Dissertations / Theses on the topic 'Aeolian transport'

Particle shape is a parameter which has been largely neglected in the study of sediment transport by wind. Many methods of measuring shape have been published. Those which characterize shape in pebble and sand sized sediments are reviewed here. In order to test the influence of shape on aspects of particle movement and on sediment transport rate, two very differently shaped populations were used, (a) a reasonably equant quartz dune sand and (b) a platy dune sand composed largely of shell fragments. Recommendations are made for reasonably fast and accurate methods of shape measurement, including Sneed and Folk's Maximum Projection Sphericity and Winklemolen's Rollability. The possible effects of shape on saltation were examined in terms of grain trajectories and the grain's interaction with the bed at collision using video and high speed photography. The latter enabled individual particles (from a coarse, medium or fine size fraction) to be followed as they impacted the bed, through to ricochet and the possible ejection of previously stationary bed grains. Experiments were conducted over horizontal and sloping beds, the latter representing different parts of the stoss face of a ripple. It was found that shape, in terms of the sphericity of the particles has a marked effect on collision. The near spherical quartz sand causes more dislodgements and more ejecta per collision than the much less spherical platy shell sand. The quartz sand is also more likely to approach and ricochet from the bed at higher angles to the horizontal than the shelly sand, and to rebound more vigorously. Thus, the bed activity generated by collision increases with an increase in particle sphericity. However, high speed photography of grain dislodgement by wind action alone indicates that, as sphericity decreases, there is a greater probability that a grain will be entrained aerodynamically. Shape also influences trajectories. Video films show that grain paths become longer and flatter as sphericity decreases. These observations indicate that the transport rate for grains with a low sphericity will be greater than those with a high sphericity, both in terms of aerodynamic entrainment and of longer trajectories. However, once collision becomes important in the dislodgement of surface grains, the greater bed activity seen with more spherical particles means that their transport rate will increase. These findings are supported by the observations of Williams (1964) and of Willetts, Rice and Swaine (1982), that sediment transport is promoted at low windspeeds by less spherical grains, while the opposite occurs at high windspeeds.

The aim of this study is to develop and test a physical model of wind blown sand transport. Once established, such a model will lead to valuable insight into the physics of sand transport by wind especially the processes that interact to produce equilibrium transport conditions. The study begins with a review of the physics of wind blown sand, beginning with Bagnold (1941). In particular, four sub-processes are discussed; aerodynamic entrainment, the grain trajectory, the grain/bed collision and the modification of the wind by the grains. The physical model is based on the coupling (or interaction) of the four sub-processes. The grain/bed collision is modelled using experimental data obtained by Willetts and Rice (1985). The wind modification is calculated from the force profile exerted by the grains and the differential fluid shear stresses induced by the grains; a mixing length model is used to calculate these stresses. The results from the model are compared with the observed features of wind flow sand transport and the agreement is encouraging. Realistic wind profiles are calculated. These profiles show a marked deceleration by the grain cloud and an increase in effective roughness due to the additional drag imposed on the wind by the grains. Moreover the horizontal mass flux profile decays exponentially from the surface in accord with experimental measurements and the sand transport rate has a roughly cubic dependence on the shear velocity. Thus, the success of the model in reproducing (spontaneously) many of the observed features of wind blown sand transport encourages confidence that the physics used to construct the model is broadly correct. A further important result emerges from the model. There appears to be two time scales associated with equilibrium saltation. Firstly, the time for the grain cloud to come into equilibrium with the surface wind; this occurs over a time of approximately 1 s. Secondly, there is an increase in the effective roughness of the surface due to the additional drag imposed on the wind by the grain cloud. The atmospheric boundary layer must come into equilibrium with this change in roughness. This second equilibrium takes place over a much longer time scale of several tens of seconds or more. It results in a gradual decay of the shear stress in time after an overshoot of the steady state. It is noted that the response in time of the boundary layer to a change in roughness is analogous to its response in distance found by Jensen (1978). It is suggested, in the concluding chapter, that the spatial and temporal variation of the saltation cloud may be related through the application of Taylor's hypothesis for turbulence. The saltation modified wind is studied with the aid of an analytical wind profile derived from an assumed fluid shear stress distribution. This distribution is chosen for its similarity to the model calculated distribution: the intension being to use the analytical wind profile as a tool to investigate the model generated wind profile. From this analytical wind profile it is shown that the 'kink' in the wind profile (first noted by Bagnold (1941)) is caused by a maximum in the force profile exerted on the wind by the grains. Such a maximum is shown to exist in the force profile generated by the saltation model. Thus, it is concluded, that the 'kink' found in many experimentally measured wind profiles is likely to be caused by a maximum in the force profile exerted by the grains on the wind. This result is important because further understanding of modification of the wind will ensure that experimental measurements made are consistent with the physics of the system: in particular that wind velocity measurements used to calculate the shear velocity should be made above a height of 2-3 cm from the surface (i.e. above the kink). In the concluding section the desirability of a multiple grain size saltation model is discussed as an important step towards more realistic modelling. Further attention is directed towards modelling sand transport in gusty winds and inclusion of interaction with a developing bed.

Of numerous aeolian studies around the world, few have been dedicated to dust trapping in the Saharan regions and none is known in Libya. This research aims to explore the extent of dust activities in the western part of Libya, the main factors that influence dust entrainment and deposition, the likely regional emission sources transporting aerosol to this part of the country and ultimately to establish base line information in space and time based on a study area larger than the size of the UK. Dust trapping was carried out at thirty sites on a monthly basis starting from the beginning June 2000 until the end of May 2001. A physical characterization of 274 aeolian sediments were based on grain size distribution analysis and the mineral composition was based on XRD testing of 24 dust samples taken from extreme locations near the Mediterranean Sea and about 800 km inland. An assessment of dust emissions within the area of study, the vertical deposition rates in relation to The Total Ozone Mapping Spectrometer (TOMS) data have been investigated. Moreover, an attempt identify potential Saharan dust emission sources that have impacted the study area has also been made utilizing TOMS data. The area of study has been divided into three regions northern, central and southern according to the dominant factors controlling mobilization of dust particles on the bed surface. This study concludes that deposition rates in the northern coastal region are largely dominated by human activities. Rates in the central region were strongly affected by topographic irregularities whereas in the south deposition rates were less affected by topography and anthropogenic activity. However, deposition rates and particle size distributions are strongly controlled by wind regimes and correlate with the average atmospheric temperatures. Local sediments seem to be strongly affected by saltating particles in most of the study area and no evidence of long range aerosol emission from western Libya was found. Nevertheless, fine dust (<10 gm) is wide spread in the local atmosphere, however it is more pronounced during late spring and summer. TOMS data and the prevailing wind directions reveal that the depressions of the Libyan Desert and the Bodele Depression in Chad were the main sources of aerosol transported over the study area during the highest months of emission, transport and deposition, July and April. It is hoped that this baseline information can pave the way for future studies on dust impacts on soil fertility, human health, desertification, climate change and the validation of present day computer models.

