a high-resolution model is used to investigate the ocean's response to the observed changes in its different freshwater inputs (sea ice melt/freeze, precipitation/evaporation, iceberg melt, ice shelf melt and glacier melt). By developing the code to enable tracing of the sources and pathways of the individual components of the freshwater budget, it is shown that sea ice dominates the seasonal changes in freshwater content, but all sources contribute approximately equally to annual-mean freshwater fluxes and interannual freshwater flux anomalies. Ice shelf melting is shown to be the largest contributor to freshwater content on the annual mean. Decadal trends in the salinity and stratification of the ocean are investigated, and a 20-year surface freshening trend is found to be predominantly driven by decreasing autumn sea ice growth. By partitioning the freshwater in this way, insight is gained into the long-term freshwater balance and variability, and therefore the potential effects of a changing climate.

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).

Many urban rivers receive significant inputs of metal-contaminated sediments from their catchments. Their restoration has the potential to increase the deposition and accumulation of these sediments from greater sediment supply and increased channel hydraulic complexity, creating a store of metals which could have negative impacts upon ecosystems and human health. Macrophytes often establish in restored channels and have the potential to stabilise these sediments and uptake metals through processes of phytoremediation, thus reducing the risk of the accumulated sediments becoming a source of metals. This thesis investigates the effects of river restoration upon sedimentation patterns and the interactions between macrophytes and sediments in terms of sediment trapping, stabilisation and metal uptake within urban river systems. At a reach scale, greater finer sediment deposition and the accumulation of sediment around in-channel vegetation was found within restored stretches of tributaries of the River Thames London, reflecting sediment availability and hydraulic conditions. These sediments were important in terms of greater metal storage within stretches, and along with gravels showed particularly high metal concentrations. Interactions between macrophytes, sediment and flow were investigated within the urban-influenced River Blackwater, Surrey. At the stand scale, the common emergent Sparganium erectum was found to significantly reduce flow velocities, accumulate fine sediments and retain them over winter. Research on individual plants revealed that, although three common emergent macrophytes (Sparganium erectum, Typha latifolia and Phalaris arundinacea) did not significantly phytoremediate metal contaminated sediments through metal uptake or bioconcentration, the reinforcement and stabilisation of these accumulated sediments (particularly by Sparganium erectum and Typha latifolia) and the creation of anoxic sediment conditions which strongly bind metals, were important in reducing the risk of metal mobilisation from the sediments. These macrophyte sediment interactions illustrate the great potential of using emergent macrophytes in the restoration and management of urban rivers with metal contaminated sediments.

Natural radioelements and inert gases in solution in groundwaters have been applied to problems of groundwater flow and age measurement. The 4He content of groundwaters generally increases with age and in the Bunter Sandstone, Nottinghamshire, the 4He contents of groundwaters have been linearly related to 14C ages. In the Lincolnshire Limestone, the He contents of the groundwaters have been used to indicate mixing of recharge water and interstitial water. In the other study areas, 4He and 40Ar in groundwaters have been used as qualitative indicators of age. The 4He contents of core samples have been related to the 4 He contents of the interstitial water and the formation depth. 4He diffusion in confined and non--confined sedimentary structures has been discussed. The amounts of non-radiogenic inert gases dissolved in groundwaters have been used to estimate groundwater recharge temperatures. In the Bunter Sandstone, these have been related to palaeoclimatic history by calibrating with the 14C ages. Estimated recharge temperatures have also been related to seasonal recharge, changes in the altitude of recharge and to variations in the hydrogen and oxygen isotopic ratios. Variation of the 222Rn contents of groundwaters has been used as an indicator of aquifer variability. The relative importance of intergranular and fissure flow and the variation in efficiency of 222Rn release into groundwaters has been investigated. The fraction of 222Rn released from rocks has been determined and the mechanisms by which 222Rn is released from sandstone, limestone and granite rock fragments has been discussed. Variability of the 226Ra contents of groundwaters has been explained in terms of the relative importance of the recoil and etch mechanisms of solution and the solubility of 226Ra salts in groundwaters.

The mechanism of Rn solution in groundwaters has been studied in both laboratory simulations and in field situations. The effect of sporadic and seasonal changes in rainfall patterns on the Rn contents of perennial springs in the Mendip Hills has been investigated. The separate contributions of surface streams, soil zone residence and percolation within the rock formation have been identified. The relative importance of fissure or conduit and percolation flow in the aquifer have been shown to determine the nature of the response of Rn content to rainfall patterns. An examination of the Rn contents of air in limestone caverns has established that intergranular diffusion of Rn from below the rock surface is the primary reason for Rn release into the air space. Rn transport and release from stream inflows is relatively unimportant. Experimental determinations of the Rn diffusion coefficient in rock sections have shown that such intergranular diffusion is much more significant than intragranular diffusion. Laboratory studies of Rn release from fragmented rock samples have been used to determine the efficiency of radon release for different rock types. These studies have also confirmed that intergranular diffusion is an important process by which groundwaters acquire high Rn contents. The Ra content of geothermal groundwaters from Iceland have been determined. The Ra and U contents of calcite deposits from various depths within these geothermal systems are discussed in relation to changes in the Ra geochemistry. Rn contents have also been determined for groundwaters from the Berkshire Chalk and are shown to be dependent upon the extent to which porewater mixing has occurred.

