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.

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 Δ<I>t</I> 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 Δ<I>t</I> 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 <I>f </I>≈ 0.5Hz the transport rate was found not to respond to the wind changes.

Le transport par charriage reste un phénomène mal compris en raison notamment de la polydispersité des particules et de la ségrégation résultante. Nous avons mené une étude expérimentale à l'échelle de la particule dans le but de comparer le mouvement de billes de verre sphériques de 4~mm et 6~mm et d'analyser la ségrégation de mélanges de deux tailles. Les particules sont entraînées dans un mouvement quasi-bidimensionnel par un écoulement turbulent et supercritique dans un canal à forte pente sur un lit mobile sous des conditions d'équilibre de charriage. Les écoulements ont été filmés sur le côté et analysés par traitement d'image afin de déterminer les trajectoires des particules, leur état de mouvement (arrêt, roulement, saltation) et la hauteur d'eau. Alors que les billes de 6~mm des cas unimodaux sont principalement en roulement, la saltation est prédominante pour les billes de 4~mm et les billes du mélange. Nos données monodisperses supportent le modèle d'érosion-dépôt de cite{Char06}. Les profils de vitesse présentent trois zones (exponentielle, linéaire, logarithmique). De même que les profils de concentration, ils ont des similitudes avec les profils en charriage côtier et en écoulement granulaire sec. Nos expériences bidisperses ont révélé un phénomène de ségrégation assimilable au processus de "kinetic sieving". / Bedload transport remains poorly understood partly due to the polydispersity of particles and resulting segregation. Experiments were carried out at the particle scale to compare the motion of spherical 4~mm and 6~mm glass beads and to analyze the segregation of two-size mixtures. Particles were entrained by a turbulent supercritical flow down a steep two-dimensional channel with a mobile bed under bedload equilibrium conditions. Flows were filmed from the side and analyzed by image processing to determine the particle trajectories, their state of motion (rest, rolling, saltation), and flow depth. Contrasting with the 6~mm uniform case where rolling was frequent, saltation was dominant for 4~mm beads and the two-size mixtures. Our one-size data support the erosion-deposition model of cite{Char06}. The velocity profiles exhibit three segments (exponential, linear, logarithmic). As well as concentration profiles, they are similar to profiles in sheet flows and dry granular flows. Our two-size experiments also revealed a phenomenon of segregation comparable to the kinetic sieving process.

Master of Science / Department of Biological & Agricultural Engineering / Ronaldo G. Maghirang / Military training lands can be significant sources of fugitive dust emissions due to wind erosion. This study was conducted to determine dust emission potential of soils due to wind erosion as affected by off-road military vehicle disturbance. Multi-pass traffic experiments using two types of vehicles (i.e., wheeled and tracked) were conducted on six soil textures at four military training facilities: Fort Riley, KS; Fort Benning, GA; Yakima Training Center, WA; and White Sands Missile Range (WSMR), NM. Prior to and after the preselected number of vehicle passes, soil samples at three locations were collected with minimum disturbance into trays. Adjacent to the location where tray samples were collected, a Portable In-Situ Wind Erosion Lab (PI-SWERL) was used to measure dust emission potential. The tray samples were tested in a laboratory wind tunnel (with sand abrader) for dust emission potential using a GRIMM aerosol spectrometer and gravimetric method with filters. Comparison of the PI-SWERL (with DustTrak™ dust monitor) and wind tunnel (with GRIMM aerosol spectrometer) measurement results showed significant difference in measured values but high correlation, particularly for soils with high sand content. Wind tunnel tests results showed that sampling locations significantly affected dust emissions for the tracked vehicles but not for the light-wheeled and heavy-wheeled vehicles. Also, soil texture, number of vehicle passes, and vehicle type significantly affected dust emissions. For the light-wheeled vehicles, dust emissions increased as the number of vehicle passes increased. From undisturbed conditions to 10 vehicle passes, there was a significant (P<0.05) increase in dust emissions (297%) on average for all light-wheeled vehicle tests. From 10 to 25 passes and 25 to 50 passes, an additional 52% and 62% increments were observed. For the tracked vehicle, for the straight section sampling location, dust emission increased as the number of vehicle passes increased. However, for the curve section, dust emissions at any level of pass were significantly higher than initial condition; beyond the first pass, no significant increase was observed.