This work is a study into the unsteady behaviour of aeolian sediment transport. A one-dimensional and a two-dimensional numerical model were developed in order to investigate the temporal behaviour of transport rate as well its spatial distribution. The numerical model of McEwan (1991) for steady state saltation served as a starting point in the development phase. Both models presented in this thesis are capable of simulating temporary varying winds. In addition, the two-dimensional model allows the relaxation of the assumption of streamwise homogeneous flow. The one-dimensional model was tested against results for steady state predicted by previous models. Further tests showed that the discretisation time step size Δt has an influence on the model's temporal behaviour. The reason for this is the better coupling of the wind-sand system when a smaller Δt is used. The implications of bed area choice on the statistical accuracy of predicted transport rate was demonstrated. In the one-dimensional case the grain cloud's total forward momentum equals transport rate, which is independent of model geometry. The initial over-shoot reported by previous investigators was found not to appear for simulation heights small than 50 to 60cm. This is due to the fast propagation of the grains' influence (momentum exchange) upward in the flow and the immediate deceleration of the wind. Confirmation of these findings comes from reports of experiments conducted in differently sized wind tunnels. Different types of wind velocity variations were investigated. The transport rate's response depends on the amplitude and frequency of the wind fluctuations. At frequencies higher than f ≈ 0.5Hz the transport rate was found not to respond to the wind changes.

The landfall of hurricane Claudette in 2003 damaged and eroded most Texas coastal counties. The residents of Pointe San Luis on the west end of Galveston Island, Texas lost their protective dune front and experienced significant shoreline erosion. Following the storm, the Pointe San Luis Property Owner's Association contacted Texas A&M University to design a dune restoration strategy. The greatest natural contributor to dune reconstruction is the available sand delivered by aeolian transport. During the course of the study it became apparent that no model or software existed capable of demonstrating the effectiveness of available dune restoration alternatives. Building Beach©, a coastal aeolian sand transport simulator, was developed in response to this need. Based on discrete dynamics and requiring a minimum of technical input, the software allows coastal property owners, consultants, and coastal developers to graphically model the effectiveness of several dune restoration options including sand fence, planted vegetation, geo-textiles, and other solid protective barriers. The graphical output of Building Beach© enables the user to compare approximations of the performance of different restoration strategies to select the most effective option for a particular beach.

Geology
M.S.
Grasslands provide fundamental ecosystem services in many arid and semi-arid regions of the world, but are experiencing rapid increases in fire activity making them highly susceptible to post-fire accelerated soil erosion by wind. A quantitative assessment that integrates fire-wind erosion feedbacks is therefore needed to account for vegetation change, soil biogeochemical cycling, air quality, and landscape evolution. We investigated the applicability of a novel tracer technique – the use of multiple rare earth elements (REE) - to quantify aeolian soil erosion and to identify sources and sinks of wind-blown sediments in a burned and unburned shrub-grass transition zone in the Chihuahuan desert, NM, USA. Results indicate that the horizontal mass flux of wind-borne sediment increased approximately three times following the fire. The REE-tracer analysis of aeolian sediments shows that an average 88% of the horizontal mass flux in the control area was derived from bare microsites, whereas at the burned site it was derived from shrub and bare microsites, 42% and 39% respectively. The vegetated microsites, which were predominantly sinks of aeolian sediments in the unburned areas, became sediment sources following the fire. The burned areas exhibited a spatial homogenization of sediment tracers, highlighting a potential negative feedback on landscape heterogeneity induced by shrub encroachment into grasslands. Though fires are known to increase aeolian sediment transport, accompanying changes in the sources and sinks of wind-borne sediments likely influence biogeochemical cycling and land degradation dynamics. Our experiment demonstrated that REEs can be used as reliable tracers for field-scale aeolian studies.
Temple University--Theses

Aeolian sediment transport threshold is commonly defined as the minimum wind speed (or shear stress) necessary for wind-driven sediment transport. Threshold is a core parameter in most models of aeolian transport. Recent advances in methodology for field-based measurement of threshold show promise for improving parameterizations; however, investigators have varied in choice of method and sensor. The impacts of modifying measurement system configuration are unknown. To address this, two field tests were performed: (i) comparison of four piezoelectric sediment transport sensors, and (ii) comparison of four calculation methods. Data from both comparisons suggest that threshold measurements are non-negligibly modified by measurement system configuration and are incomparable. A poor understanding of natural sediment transport dynamics suggests that development of calibration methods could be difficult. Development of technical standards was explored to improve commensurability of measurements. Standards could assist future researchers with data syntheses and integration.
xi, 108 leaves : ill. ; 29 cm

