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No description available.

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The prediction of grain entrainment rates in river flows is of great importance to an engineer, as the movement of sediment can cause permanent changes to the form and character of a river. One of the key elements in achieving accurate prediction of grain entrainment rates is the understanding of the near bed flow regime, as research has revealed the existence of a number of organised flow structures which are thought to control the magnitude of shear stress applied to the bed surface. However, most of the knowledge gained on these turbulent flow structures has been from experiments conducted over smooth walls often at low Reynolds numbers. Many geophysical flows differ greatly from this scenario, having flows with high Reynolds numbers and boundaries which are rough and deformable. A series of flume experiments have therefore been conducted to investigate whether the flow structures identified over smooth walls are also present over mixed grain sized sediment beds. The first experiment used a new flow measurement technique known as particle image velocimetry to record flow field measurements over a fixed bed. Both horizontal and vertical planes of the flow were investigated (at different times) in order to obtain an understanding of the three dimensional nature of the flow. A second set of experiments was conducted over the same fixed bed, but also involved the introduction of sediment grains into the flow upstream of the measurement area. The same techniques were used to examine the flow in this case, thus enabling significant differences between the circumstances to be identified. A third set of experiments was conducted over a natural sediment and in this case flow field measurements were taken at periods of high and low transport rates, a difference in transport rate being associated with bed armouring. A short time series of near instantaneous velocity field measurements were recorded for each experimental condition to aid the identification of the flow structures. Visual inspection of the time series revealed that sweeps and ejections occur at different scales, the smallest scaling with grain size or bed features and the largest scaling with flow depth.

A dissertation submitted to the Faculty of Science in fulfilment of the requirements for the degree of Master of Science November 2015, Johannesburg / South Africa is faced with water quantity and quality issues in most catchments. Intensification of coal-based industrial activity in the Waterberg, Limpopo and the concomitant emissions of oxides of nitrogen (NOX) and sulphur (SOX) pose potential ecological impacts to regional freshwater systems. Some research indicates that a significant proportion of minerals in the Waterberg parent rock may be prone to acid generation with catchment soils being potentially susceptible to acidification via NOX and SOX deposition. Cultural and recurrent nutrient loading of freshwater bodies also impacts on primary production and can ultimately alter the natural structure and functioning of these ecosystems. Trend analyses on historical hydrological data from 1982 to 2013 were carried out for several response water quality variables from six quaternary Waterberg catchments. Results were assessed for possible changes attributable to increased NOX and SOX loading post commencement of large-scale coal combustion. Historical inorganic N:P ratios were calculated in conjunction with a series of nutrient (N and P) bioassay experiments to predict which nutrient may be limiting growth of stream periphyton. Although trends were identified in most catchments for several of the water quality variables, the notion that the onset of large scale coal combustion has led to noticeable downward trends in pH and upward trends in either inorganic N or sulphate is not unequivocal. Patterns in trends were not distinct for catchments situated in close proximity to the primary emission source and those further away. Nor were there any distinct differences in trends between upwind and downwind catchments. Climate and geo-hydrological factors are likely to still function as the primary drivers of spatial and temporal variation in past and present catchment water quality. Contrary to the view that stream primary production is limited largely by the availability of P, predictions based on N:P ratios calculated in this study suggest N to be the limiting nutrient. This was shown to be the case in four of the five study-site rivers. N-limitation increased by 18% (67% to 85%) in the Matlabas River post-commencement of large-scale coal combustion. A greater increase of 24% (60% to 84%) was observed in the Middle Mokolo. Although cultural eutrophication levels in the Waterberg do not yet exceed management-set targets, the cumulative effect of industrial-derived nutrient inputs remains a threat to the nearshore marine ecosystem and human communities living downstream.