This dissertation investigates characteristics of the vertical distributions of mass flux observed in field and laboratory experiments. Thirty vertical mass flux profiles were measured during a field experiment in Jericoacoara, Brazil from October to November, 2008. These data were supplemented with 621 profiles gathered from an extensive review of the aeolian literature. From the field experiment, the analysis of the grain-size statistics for the flux caught in each trap shows that a reverse in grain-size trends occurs at an inflection zone located 0.05 ? 0.15 m above the bed. Below this inflection, mean grain-size decreases steeply with elevation in the near bed region dominated by reptation and saltation modes of transport. Above the inflection there is a coarsening of grain size with elevation; as saltation becomes the dominant transport mode. These results indicate that the coarsest grains are found close to and farthest from the bed. Using a data set comprising 274 vertical flux profiles, the performance of the exponential, power and logarithmic functions were tested to see which provided the best fit to the vertical flux distributions. The exponential function performed best 88% of the time. The average r2 value for the grouped exponential, logarithmic, and power function fits are 0.98, 0.85 and 0.91, respectively. The populations of the exponent coefficients, representing the relative rate of decrease with height above the surface, or slope of the vertical mass flux profiles, are statistically different in wind tunnels and field experiments. The slopes of the vertical flux profiles observed in wind tunnel experiments are steeper compared to field environments, which infers that saltation is suppressed in wind tunnels. These differences are magnified in wind tunnels with small working cross section areas, and in wind tunnel experiments that use extreme environmental conditions, such as very high shear velocities. The Rouse concentration model, widely used in water studies, was tested to see if it could replicate the observed vertical flux distributions and transport rates. A fall velocity (w0) equation for particles falling in air was derived using a grain size (d) dependency: w0 (in m/s) = 4.23d (in mm) + 0.1956 (r^2=0.88). The Rouse model performs poorly when the value of the beta (a form of the Schmidt number in the Rouse number exponent) is assumed to be unity. The values of beta were modeled using a relationship derived from a dependency of beta on the w0/u* ratio: beta = 3.2778(w0/u*) - 0.4133 (r^2=0.65). The values of beta ranged from 6.11 ? 17.83 for all the experiments. The Rouse profiles calculated using this approach predict very similar vertical distributions to the observed data and predicted 86% and 81% of the observed transport rate in field and wind tunnel experiments respectively. The Rouse approach is more physically meaningful than current approaches that use standard curve fitting functions to represent the vertical flux data but do not provide any explanatory power for the shape or magnitude of the profile.

Le travail présenté dans cette thèse s'inscrit dans la problématique du transport éolien du sable.<br />L'étude principale porte sur un aspect particulier du transport par saltation. La saltation désigne le mouvement par saut des grains de sable emportés par le vent. Lors du rebond du grain en saltation sur le lit de sable, certains grains peuvent ètre éjectés. Ce mécanisme d'éjection a été étudié à partir d'une expérience modèle de collision d'une bille sur un empilement bidimensionnel de mêmes billes. Une étude détaillée de ce processus a été réalisée, en fonction de la vitesse d'impact de la bille incidente, de la hauteur de l'empilement, des propriétés élastiques des billes ainsi que de la géométrie de l'empilement. Des lois de distribution des vitesses d'éjection ont été établies. Par ailleurs, nous avons montré que le processus d'éjection est extrèmement sensible au désordre au sein de l'empilement: dans le cas d'un empilement désordonné, un plus grand nombre de billes est éjecté. Enfin, nous avons développé un modèle phénoménologique basé sur une description simplifiée de la propagation à travers l'empilement, de l'onde de déformation générée par le choc. Ce modèle permet d'interpréter certains résultats expérimentaux du processus d'éjection. <br />La dernière partie de cette thèse aborde certains aspects du transport éolien à une autre échelle: celle des structures éoliennes telles que les rides et les dunes. Un modèle de formation des rides éoliennes est proposé qui est capable de prédire l'évolution spatio-temporelle de ces structures. Les paramètres du modèle sont reliés directement aux lois d'éjection déterminées expérimentalement. Nous avons mis en évidence notamment un phénomène de murissement des rides: la longueur d'onde augmente au cours du temps, ce qui est observé expérimentalement.

abstract: The morphology of mountainous areas is strongly influenced by stream bed incision rates, but most studies of landscape evolution consider erosion at basin scales or larger. The research here attempts to understand the smaller-scale mechanics of erosion on exposed bedrock channels in the conceptual framework of an established saltation-abrasion model by Sklar and Dietrich [2004]. The recirculating flume used in this experiment allows independent control of bed slope, water discharge rate, sediment flux, and sediment grain size – all factors often bundled together in simple models of river incision and typically cross-correlated in natural settings. This study investigates the mechanics of erosion on exposed bedrock channels caused by abrasion of transported particles. Of particular interest are saltating particles, as well as sediment near the threshold between saltation and suspension - sediment vigorously transported but with significant interaction with the bed. The size of these erosive tools are varied over an order of magnitude in mean grain diameter, including a sand of D¬50 = 0.56 mm, and three gravel sizes of 3.39, 4.63, and 5.88 mm. Special consideration was taken to prevent any flow conditions that created a persistent alluvial cover. The erodible concrete substrate is fully exposed at all times during experiments reported here. Rates of erosion into the concrete substrate (a bedrock proxy) were measured by comparing topographic data before and after each experimental run, made possible by a precision laser mounted on a high speed computer-controlled cart. The experimental flume was able to produce flow discharge as high as 75 liters per second, sediment fluxes (of many varieties) up to 215 grams per second, and bed slopes up to 10%. I find a general positive correlation is found between erosion rate and bed slope, shear stress, grain size, and sediment flux. / Dissertation/Thesis / Masters Thesis Geological Sciences 2016