LUNA, Marco César Monteiro de Morais. Simulações de campos de dunas sob a influência do crescimento de vegetação e da exposição do nível de água do terreno. 2010. 129 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010.
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In this work we study the genesis of coastal dune fields using the DUNE model developed by Sauermann et al. (2001) and Kroy et al. (2002), and later perfected by Schwämmle and Herrmann (2004). The model was developed to calculate the transport of grains by saltation and the formation of sand dunes, and then it takes account the growth of vegetation (Durán and Herrmann, 2006a). Initially we investigate the genesis and the early stages of development of coastal dune fields in the presence of vegetation growth. The model is applied to calculate the evolution of sand transported in the same direction of the wind into a land where there is vegetation growth. In previous work, the model reproduced the fixation of sand grains and the stabilization of dunes. Consequently, the barchans dunes in the presence of plants can be transformed into parabolic dunes (Durán et al., 2006a). Here, we seek the understanding of competition between the sand transport and vegetation growth in the development of transverse dunes under the influence of plant growth, when we find a maximum height for transverse dunes under the influence of vegetation. Then we conduct a detailed study of the development of dune fields taking into account changes in the relevant parameters: the width of the beach, availability of sand sediment, wind intensity, the vegetation growth rate and the maximum height reached by the plants. Different morphologies are obtained for the coastal dune fields which depend crucially on the characteristic vegetation growth rate and the wind shear velocity, which can reproduce patterns found in actual fields. After we investigate, the genesis and development of coastal dune fields in the presence of a dynamic water level. In other to do it we complement this model with a dynamic water surface. Here we try to obtain and understand the influence of a exposed water surface over the ground in the development and morphology of the dunes. In this way, we also try a quantitative understanding of the dynamics of coastal dune fields as a function of local climatic conditions, since the dynamics of the water level is linked to rainfall (Jimenez et al., 1999, Levin et al., 2009). The results show that for a constant wind shear velocity u∗ the main effect of the presence of water is to produce the dunes in the field more regular in their shape and size. As the shift Aw in the water level becomes larger, the dunes undergo a transition from barchans dunes to chains of barchans dunes, and then to transverse dunes, reducing the maximum height reached by the dunes in the field. The water spreads the sand across the field and reduces the existence of areas which are free of the sand presence, preventing the formation of dunes and leading to the formation of a sandy bed with a smooth surface. When we increase the time which the water level tew is above the ground, the dunes in the field pattern are more regular in shape and height in the same way as when we increase the shift in water level. For a unidirectional wind sinusoidally oscillating out of phase with respect to fluctuation of water level, in opposition to the constant wind, as Aw becomes larger, higher are the dunes in the field, and they undergo a transition from chains of barchans dunes to isolated barchans dunes. In this case, the sand is retained in the dunes and we do not observe the formation of a sand bed as Aw grows. In the case of a sinusoidal wind in phase with the oscillation of the water level, the dunes appear only for small shifts in the water level. These results agree with the observations of real sand dune fields (Kocurek et al., 1992), reproducing the constructive and destructive phases which alternate in dune fields under the influence of an oscillating water level due to seasonal climatic changes. In the simulations we reproduce some morphological patterns of sand dunes that are found in the Lençóis Maranhenses sand dune field. Comparison between simulation results and satellite images of this region is quite satisfactory from a qualitative point of view and from a quantitative point of view.
Neste trabalho estudamos a gênese de campos de dunas costeiras através do modelo DUNE desenvolvido por Sauermann et al. (2001) e Kroy et al. (2002), e posteriormente aprimorado por Schwämmle e Herrmann (2004). O modelo DUNE foi desenvolvido para calcular o transporte de grãos por saltação e para a formação de dunas de areia. Depois, passou a levar em consideração também o crescimento da vegetação (Durán e Herrmann, 2006a). Inicialmente investigamos a gênese e os primeiros estágios de desenvolvimento de campos costeiros de dunas na presença do crescimento da vegetação. O modelo é aplicado para calcular a evolução da areia transportada na mesma direção do vento para dentro de um terreno onde ocorre o crescimento da vegetação. Antes, o modelo reproduziu a fixação da areia e consequente estabilização de dunas barcanas na presença das plantas, resultando na formação de dunas parabólicas (Durán et al., 2006a). Aqui, buscamos o entendimento dessa competição entre o transporte de areia e o crescimento da vegetação no desenvolvimento de dunas transversais na presença do crescimento de plantas, quando encontramos uma altura máxima para a duna transversal sob a influência da vegetação. Em seguida realizamos um estudo detalhado do desenvolvimento de campos de dunas levando em consideração variações nos parâmetros relevantes: largura da praia, disponibilidade de sedimentos, intensidade do vento, a velocidade de crescimento da vegetação e a altura máxima atingida pelas plantas. Diferentes morfologias foram obtidas para os campos de duna costeiras que dependem fundamentalmente da taxa de crescimento característica da vegetação e da velocidade de cisalhamento do vento, e que conseguem reproduzir padrões encontrados em campos reais. Depois investigamos no trabalho, a gênese e o desenvolvimento de campos de dunas costeiros na presença de um nível de água dinâmico. Para isso complementamos o modelo com uma superfície de água dinâmica. Buscamos aqui, obter e compreender a influência de uma superfície de água exposta no terreno no desenvolvimento e na morfologia das dunas. De maneira que, buscamos também um entendimento quantitativo da dinâmica de campos de dunas costeiros como função das condições climáticas locais, já que a dinâmica do nível de água está ligada ao regime de chuvas (Jimenez et al., 1999; Levin et al., 2009). Os resultados mostram que, para ventos com velocidade de cisalhamento u∗ constante, o principal efeito da presença da água é tornar as dunas no campo mais regulares em sua forma e tamanho. À medida que o deslocamento no nível de água Aw se torna maior, as dunas sofrem uma transição, passando de barcanas para cadeias de dunas barcanóides e depois para dunas transversais, reduzindo as alturas máximas atingidas pelas dunas. A água espalha a areia por todo campo e diminui a existência de áreas livres da presença de areia, impedindo a formação de dunas e levando à formação de um leito arenoso de relevo suave. Quando fixamos a altura máxima atingida pelo nível de água e aumentamos o seu tempo de exposição tew acima do chão, as dunas no campo apresentam padrão mais regular na forma e na altura da mesma maneira que acontece quando cresce o deslocamento no nível de água. Para um vento unidirecional oscilando senoidalmente fora de fase em relação a oscilação do nível de água, de maneira contrário ao vento constante, à medida que Aw se torna maior, maiores são as dunas no campo, e as dunas passam de cadeias de dunas barcanóides para dunas barcanas isoladas. Nesse caso, a areia fica retida nas dunas e não temos a formação de um leito de areia à medida que Aw cresce. Já no caso de um vento senoidal em fase com a oscilação do nível de água as dunas somente surgem para pequenas variações no nível de água. Esses resultados concordam com observações realizadas em campos reais por Kocurek et al. (1992), reproduzindo as fase destrutivas e construtivas que se alternam em campos de dunas sob a influência de um nível de água oscilante, devido às mudanças climáticas sazonais. Conseguimos com o modelo reproduzir alguns padrões de morfológicos de dunas que são encontrados nos Lençóis Maranhenses. A comparação dos resultados obtidos nas simulações com as imagens de satélite da região é bastante satisfatória, tanto do ponto de vista qualitativo, quanto do ponto de vista quantitativo.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
Neste trabalho estudamos a gÃnese de campos de dunas costeiras atravÃs do modelo DUNE desenvolvido por Sauermann et al. (2001) e Kroy et al. (2002), e posteriormente aprimorado por SchwÃmmle e Herrmann (2004). O modelo DUNE foi desenvolvido para calcular o transporte de grÃos por saltaÃÃo e para a formaÃÃo de dunas de areia. Depois, passou a levar em consideraÃÃo tambÃm o crescimento da vegetaÃÃo (DurÃn e Herrmann, 2006a). Inicialmente investigamos a gÃnese e os primeiros estÃgios de desenvolvimento de campos costeiros de dunas na presenÃa do crescimento da vegetaÃÃo. O modelo à aplicado para calcular a evoluÃÃo da areia transportada na mesma direÃÃo do vento para dentro de um terreno onde ocorre o crescimento da vegetaÃÃo. Antes, o modelo reproduziu a fixaÃÃo da areia e consequente estabilizaÃÃo de dunas barcanas na presenÃa das plantas, resultando na formaÃÃo de dunas parabÃlicas (DurÃn et al., 2006a). Aqui, buscamos o entendimento dessa competiÃÃo entre o transporte de areia e o crescimento da vegetaÃÃo no desenvolvimento de dunas transversais na presenÃa do crescimento de plantas, quando encontramos uma altura mÃxima para a duna transversal sob a influÃncia da vegetaÃÃo. Em seguida realizamos um estudo detalhado do desenvolvimento de campos de dunas levando em consideraÃÃo variaÃÃes nos parÃmetros relevantes: largura da praia, disponibilidade de sedimentos, intensidade do vento, a velocidade de crescimento da vegetaÃÃo e a altura mÃxima atingida pelas plantas. Diferentes morfologias foram obtidas para os campos de duna costeiras que dependem fundamentalmente da taxa de crescimento caracterÃstica da vegetaÃÃo e da velocidade de cisalhamento do vento, e que conseguem reproduzir padrÃes encontrados em campos reais. Depois investigamos no trabalho, a gÃnese e o desenvolvimento de campos de dunas costeiros na presenÃa de um nÃvel de Ãgua dinÃmico. Para isso complementamos o modelo com uma superfÃcie de Ãgua dinÃmica. Buscamos aqui, obter e compreender a influÃncia de uma superfÃcie de Ãgua exposta no terreno no desenvolvimento e na morfologia das dunas. De maneira que, buscamos tambÃm um entendimento quantitativo da dinÃmica de campos de dunas costeiros como funÃÃo das condiÃÃes climÃticas locais, jà que a dinÃmica do nÃvel de Ãgua està ligada ao regime de chuvas (Jimenez et al., 1999; Levin et al., 2009). Os resultados mostram que, para ventos com velocidade de cisalhamento u∗ constante, o principal efeito da presenÃa da Ãgua à tornar as dunas no campo mais regulares em sua forma e tamanho. à medida que o deslocamento no nÃvel de Ãgua Aw se torna maior, as dunas sofrem uma transiÃÃo, passando de barcanas para cadeias de dunas barcanÃides e depois para dunas transversais, reduzindo as alturas mÃximas atingidas pelas dunas. A Ãgua espalha a areia por todo campo e diminui a existÃncia de Ãreas livres da presenÃa de areia, impedindo a formaÃÃo de dunas e levando à formaÃÃo de um leito arenoso de relevo suave. Quando fixamos a altura mÃxima atingida pelo nÃvel de Ãgua e aumentamos o seu tempo de exposiÃÃo tew acima do chÃo, as dunas no campo apresentam padrÃo mais regular na forma e na altura da mesma maneira que acontece quando cresce o deslocamento no nÃvel de Ãgua. Para um vento unidirecional oscilando senoidalmente fora de fase em relaÃÃo a oscilaÃÃo do nÃvel de Ãgua, de maneira contrÃrio ao vento constante, à medida que Aw se torna maior, maiores sÃo as dunas no campo, e as dunas passam de cadeias de dunas barcanÃides para dunas barcanas isoladas. Nesse caso, a areia fica retida nas dunas e nÃo temos a formaÃÃo de um leito de areia à medida que Aw cresce. Jà no caso de um vento senoidal em fase com a oscilaÃÃo do nÃvel de Ãgua as dunas somente surgem para pequenas variaÃÃes no nÃvel de Ãgua. Esses resultados concordam com observaÃÃes realizadas em campos reais por Kocurek et al. (1992), reproduzindo as fase destrutivas e construtivas que se alternam em campos de dunas sob a influÃncia de um nÃvel de Ãgua oscilante, devido Ãs mudanÃas climÃticas sazonais. Conseguimos com o modelo reproduzir alguns padrÃes de morfolÃgicos de dunas que sÃo encontrados nos LenÃÃis Maranhenses. A comparaÃÃo dos resultados obtidos nas simulaÃÃes com as imagens de satÃlite da regiÃo à bastante satisfatÃria, tanto do ponto de vista qualitativo, quanto do ponto de vista quantitativo.
In this work we study the genesis of coastal dune fields using the DUNE model developed by Sauermann et al. (2001) and Kroy et al. (2002), and later perfected by SchwÃmmle and Herrmann (2004). The model was developed to calculate the transport of grains by saltation and the formation of sand dunes, and then it takes account the growth of vegetation (DurÃn and Herrmann, 2006a). Initially we investigate the genesis and the early stages of development of coastal dune fields in the presence of vegetation growth. The model is applied to calculate the evolution of sand transported in the same direction of the wind into a land where there is vegetation growth. In previous work, the model reproduced the fixation of sand grains and the stabilization of dunes. Consequently, the barchans dunes in the presence of plants can be transformed into parabolic dunes (DurÃn et al., 2006a). Here, we seek the understanding of competition between the sand transport and vegetation growth in the development of transverse dunes under the influence of plant growth, when we find a maximum height for transverse dunes under the influence of vegetation. Then we conduct a detailed study of the development of dune fields taking into account changes in the relevant parameters: the width of the beach, availability of sand sediment, wind intensity, the vegetation growth rate and the maximum height reached by the plants. Different morphologies are obtained for the coastal dune fields which depend crucially on the characteristic vegetation growth rate and the wind shear velocity, which can reproduce patterns found in actual fields. After we investigate, the genesis and development of coastal dune fields in the presence of a dynamic water level. In other to do it we complement this model with a dynamic water surface. Here we try to obtain and understand the influence of a exposed water surface over the ground in the development and morphology of the dunes. In this way, we also try a quantitative understanding of the dynamics of coastal dune fields as a function of local climatic conditions, since the dynamics of the water level is linked to rainfall (Jimenez et al., 1999, Levin et al., 2009). The results show that for a constant wind shear velocity u∗ the main effect of the presence of water is to produce the dunes in the field more regular in their shape and size. As the shift Aw in the water level becomes larger, the dunes undergo a transition from barchans dunes to chains of barchans dunes, and then to transverse dunes, reducing the maximum height reached by the dunes in the field. The water spreads the sand across the field and reduces the existence of areas which are free of the sand presence, preventing the formation of dunes and leading to the formation of a sandy bed with a smooth surface. When we increase the time which the water level tew is above the ground, the dunes in the field pattern are more regular in shape and height in the same way as when we increase the shift in water level. For a unidirectional wind sinusoidally oscillating out of phase with respect to fluctuation of water level, in opposition to the constant wind, as Aw becomes larger, higher are the dunes in the field, and they undergo a transition from chains of barchans dunes to isolated barchans dunes. In this case, the sand is retained in the dunes and we do not observe the formation of a sand bed as Aw grows. In the case of a sinusoidal wind in phase with the oscillation of the water level, the dunes appear only for small shifts in the water level. These results agree with the observations of real sand dune fields (Kocurek et al., 1992), reproducing the constructive and destructive phases which alternate in dune fields under the influence of an oscillating water level due to seasonal climatic changes. In the simulations we reproduce some morphological patterns of sand dunes that are found in the LenÃÃis Maranhenses sand dune field. Comparison between simulation results and satellite images of this region is quite satisfactory from a qualitative point of view and from a quantitative point of view.