The occurrence of the emerging weed Glyceria maxima (Reed sweet-grass) in KwaZulu-Natal and Mpumalanga Province is possibly a threat to aquatic ecosystems. The aim of this study was to determine if G. maxima exhibits traits that indicate its potential for invasiveness. Additional aims included determining environmental factors that influence G. maxima establishment, preferred habitat and its impacts on plant species diversity in the invaded sites. The study was carried out at a dam at Mt. Shannon, Boston, and the Luhane River, Bulwer, both in the KwaZulu-Natal midlands. The littoral zone was divided into five elevational sampling zones in relation to water depth (i.e. from the terrestrial to aquatic habitat). Vegetation sampling was carried out twice in the 2008/09 and 2009/10 growing seasons. Water depth was found to be the major factor that determines the distribution patterns of G. maxima. Glyceria maxima’s preferred habitat was found to be semi-aquatic and aquatic habitats with a water depth of 130 cm. An unplanned fire event at Mt. Shannon Dam resulted in an increase in G. maxima vegetative recruitment. Glyceria maxima’s mode of spread within the study sites was found to be mainly vegetative reproduction as no seedlings were found. This was supported by a very low seed germination percentage (0.3-2%) in the laboratory germination tests although tetrazolium test showed that a considerable percentage of seeds was viable (25-89%; 53% av.). In the 2009/10 season G. maxima tiller density, percentage basal cover and height increased relative to 2008/09 which corresponded with a decline in the abundance of other species growing in the G. maxima stands. This shows that the habitat in which G. maxima is growing is ideal for its spread. Water depth, fire and vegetative reproduction are the main factors that influence G. maxima invasiveness in the littoral zone. Immediate attention should be given to the eradication of G. maxima while the number of the known infestations remains relatively small and fragmented, rather than waiting until the species is having a noticeable impact on riparian and wetland functioning. Glyceria maxima is likely to pose a threat to montane wetlands, which are generally regarded as biodiversity hotspots and water production regions. Keywords: Glyceria maxima, aquatic ecosystems, water depth, vegetative reproduction, seed germination and viability, Invasive.

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.

The benefits and costs of controlling rivers - building dams, controlling floods, extracting water - are constantly contested. Modifying rivers has brought great benefit to communities, fulfilling important community goals - supporting profitable commercial activities and providing a basis for vibrant communities. However modifying rivers has also had negative consequences - in particular, a decline in the quality and quantity of water. These impacts have undermined valued aspects of rivers (such as fish habitat) and have also caused decline in commercial activities (such as fishing and floodplain grazing). This thesis explores the ways that these contending perceptions of modification work out on the ground in rural communities. How are the benefits of modification recognised? How are the negative consequences of modification noticed and measured? Under what conditions are the benefits of modification reassessed? These are important questions in the current moment as our society reassesses the past modification of rivers and attempts to move towards more sustainable use of natural resources. This thesis explores this topic by undertaking in depth case-studies of two distinctive riverine environments: one coastal, the Clarence River in luscious coastal northern New South Wales; and one inland, the Balonne River, at the top of the Murray-Darling Basin, in semi-arid south-west Queensland. The case studies explore responses to modification of the rivers in two periods: the post-war decades - a time of widespread support for modification, and recent decades - a time of widespread recognition of the negative consequences of development. The thesis investigates perceptions of modification at three different scales: (i) groups within localities - the ways that modification is perceived by local groups with contrasting physical and conceptual interactions with the rivers (such as graziers, fishers, irrigators, Aboriginal people, ecologists and engineers); (ii) regional communities - which are constituted by groups with differing interests, and (iii) governments - which have the role of managing the long-term health of the economy and the environment, despite the long-term goals often being contested. This thesis provides insights into the ways that our complex society grapples with the possibility, and effects, of modifying the natural environment. This thesis suggests that local conditions - the actual local physical environment and local social conditions - shape the ways that modification of rivers is supported, challenged and reassessed. However, both local social conditions and the environment are constantly changing, often in surprising ways. Therefore outcomes are always an interaction between different levels of interest groups and the environment itself.