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.

A erosão do solo tem sido extensamente estudada dentro do ponto de vista agrícola, pois está relacionada a sérias perdas em termos de produtividade das mais variadas culturas e conseqüentemente, é causa de prejuízos econômicos. Recentemente tem-se dado atenção especial aos problemas ambientais relacionados à erosão, transporte e deposição de sedimentos, já que são, provavelmente, os mais significativos de todos os poluentes em termos de sua concentração na água e dos seus impactos e efeitos no transporte de substâncias químicas, entre elas o nitrogênio aplicado na forma de fertilizantes. O foco central do presente trabalho foi a investigação dos padrões de transporte de sedimentos por erosão hídrica e suas influências na transferência de nitrogênio aos cursos d\'água. A fonte de nitrogênio avaliada foi o sulfato de amônio aplicado superficialmente como fertilizante em um latossolo vermelho amarelo, em diferentes preparos do solo, sem cobertura vegetal e sob chuva simulada em diferentes intensidades. Os resultados permitiram identificar a fração argilosa dos sedimentos erodidos transportados em suspensão na água da enxurrada como a principal via de transporte/transferência de nitrogênio proveniente do fertilizante. Este processo de transferência deu-se mais intensamente nas primeiras chuvas. Constatou-se que os valores de perdas de solo e água não influenciam a capacidade de transporte de nitrogênio entre a área de cultivo e os cursos d\'água. O preparo do solo influenciou as concentrações de nitrogênio transportadas pela enxurrada. As concentrações de nitrogênio nos sedimentos são altas mesmo com baixo volume e intensidade de chuva. / Soil erosion has been extensively studied from the agricultural point of view, because it is related with serious damages in terms of a huge variety of plantations productivity and its consequent economic detriments. Currently, special attention is given to the environmental problems caused by erosion processes and the transport and deposition of the sediments, since they are, probably, the most significant pollutants in terms of its concentration in the water and its impacts and effects in the chemical substances transport, such as nitrogen which is applied in the form of fertilizer. The central focus of the present work was the research of the standards sediment transport origined by the water erosion and their influences in the transference of nitrogen to the water bodies. The evaluated nitrogen source was the ammonium sulphate applied superficially as a fertilizer in a latossolo vermelho amarelo soil, under different soil tillage systems, without vegetal covering and under different intensities of simulated rain. The results allowed to identified that the argillaceous fraction of the eroded sediments carried in the runoff suspension was the main way of the nitrogen transport/transference derived from the fertilizer. This transference process was more intensely observed in the first rains. It was evidenced too, that the losses values of soil and water did not influence the nitrogen transport capacity between the planted areas and the water bodies. Aditionally, soil tillage influenced in nitrogen concentrations carried by the runoff. The nitrogen concentrations in the sediments were high, in spite of the low volumes and low intensities of the rain.