Geology
M.S.
Grasslands provide fundamental ecosystem services in many arid and semi-arid regions of the world, but are experiencing rapid increases in fire activity making them highly susceptible to post-fire accelerated soil erosion by wind. A quantitative assessment that integrates fire-wind erosion feedbacks is therefore needed to account for vegetation change, soil biogeochemical cycling, air quality, and landscape evolution. We investigated the applicability of a novel tracer technique – the use of multiple rare earth elements (REE) - to quantify aeolian soil erosion and to identify sources and sinks of wind-blown sediments in a burned and unburned shrub-grass transition zone in the Chihuahuan desert, NM, USA. Results indicate that the horizontal mass flux of wind-borne sediment increased approximately three times following the fire. The REE-tracer analysis of aeolian sediments shows that an average 88% of the horizontal mass flux in the control area was derived from bare microsites, whereas at the burned site it was derived from shrub and bare microsites, 42% and 39% respectively. The vegetated microsites, which were predominantly sinks of aeolian sediments in the unburned areas, became sediment sources following the fire. The burned areas exhibited a spatial homogenization of sediment tracers, highlighting a potential negative feedback on landscape heterogeneity induced by shrub encroachment into grasslands. Though fires are known to increase aeolian sediment transport, accompanying changes in the sources and sinks of wind-borne sediments likely influence biogeochemical cycling and land degradation dynamics. Our experiment demonstrated that REEs can be used as reliable tracers for field-scale aeolian studies.
Temple University--Theses