An understanding of the longstream distribution of hydraulic processes is important for evaluating the mechanisms which control the catchment-scale variation of channel stability, sediment transport and siltation, flood generation and aquatic habitats. However, little attention has been given to quantifying the detailed spatial and temporal variability of channel hydraulic parameters, despite considerable attention to reach-scale processes. This study uses a combined field and modelling approach to define the distributions of channel hydraulic parameters between the source and the near-tidal limit of the River Severn, UK. Field measurements were made at 25 logarithmically-spaced sites along the Severn, under 3 flow conditions (low, medium and high) defined by exceedence frequencies. A flow event, occurring between 15-24 February 1989, was simulated by the 1-D hydraulic model, MIKE11. The simulated reach consists of cross sections spaced at 1 km intervals between 4 km and 254 km downstream from the source. Channel hydraulic parameters showed considerable variability in both space and time, reflecting cross section geometry variation downstream. Mean velocity increased with distance downstream from 0.23 m s\(^{-1}\) to 1.72 m s\(^{-1}\) under steady, bankfull flow conditions. However, unsteady flows simulated by MIKE11 demonstrated a longitudinal decline in the mean velocity of the wave peak associated with the rapid movement and minimal attenuation of the flood wave through the unconfined upper Severn. Flow resistance (Manning's n and Darcy-Weisbach f) decreased downstream from the source (0.32 - 0.06), although under low flow conditions it increased from 0.3 to 1.1 downstream to the non-alluvial - alluvial transition at Llanidloes; thereafter it exhibited a steady downstream decline. Reach mean shear stress and unit stream power peaked near the source (5-10 km downstream; drainage area < 50 km\(^2\)) at 120 N m\(^{-2}\) and 290 W m-2 and further downstream at the Ironbridge Gorge (170 km) (38 N m\(^{-2}\); 40 W m\(^{-2}\)) in response to the lithological controls on channel slope and valley width.

This thesis assesses long-term runoff projections from global multi-model ensembles used in hydrological impact studies. Firstly, the study investigates global-scale changes in frequency of high and low flow days towards the end of the current century, quantifying the relative contribution to uncertainty from global climate (GCMs) and global impact models (GIMs). Results show increases in high flows for northern latitudes and in low flows for several hotspots worldwide. Overall, GCMs provide the largest uncertainty; but GIMs are the greatest source of uncertainty in snow-dominated regions. Secondly, the ability of a set of GIMs to reproduce observed runoff is evaluated at the regional scale, indicating that GIMs capture well trends in low, medium, and high flows, but differ from observations with respect to medium and high flows timing. Thirdly, the contribution to uncertainty from GCMs, GIMs, Representative Concentration Pathways (RCPs), and internal variability is quantified for transient runoff until 2099. Over the USA, GCMs and GIMs are responsible for the largest uncertainty. Efforts to improve runoff projections should thus focus on GCMs and GIMs. In particular, GIMs should be evaluated in the region of study, so that models reproducing unrealistic runoff can be excluded, potentially yielding greater confidence in ensemble projections.

Although the Arctic is one of the most vulnerable regions to climate change, links between hydrology and ecology in high-latitude northern river basins are not well understood. Interdisciplinary research over three summer melt seasons (2010 to 2012) in the Kongsfjorden area of northwest Svalbard identified process connections between conceptual water sources, physicochemical habitat, and ecological structure and functioning in river basins. Water source dynamics determined from hydrochemical and isotopic data indicated differences in meltwater and groundwater contributions to river flow which varied both spatially and temporally at seasonal and year-to-year timescales. Non-glacier-fed rivers were characterised by less variable flow regimes, warmer water temperature, lower suspended sediment concentration and more stable channel morphologies. Several physicochemical habitat variables, notably water temperature and channel stability, were related significantly to rates of nutrient uptake and macroinvertebrate community structure. These data suggest that a future shift towards groundwater-dominated flow regimes may increase biotic diversity and rates of nutrient cycling in some high-latitude rivers. Key research findings are synthesised in conceptual models and provide a framework to understand the hydroecological response of these Arctic river systems to climate change.

Meandering rivers create some of the most intricate and diverse regions on the planet. Lateral migration excavates floodplain material from the outer banks of meanders and subsequently deposits it at the hydrodynamically calmer inner bank region, creating point bars. Point bars are constructed by the progressive attachment of sediment in the form of scroll bar deposits: sediment shoals up and becomes deposited on the margins of the bar, increasing its aerial extent, which becomes stabilised by vegetation once regular inundation ceases. Point bars have the potential to affect flow as it is routed through the meander by both increasing the curvature of the bend and topographically forcing water to flow outwards, increasing boundary shear stress and rates of bank erosion. A combination of remotely-sensed data, GIS, and a 2D morphodynamic flow model was used to examine the role of externally-imposed sediment supply on point bar growth and floodplain evolution in the Amazon Basin. Externally-derived sediment supply is important since it provides the material used to build point bars – of course supplemented by internal sediment sources. A simulated reach on the Sacramento River, USA was used to investigate the role of increased sediment supply on bar growth and meander evolution through time. It is demonstrated that rivers characterised by high sediment loads have greter migration rates, rates of cutoff production, and larger populations of oxbow lakes driven by the maintenance of a steady-state sinuosity on the rivers through time. Channel sinuosity increases with migration rate, although the rate of sinuosity increase is determined by the type of meander deformation: downstream translating bends increase their length more quickly than their upstream translating or extensional counterparts. Point bar growth was observed to occur under high sediment loading conditions when modelled using a 2D morphodynamic model. The bar sequestered sediment at the upstream head of the bar causing it to grow ii upstream. This increased the distance of outer bank subjected to bank erosion and also increased to magnitude of bank erosion. a sediment-driven control on sinuosity increases manifested through bend deformation style, and the simulated growth of point bars in the presence of enhanced sediment loading which resulted in increased rates of bank erosion. These results are of significance for meandering theory and particularly indicate the importance of point bars in effecting the morphodynamic evolution of meandering rivers.