A erosão do solo tem sido extensamente estudada dentro do ponto de vista agrícola, pois está relacionada a sérias perdas em termos de produtividade das mais variadas culturas e conseqüentemente, é causa de prejuízos econômicos. Recentemente tem-se dado atenção especial aos problemas ambientais relacionados à erosão, transporte e deposição de sedimentos, já que são, provavelmente, os mais significativos de todos os poluentes em termos de sua concentração na água e dos seus impactos e efeitos no transporte de substâncias químicas, entre elas o nitrogênio aplicado na forma de fertilizantes. O foco central do presente trabalho foi a investigação dos padrões de transporte de sedimentos por erosão hídrica e suas influências na transferência de nitrogênio aos cursos d\'água. A fonte de nitrogênio avaliada foi o sulfato de amônio aplicado superficialmente como fertilizante em um latossolo vermelho amarelo, em diferentes preparos do solo, sem cobertura vegetal e sob chuva simulada em diferentes intensidades. Os resultados permitiram identificar a fração argilosa dos sedimentos erodidos transportados em suspensão na água da enxurrada como a principal via de transporte/transferência de nitrogênio proveniente do fertilizante. Este processo de transferência deu-se mais intensamente nas primeiras chuvas. Constatou-se que os valores de perdas de solo e água não influenciam a capacidade de transporte de nitrogênio entre a área de cultivo e os cursos d\'água. O preparo do solo influenciou as concentrações de nitrogênio transportadas pela enxurrada. As concentrações de nitrogênio nos sedimentos são altas mesmo com baixo volume e intensidade de chuva. / Soil erosion has been extensively studied from the agricultural point of view, because it is related with serious damages in terms of a huge variety of plantations productivity and its consequent economic detriments. Currently, special attention is given to the environmental problems caused by erosion processes and the transport and deposition of the sediments, since they are, probably, the most significant pollutants in terms of its concentration in the water and its impacts and effects in the chemical substances transport, such as nitrogen which is applied in the form of fertilizer. The central focus of the present work was the research of the standards sediment transport origined by the water erosion and their influences in the transference of nitrogen to the water bodies. The evaluated nitrogen source was the ammonium sulphate applied superficially as a fertilizer in a latossolo vermelho amarelo soil, under different soil tillage systems, without vegetal covering and under different intensities of simulated rain. The results allowed to identified that the argillaceous fraction of the eroded sediments carried in the runoff suspension was the main way of the nitrogen transport/transference derived from the fertilizer. This transference process was more intensely observed in the first rains. It was evidenced too, that the losses values of soil and water did not influence the nitrogen transport capacity between the planted areas and the water bodies. Aditionally, soil tillage influenced in nitrogen concentrations carried by the runoff. The nitrogen concentrations in the sediments were high, in spite of the low volumes and low intensities of the rain.

The demand of electric power has rapidly increased in China along with the financial development. High pollution rates have pushed the development towards more renewable energy generating alternatives. In recent years larger investments of hydropower- generated electricity have been to satisfy the increased demand. Hydropower can cause non-energy related problems when implemented in a large scale. In this project the Nanjing Reach of Yangtze River was investigated through a hydrodynamic and sediment transport perspective. Nanjing Reach consists of two river branches and the upstream construction of the Three Gorges Dam has caused an imbalance of sediment composition that leads to an increasing difference of the diversion ratio between the branches. This is causing a problematic navigational situation for larger vessels along the north branch. In this project non-submerged water diverting dykes were modelled using two-dimensional depth averaged finite element method to estimate the impact on the hydrodynamic and sediment transport conditions. The examination showed that engineering measures at the downstream part of the reach were more effective than structures at the front, when taking both hydrodynamic and the more long-term impacts from sediment transport into account. Although the results of the sediment transportation are uncertain, the long- term consequences with investigated engineering measures can be concluded to have negative impact on the sustainability of the reach, however short-term improvements can be achieved. Other factors such as stability, material use, economics and marine conditions were not treated in this project but are vital for a reliable solution.