Le vieillissement des lignes de transport d’énergie électrique est une problématique majeure des réseaux. D’ailleurs, des problèmes se posent au plan de l’évaluation de l’état des conducteurs qui, soumis aux vibrations éoliennes, sont vulnérables à l’endommagement en fatigue. Surtout présent aux pinces de suspension, ce phénomène est encore difficile à quantifier, notamment quant à la prédiction de la durée de vie résiduelle des conducteurs. D’autre part, avec le besoin croissant d’optimiser l’exploitation du réseau tout en maintenant sa fiabilité, une estimation précise de l’état d’endommagement des conducteurs est primordiale. Pour cela, une caractérisation des sollicitations à l’échelle des brins est d’abord requise. L’objectif principal de cette thèse vise donc le développement d’une stratégie de modélisation et d’analyse des conducteurs sollicités en vibrations éoliennes permettant une évaluation précise des conditions de chargement locales à l’échelle des brins, tout en tenant compte de l’effet de la géométrie des pinces de suspension. Une stratégie de modélisation 3D des solides toronnés est d’abord développée avec la méthode des éléments finis selon une discrétisation individuelle des brins par éléments poutres, capable de traiter toutes les interactions inter-filaires en frottement. Cette modélisation traduit efficacement la cinématique des torons tout en donnant accès aux charges locales. Son caractère général lui permet aussi d’être appliquée à tout problème impliquant des torons. Appliquée à l’étude des conducteurs sous l’effet des vibrations éoliennes, la stratégie conduit à une description précise de leur comportement tant au plan global en flexion que de la description des contraintes aux brins. Des estimations réalistes de durées de vie en fatigue des conducteurs sont même possibles par l’application de critères d’endommagement aux contraintes. Ensuite, les pinces de suspension sont intégrées à la stratégie de modélisation selon une représentation surfacique traitant le contact pince/conducteur. Une comparaison à des mesures expérimentales met en relief la précision de l’approche. L’analyse de la solution numérique permet l’identification des zones critiques d’endommagement en contact à chacune des couches du conducteur et révèle des informations nouvelles quant à la nature de la sollicitation des brins à la pince de suspension. Finalement, des travaux exploratoires proposent un nouveau concept d’analyse multi-échelles en combinant la modélisation numérique d’un système pince/conducteur à des essais de fatigue sur brins individuels. Une mise en œuvre préliminaire de l’approche permet de valider le concept et en jette les bases en vue de son application future. En somme, la stratégie de modélisation développée dans cette thèse constitue un puissant outil d’analyse qui ouvre maintenant la voie à une caractérisation appropriée de la fatigue des conducteurs en vue ultimement de prédire leur durée et vie résiduelle.
Abstract : The aging of overhead transmission lines is a major concern for utilities. In particular, problems arise in assessing the integrity of conductors whose exposure to Aeolian vibrations renders them vulnerable to fatigue damage. Occurring mainly at the suspension clamps, conductor fatigue is still difficult to quantify, especially regarding the prediction of their residual life. With the increasing need to optimize the power grid while maintaining its reliability, accurate evaluations of the conductor damage state become crucial. To this matter, a characterization of the stress levels at the wire scale is first required. The main objective of this thesis is therefore to develop a strategy for the modeling and analysis of conductors subjected to wind induced vibrations, allowing an accurate description of the local load conditions, while accounting for the effects of the suspension clamps. A finite element wire strand modeling strategy is first developed based on a 3D beam element discretization, considering all frictional wire interactions. The modeling approach efficiently reproduces the wire strand kinematics while giving access to the local loads. Its general formulation also allows it to be applied to any problem involving strands. Applied to the study of conductors subjected to Aeolian vibrations, the strategy leads to an accurate description of their behavior at both the global strand deformations and the wire stress description. Realistic conductor residual life estimates are even possible with the use of common damage criteria. The suspension clamps are then incorporated into the modeling strategy using a surface representation of the conductor/clamp contact. Comparisons with experimental measurements highlight the precision of the approach. The model response analysis allows now the identification of the critical damage zones within each conductor layers and reveals new information about the nature of the wire stresses at the suspension clamp. Finally, exploratory works propose a new concept of multi-scale analysis combining the numerical conductor/clamp modeling strategy to experimental fatigue tests on individual wires. A preliminary implementation of the approach validates the concept and lays the foundations for its future application. In summary, the modeling strategy developed in this thesis constitutes a powerful analytical tool which now opens the way to an appropriate characterization of conductor fatigue with the ultimate objective to eventually predict their residual life.

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Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
This study analyzed measurements of the aeolian transport using vertical sand traps across the field dunes of Jenipabu, in the municipality of Extremoz, Rio Grande do Norte state to the North of Natal city. These measurements were used as parameters for the sand aeolian transport in the region. Before the field trips a map of landscape units was made. Three visits to the field were done in September 2011 (field a - the 13th, field b - the 21st, field c - the 29th), period of the year with the highest wind speed, and another in December 8th, 2011 (field d ) when the wind speed starts to decrease. The sand traps used were of the type "I" with collecting opening of 25 cm from the surface level, and type "S" with collecting opening of 25 cm located 25 cm from the surface level in six collecting points in two of the visits (fields a and d ), and sand traps of the type "T" with the collecting opening of 50 cm from the surface level in the other field trips (fields "b" and "c"). A set of records was also collected by using a portable meteorological station complemented with information such as frequency and intensity of winds, precipitation and relative air humidity in the region, from the Esta??o Meteorol?gica de Natal , located 12 km from the study area. The sediments collected were treated and the data obtained permitted calculating the ratio of sediment transport. In September, the sedimentation ratio varied from 0.01 to 11.39 kg.m-1.h-1 and in December this ratio varied from 0.33 to 1.30 kg.m-1.h-1 in the type T collectors. In type I collectors they ranged from 0.01 to 11.39 kg.m-1.h-1, while the same parameters varied from 0.01 to 0.73 kg.m-1.h-1 in type S collector. Based on the statistical analysis done, we concluded that the sediment transport increased proportionally to the wind speed 25 cm from the surface. However, this is not true above 25 cm from the surface. The transport of sediments is more intense near the surface where sedimentation ratios greater than 10 kg.m-1.h-1 were found, whilst a maximum value of 3 kg.m-1.h-1 was observed 25 cm below the surface. The volume of sediments collected increases with the increasing wind speed at the surface level, whereas this relationship is opposed far away from that surface
O presente estudo fez a an?lise de medidas de transporte e?lico utilizando coletores de areia verticais (Sand Traps) ao longo de um campo de dunas na praia de Jenipabu, munic?pio de Extremoz no estado do Rio Grande do Norte ao Norte da cidade de Natal. Estas medidas foram utilizadas como par?metro de avalia??o da movimenta??o e?lica nesta regi?o. Tr?s visitas a campo foram realizadas no m?s de setembro de 2011 nos dias 13 (campo "a"), 21 (campo "b") e 29 (campo "c"), per?odo do ano com maiores velocidades de vento e uma em dezembro de 2011 no dia 08 (campo "d"), per?odo onde a velocidade do vento come?a a diminuir. Utilizaram-se coletores tipo "I" com abertura de 25 cm a partir do n?vel da superf?cie e "S" com abertura de 25 cm a 25 cm da superf?cie em seis pontos de coleta em duas das visitas ? campo nos dias 13 de setembro (campo "a") e 08 de dezembro (campo "d"), e tipo "T" com abertura de 50 cm a partir do n?vel da superf?cie nas demais visitas 21 e 29 de setembro (campo "b" e "c") ao longo do Campo de Dunas de Jenipabu. Foram tamb?m coletados dados meteorol?gicos com a utiliza??o de esta??o meteorol?gica port?til e complementados com dados da Esta??o Meteorol?gica de Natal que est? localizada a cerca de 12 km da ?rea de estudo para obter informa??es sobre a frequ?ncia e intensidade dos ventos, precipita??o e umidade relativa do ar da regi?o. Os sedimentos coletados foram tratados e foi calculada a vaz?o de fluxo de sedimentos (taxa de sedimenta??o). Em setembro a taxa sedimenta??o variou de 0,01 at? 11,39 Kg m-1 h-1 e em dezembro entre 0,33 e 1,30 Kg. m-1. h-1 nos coletores tipo "T". Nos coletores tipo "I" esses valores variaram entre 0,01 e 11,39 Kg m-1 h-1, enquanto que nos coletores tipo "S" de 0,01 a 0.73 Kg. m-1. h-1. De acordo com a an?lise estat?stica realisada percebe-se que o transporte de sedimento aumentou proporcionalmente a velocidade do vento em at? 25 cm da superf?cie. Por?m, essa rela??o n?o ? verdadeira acima de 25 cm do n?vel da superf?cie. O transporte de sedimentos ? mais intenso mais pr?ximo ? superf?cie onde foram encontradas taxas de sedimenta??o maiores que 10Kg. m-1. h-1 enquanto que a 25cm de dist?ncia o m?ximo encontrado foi menor que 3Kg. m-1. h-1, a quantidade de sedimento coletados aumenta ? medida que aumenta a velocidade no n?vel da superf?cie, ao se afastar da superf?cie a quantidade de sedimento essa rela??o n?o ? verdadeira

Le transport de la neige par le vent est une composante importante de l'interaction entre l'atmosphère et la cryosphère. En zone de montagne, il influence la distribution temporelle et spatiale de la couverture neigeuse au cours de l'hiver et a en premier lieu des conséquences sur le danger d'avalanche. La modélisation numérique de ce phénomène permet d'étudier les interactions complexes entre le manteau neigeux et le vent et d'en estimer les conséquences de manière distribuée. Dans ce contexte, cette thèse décrit le développement et l'évaluation d'un modèle couplé atmosphère/manteau neigeux dédié à l'étude du transport de la neige par le vent en zone de montagne reposant sur le modèle atmosphérique Meso-NH et le modèle détaillé de manteau neigeux Crocus. Le transport de la neige par le vent a été étudié sur le site expérimental du Col du Lac Blanc (massif des Grandes Rousses, France). Une base de données d'épisodes de transport couvrant dix hivers a tout d'abord été utilisée pour déterminer les caractéristiques principales de ces épisodes. Des simulations avec le modèle Crocus (non couplé à Meso-NH) ont ensuite montré qu'il était nécessaire de tenir compte des transformations mécaniques des grains de neige induites par le vent afin de simuler une évolution réaliste de la vitesse seuil de transport. Le site expérimental a également été le siège de deux campagnes de mesures en 2011 et 2012 visant à collecter de données de validation pour le modèle. Elles renseignent sur les conditions météorologiques près de la surface, sur les quantités de neige transportées et sur la localisation des zones d'érosion et de dépôt de la neige grâce à l'utilisation d'un laser terrestre. Le modèle de transport de neige par le vent Meso-NH/Crocus a été développé. Il intègre le transport de la neige en saltation et en suspension turbulente ainsi que la sublimation des particules de neige transportée. Un schéma à deux moments permet de simuler l'évolution spatiale et temporelle de la distribution en taille des particules. L'utilisation d'un schéma de couche limite de surface à l'interface entre Meso-NH et Crocus s'est révélé nécessaire pour représenter les forts gradients de concentration en particules de neige observés près de la surface. Meso-NH/Crocus est le premier modèle couplé atmosphère/manteau neigeux capable de simuler de manière interactive le transport de la neige par le vent en zone alpine. Meso-NH/Crocus a été évalué en relief réel grâce aux données collectées lors de la première campagne de mesure en 2011. La simulation d'un épisode de transport sans chute de neige simultanée montre que le modèle reproduit de manière satisfaisante les principales structures d'un écoulement en relief complexe ainsi que les profils verticaux de vitesse de vent et de flux de particules de neige en suspension près de la surface. En revanche, la résolution horizontale de 50 m est insuffisante pour reproduire avec précision la localisation des zones d'érosion et de dépôt autour du Col du Lac Blanc. La prise en compte de la sublimation réduit la quantité de neige déposée de l'ordre de 5%.Les techniques de descente d'échelle dynamique (grid nesting) ont ensuite été utilisées pour simuler un second épisode de transport avec chute de neige. L'augmentation de la résolution horizontale intensifie les contrastes de vitesse de vent entre versants au vent et sous le vent. En revanche, elle modifie peu les quantités et les structures spatiales des précipitations solides autour du Col du Lac Blanc. Lorsqu'il est activé, le transport devient la principale source d'hétérogénéités des accumulations neigeuses