The erosion and transport of fertile topsoil is a serious problem in the U.S., Australia, China and throughout Europe. It results in extensive environmental damage, reduces soil fertility and productivity, and causes significant environmental loss. It is as big a threat to the future sustainability of global populations as climate change, but receives far less attention. With both chemicals (fertilizers, pesticides, herbicides) and biological pathogens (bacteria, viruses) preferentially sorbing to silt and clay sized soil particles, estimating contaminant fluxes in eroded soil also requires predicting the transported soils particle size distribution. The Hairsine-Rose (HR) erosion model is considered in this thesis as it is one of the very few that is specifically designed to incorporate the effect of particle size distribution, and differentiates between non-cohesive previously eroded soil compared with cohesive un-eroded soil. This thesis develops a new extended erosion model that couples the HR approach with the one-dimensional St Venant equations, and an Exner bed evolution equation to allow for feedback effects from changes in the local bed slope on surface hydraulics and erosion rates to be included. The resulting system of 2I +3 (where I = number of particle size classes) nonlinear hyperbolic partial differential equations is then solved numerically using a Liska-Wendroff predictor corrector finite difference scheme. Approximate analytical solutions and series expansions are derived to overcome singularities in the numerical solutions arising from either boundary or initial conditions corresponding to a zero flow depth. Three separate practical applications of the extended HR model are then considered in this thesis, (i) flow through vegetative buffer strips, (ii) modelling discharge hysteresis loops and (iii) the growth of antidunes, transportational cyclic steps and travelling wave solutions. It is shown by comparison against published experimental flume data that predictions from the extended model are able to closely match measurements of deposited sediment distribution both upstream and within the vegetative buffer strip. The experiments were conducted with supercritical inflow to the flume which due to the increased drag from the vegetative strip, resulted in a hydraulic jump just upstream of the vegetation. As suspended sediment deposited at the jump, this resulted in the jump slowly migrating upstream. The numerical solutions were also able to predict the position and hydraulic jump and the flow depth throughout the flume, including within the vegetative strip, very well. In the second application, it is found that the extended HR model is the first one that can produce all known types of measured hysteresis loops in sediment discharge outlet data. Five main loop types occur (a) clockwise, (b) counter-clockwise, (c,d) figure 8 of both flow orientations and (e) single curve. It is clearly shown that complicated temporal rainfall patterns or bed geometry are not required to developed complicated hysteresis loops, but it is the spatial distribution of previously eroded sediment that remains for the start of a new erosion event, which primarily governs the form of the hysteresis loop. The role of the evolution of the sediment distribution in the deposited layer therefore controls loop shape and behavior. Erosion models that are based solely on suspended sediment are therefore unable to reproduce these hysteretic loops without a priori imposing a hysteretic relationship on the parameterisations of the erosion source terms. The rather surprising result that the loop shape is also dominated by the suspended concentration of the smallest particle size is shown and discussed. In the third application, a linear stability analysis shows that instabilities, antidunes, will grow and propagate upstream under supercritical flow conditions. Numerical simulations are carried out that confirm the stability analysis and show the development and movement of antidunes. For various initial parameter configurations a series of travelling antidunes, or transportational cyclic steps, separated by hydraulic jumps are shown to develop and evolve to a steady form and wave speed. Two different forms arise whereby (a) the deposited layer completely shields the underlying original cohesive soil so that the cohesive layer plays no role in the speed or shape of the wave profile or (b) the cohesive soil is exposed along the back of the wave such that both the non-cohesive and cohesive layers affect the wave profile. Under (a) the solutions are obtained up to an additive constant as the actual location of the boundary of the cohesive soil is not required, whereas for (b) this constant must be determined in order to find the location on the antidune from where the cohesive soil becomes accessible. For single size class soils the leading order travelling wave equations are fairly straightforward to obtain for both cases (a) and (b). However for multi-size class soils, this becomes much more demanding as up to 2I + 3 parameters must be found iteratively to define the solution as each size class has its own wave profile in suspension and in the antidune.

Barrier island restoration and nourishment is necessary for sustaining coastal systems worldwide. In the Mississippi River Delta Plain, the lack of sediment supply, relative sea level rise, and reworking of abandoned delta lobes promote rapid disintegration of barriers, which can contribute to mainland storm impacts. Barrier island restorations that utilize higher quality sediments (Outer Continental Shelf- OCS) are expected to exhibit higher resiliency, withstanding coastal erosion, event-induced erosion, and ongoing transgression when compared to barriers nourished using lower quality nearshore (NS) sands. Additionally, use of OCS sediments increases sediment supply by adding material to the system supporting increased barrier longevity by maintaining a subaerial footprint longer compared to NS sediments. We used the Delft3D modeling suite to study barrier geomorphic trajectories nourished using OCS/NS sands, compared with control simulations with no nourishment. Resulting morphologies from 18 simulations with forcing that included annualized forcing, storms, and SLR are evaluated and compared.