La façade littorale du Grand Port Maritime de Dunkerque est une cellule sédimentaire quasi-fermée où les formes dunaires résultent de l'imbrication de facteurs humains et naturels. Ce littoral peut être divisé en deux parties : la partie Ouest est constituée d'un cordon bordier sur une longueur de 7 km (plage du Clipon) et le long de la partie Est, la plage est adossée à une digue en enrobé bitumineux de 6 km de long (digue du Braek) où des transferts sableux se produisent. L'objectif de cette étude est de (1) caractériser les formes dunaires et leur évolution sur un site artificiel, (2) quantifier le transport éolien sur le haut de plage, le versant externe de la dune bordière et la digue du Braek, et (3) proposer des mesures de gestion afin de remédier aux contraintes d'exploitation engendrées par les dépôts éoliens sur les infrastructures. A moyen terme (28 ans), l'analyse diachronique de photographies aériennes montre que les superficies dunaires ont fortement augmenté depuis 1983. A court terme (2 ans), des mesures topographiques révèlent que les dunes continuent de se développer aujourd'hui mais gardent la marque des interventions anthropiques passées. Sur la digue du Braek, des formes dunaires se sont naturellement constituées sur l'asphalte. Le sable a été colonisé par Ammophila arenaria qui s'est enraciné sous l'enrobé par l'intermédiaire de fissures. Une typologie de ces dunes de digue a été proposée. A l'Est, des dunes se développent en pied de digue en raison de l'interruption des transferts sédimentaires longitudinaux par une jetée. Des piégeages éoliens in situ ont montré que le transport éolien était essentiellement contrôlé par la vitesse du vent et la source de sable disponible. Par vents frontaux et obliques, les dunes de pied de digue constituent la principale source des envols, et le transport éolien est favorisé par une accélération du vent sur le versant exposé. Différents types de brise-vent ont été testés sur l'ensemble du site afin de trouver le meilleur mode de gestion souple de ces envols de sable. Ces analyses montrent que si la localisation des structures brise-vent a une grande influence sur leur efficacité, les systèmes habituellement utilisés sur les plages (ganivelles, filets synthétiques) peuvent aussi favoriser le développement de dunes sur une digue en asphalte. Les mesures de gestion envisagées sont la stabilisation des principales sources de sable en pied de digue ainsi que le renforcement de la dune bordière aux endroits les plus fragiles.

A description and some results of the calculated aeolian sand transport including the turbulence intensity and the autocorrelation of the wind field in the transport equation’s are presented in this study. The simulated transport related to Sørensen (2004) transport equation varies by more than 100% for one and the same friction velocity and one and the same sand type near the thresholds by variation of turbulence intensity and/or the autocorrelation of the wind. Published results of sand transport measurements are imperfect and not comparable without information about the characteristics of the turbulent wind. An indication of friction velocity, threshold friction velocity and measured transport must be supplement by the specification of the turbulence intensity and the autocorrelation of the wind velocity.

The formation of aeolian ripples has been modeled, quite successfully, using discrete approaches like cellular automaton models. Numerical analysis of continuum models to obtain similar success in modeling ripple evolution, however, has not been studied extensively. A numerical model based on continuum theories expedites calculations, as opposed to discrete approaches which model trajectory of each and every sand grain, and are hence relatively more economical. The numerical analysis strives to contribute to the field of study of aeolian ripple migration by an extensive comparison and discussion of modeled ripple evolution results with those of a particular laboratory based wind-tunnel experiment. This research also endeavors to under- stand the physics behind ripple generation and what parameters to be modified to account for multiple grain sizes. Incorporation of multiple grain sizes would enable us to study the stratigraphy of the generated bed-forms. To obtain smoother and realistic ripple surfaces, a sixth-order compact finite difference numerical scheme is used for spatial derivates and fourth-order Runge-Kutta scheme for time derivates. The boundary conditions incorporated are periodic and the initial condition employed to generate ripple is a rough sand surface. The numerical model is applied to study the effect of varying the angle, at which the sand bed gets impacted by sand grains, on the evolution of ripples. Ripples are analyzed qualitatively and quantitatively by considering the contribution of processes involved in the evolution process. The ripple profiles and the time taken to reach equilibrium state, obtained by numerical experiments, are in close agreement with the ones obtained by the wind-tunnel experiment.

In the present study is attempted to examine aeolian sediment transport potential and to determine the constituents that promote/restrict the dune system development of Ancão Peninsula, where, apparent alongshore geomorphological differences, i.e. western and eastern sector, exist. In this study were investigated: wind potential for sediment transfer over the last 20 years (1997-2016) using recent granulometric data, dune morphology and elevation changes obtained from RTK-GPS beach profiles (2016), volumetric changes in three backshore polygons extracted from Lidar and UAV datasets (2009-2011 and 2011-2016) in both sectors. Presence of species communities of vegetation and shoreline evolution rates (1958-2014) as an indirect indicator for dune morphology were also used. Additional key parameters, namely littoral drift, updrift nourishment interventions and effect of Ancão Inlet were also considered. A windy event was defined at velocity above 8.9 m/s for a mean beachface grain size of 466 μm. Accounting for onshore winds and steering of the peninsula, only 1.7% of the winds was responsible for potential sediment transport. Moreover, the wind records, revealed that the windier years for potential transfer were 2002 (1.7 m3/m), 2009 (2.1 m3/m) and 2010 (1.8 m3/m). Shoreline evolution showed negative values (retreat) for the western and central sector of the peninsula (-0.2 m/yr and -0.25 m/yr respectively) and accretional (+0.79 m/yr) for the eastern. The volume change rates for 2009-2011 show accumulation with very low positive values for the western sector that progressively increase towards the eastern (from eastern polygon: 0.85 m3/(m.yr) to western: 7.32 m3/(m.yr)). The vegetational communities in the two sectors are compatible to the respective morphologies. In the western sector the absence of typical embryo dune vegetation (e.g. Elymus farctus, Eryngium maritimum) conforms with the absence of new foredune development. In the western part of the eastern sector, sparse embryo dune vegetation with low density is present, whereas in the most eastward part of the peninsula -in the inlet vicinity- two seaward developing foredunes are formed. Assembling all elements, it is concluded that the mild wind regime in conjunction with the existing coarse grain size permits the development of relatively small dunes. In the western sector the vegetation suggests dune maturity, however the updrift nourishments have contributed to shoreline stabilization and subsequently to restriction of dune erosion. On the eastern sector nevertheless, the presence of Ancão inlet fostered creation of accommodation space through shoreline progradation, and hence incipient dune vegetation and new embryo dunes formation.
No presente estudo avaliou-se o potencial eólico de transporte sedimentar e determinaram-se os fatores responsáveis pela promoção/restrição do desenvolvimento do sistema dunar da Península do Ancão, onde se verificam diferenças geomorfológicas, por exemplo entre os sectores este e oeste. Este estudo analisou o potencial eólico na transferência de sedimentos nos últimos 20 anos (1997-2016), utilizando para tal dados granulométricos recentes, a morfologia dunar e variações de elevação obtidas a partir de perfis de praia efetuados com RTK-GPS (2016) e a determinação de mudanças volumétricas em três polígonos costeiros extraídos de conjuntos de dados de Lidar e UAV (2009-2011 e 2011-2016). Utilizou, ainda, a presença de comunidades de espécies de vegetação dunar e taxas de evolução da linha de costa (1958-2014) como indicadores indiretos para a evolução da morfologia das dunas. Parâmetros-chave adicionais, nomeadamente a deriva litoral, as intervenções de realimentação de praia e o efeito da Barra do Ancão foram também considerados na análise. Foram definidos como eventos com capacidade de transporte eólica os registos em que a velocidade do vento foi superior a 8,9 m/s, para um tamanho médio de grão de praia de 466 μm. Tendo em consideração os ventos dirigidos para terra e a direção da península, verifica-se que apenas 1,7 % dos ventos foram responsáveis por transporte potencial de sedimentos. Os registos de vento revelaram que os anos com maior potencial de transporte eólico foram 2002 (1,7 m3/m), 2009 (2,1 m3/m) e 2010 (1,8 m3/m). A evolução da linha de costa mostrou valores negativos (recuo) para os setores ocidental e central da península (-0,2 m/ano e -0,25 m/ano, respetivamente) e avanço da linha de costa (+0,79 m/ano) no sector este. As taxas de variação volumétrica, entre 2009 e 2011, mostram acumulação, com valores positivos muito baixos para o setor ocidental que aumentam progressivamente em direção a leste (R5: 0,85 m3/m ano, R3: 1,2 m3/m ano, R2: 7,32 m3/m ano). As comunidades vegetais dos dois setores são compatíveis com as respetivas morfologias. No sector ocidental, a ausência de vegetação típica das dunas embrionárias (por exemplo Elymus farctus, Eryngium maritimum) está em conformidade com a ausência de desenvolvimento de um novo cordão dunar frontal. Para oeste do setor oriental, verifica-se a existência de vegetação de dunas embrionárias, com baixa densidade, enquanto que na parte mais a leste da península - na vizinhança da barra de maré – se desenvolveram duas crista de dunas frontais, em direção ao mar. Conclui-se que o regime de ventos moderado, em conjunção com o tamanho de grão grosseiro existente, limita a promoção do crescimento dunar. No setor ocidental, a vegetação sugere que as dunas apresentam um elevado estado de maturidade. As realimentações de praia efetuadas a barlamar contribuíram para a estabilização da evolução da linha de costa e, subsequentemente, para a redução da erosão das dunas. No sector oriental, a presença da Barra do Ancão fomentou a criação de espaço de acomodação, através da progradação da linha de costa e permitiu, consequentemente, a formação de vegetação dunar incipiente e a formação de novas dunas embrionárias.

This dissertation combines studies of aeolian bedforms and aeolian dune-field patterns to create a comprehensive set of tools that can be used in tandem (or separately) to extract information about climate change and landscape evolution, and to identify the controls on formation for specific modern dune fields or ancient aeolian sequences. The spatial distribution of surface processes, erosion/deposition rates, and lee face sorting on aeolian dunes are each a function of the incident angle. This correlation between stratification style and incidence angle can be used to develop a “toolbox” of methods based on measurements of key suites of parameters found in ancient aeolian deposits. Information obtained from the rock record can be used as input data for different kinds of numerical models. Regional-scale paleowind conditions can be used to validate paleoclimate and global circulation models. Understanding the natural variability in the Earth’s climate throughout its history can help predict future climate change. Reconstructed wind regimes and bedform morphologies can be used in numerical models of aeolian dune-field pattern evolution to simulate patterns analogous to those reconstructed from ancient aeolian systems. Much of the diversity of aeolian dune-field patterns seen in the real world is a function of the sediment supply and transport capacity, which in turn determine the sediment availability of the system. Knowledge of the sediment supply, availability, and transport capacity of aeolian systems can be used to predict the amount of sand in the system and where it might have migrated. This information can be extremely useful for development and production of oil and gas accumulations, where a discovery has been made but the spatial extent of the aeolian reservoir is unknown.
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Coastal foredune evolution involves complex processes and controls. Although a great deal is known about the effects of vegetation cover, moisture, and fetch distance on sediment supply, and of topographic forcing on airflow dynamics, the role of large woody debris (LWD) as a modulator of sediment supply and a control on foredune growth is understudied. Large assemblages of LWD are common on beaches near forested watersheds and collectively have a degree of porosity that increases aerodynamic roughness and provides substantial sand trapping volume. To date, no research has attempted to understand the geomorphic role that LWD matrices, as a whole, have as roughness elements affecting airflow and sediment transport across a beach-dune system, or, what the long-term implications of these impacts are on beach and foredune erosion recovery and evolution. This four-year research initiative investigated the role of a LWD matrix on beach-dune morphodynamics on West Beach, Calvert Island on the central coast of British Columbia, Canada. This study integrated data from research that spanned three temporal scales, 1) event-scale (10 min) flow and sediment transport patterns, 2) daily frequency and relative magnitude of landscape changing events, 3) seasonal to interannual-scale volumetric and LWD changes. An event-scale experiment to characterise airflow dynamics and related sand transport patterns showed that LWD distinctly alters wind flow patterns and turbulence levels from that of incoming flow over a flat beach. Overall, mean wind speed and fluctuating flow properties declined as wind transitioned across the LWD. Streamwise mean energy was converted to turbulent energy, however, the reductions in mean flow properties were too great for the increased streamwise turbulence to have an effect on transport. In response to these flow alterations and more limited sand transport pathways to the foredune, sediment flux was reduced by 99% in the LWD compared to the open beach, thereby reducing sand supply to the foredune. Sand grains rebounding off of the LWD were carried higher into the flow field resulting in greater mass flux recorded at 20-50 cm in the LWD as opposed to the flat beach. This effect was only recorded 6 m into the LWD. As such, LWD has the potential to modulate rates of foredune recovery, growth, and evolution. Time-lapse photography collected at 15 min intervals during the study revealed that storm events lead to wave-induced erosion of the backshore and reworking of the LWD matrix. The exposed LWD matrix subsequently traps aeolian sediment that leads to rapid burial of the LWD and building of a raised platform for emergent vegetation. However, infilling of the accommodation space within the LWD matrix is so rapid, that sediment starvation of the foredune is short-lived. While the LWD at this site does trap sediment in the backshore, helping to protect the dune from scarping, LWD at this study site maintains an overall lower impact on transport to the foredune. Critical to this relationship is the frequency and magnitude of nearshore events that erode the beach periodically and re-organize the LWD matrix, which directly impacts the ability of LWD to store sediment and modulate transport to the foredune. A conceptual model exploring these relationships is presented.
Graduate