Diese Studie soll einen näheren Aufschluss über die Ermittlung der mittleren Saltationslänge und somit über den Sandtransport geben. Die Bestimmung der vertikalen und horizontalen Verteilung des Sedimenttransports erfolgte mittels MWAC- und Quaderfallen-Messungen. Das dazugehörige Feldexperiment fand im März 2008 am Zingster Strand (Ostsee/Deutschland) statt. Des Weiteren wurde von Namikas im Juni 1997 in der 'State Vehicular Recreation Area' (SVRA) an der kalifornischen Küste der Transport mithilfe von Vertikal- (VTRAP) und Horizontalfallen (HTRAP) ermittelt (Namikas, 2003). Die Experimente der Quaderfallen und der HTRAP wurden numerisch simuliert und auf Tauglichkeit zur Sprunglängenberechnung geprüft. Die Analyse brachte hervor, dass die vier benutzten Quaderfallen mit unterschiedlicher Länge eine unzureichende Vorhersage für den Massenfluss liefern. Die HTRAP gibt erst durch Differenz der einzelnen Fächer eine nahezu korrekte Prognose. Folglich kann durch eine Korrektur der ermittelten Werte eine zutreffende mittlere Saltationslänge errechnet werden. / This study is giving a near information about ascertaining of mean saltation lenght and consequently about sand transport. The vertical and horizontal distribution of transport of sediment was determined by using MWAC-sampler and cuboid traps measurements. The respective field experiment was processed on the beach of Zingst (Baltic Sea/Germany) in March 2008. Furthermore, the transport was established by using vertical (VTRAP) and horizontal traps (HTRAP) in the 'State Vehicular Recreation Area' (SVRA) on the California coast in July 1997 (Namikas, 2003). The experiments of cuboid traps and HTRAPs have been simulated numerical and checked for efficiency to evaluation of jump lenght. The results generated an insufficient prediction of mass flux by using four cuboid traps with different length. The HTRAP delivered an almost correct prognosis when seperate compartments are subtracted. Consequently a true mean saltation length is calculated by an adjustment of determined values.

Au cours de cette thèse, le rôle joué par la turbulence de paroi dans le transport de particules a été étudié. Pour ce faire, deux types d'expériences ont été réalisés. Les premières ont porté sur un écoulement de turbulence de paroi naturelle chargé en billes de verre et de céramique, pour 4 conditions hydrauliques données. Les secondes expériences ont été réalisées en turbulence contrôlée. En effet, nous avons choisi de générer artificiellement des éjections en créant des hairpin vortex. Les premières expériences sont réalisées dans des conditions de fort transport par suspension et les secondes sont faites pour des conditions très proches du seuil de suspension, les particules étant majoritairement transportées par saltation.<br />Afin de mesurer simultanément le champ de vitesse de l'écoulement ainsi que la position et la vitesse des particules, nous avons utilisé la PIV (Vélocimétrie par Image de Particules) qui permet d'avoir sur la même paire d'image l'information liquide et solide. Nous pouvons ainsi analyser l'écoulement autour de chaque particule et mettre en évidence l'action locale du fluide sur les particules.<br />L'analyse de l'écoulement se fait dans le voisinage de chaque particule, dont la taille varie entre 5 à 10d. Tout d'abord, l'analyse par quadrants montre que les éjections sont fortement impliquées dans le transport par suspension et par saltation, notamment dans la montée des particules. Elle indique aussi que certaines particules qui descendent sont dans des éjections. Nous cherchons donc à déterminer ce qui fait qu'une éjection est capable ou non de contrer la gravité et de faire monter les particules. <br />L'étude du flux de quantité de mouvement des éjections montre que plus ce flux est important, plus les particules ont tendance à monter dans les éjections. Nous déterminons donc un seuil permettant de détecter les éjections faisant monter systématiquement les particules. Ce seuil, adimensionalisé , est compris de l'ordre de 0,5 et est quasiment constant en fonction du nombre de Reynolds de l'écoulement, de la hauteur et des caractéristiques des particules. <br />Nous observons qu'une différence importante entre la suspension et la saltation est le temps de résidence des particules dans les éjections. En effet, plus ce temps est long, plus la particule va pouvoir monter dans l'éjection et plus elle sera transportée par suspension. En revanche quand elle ne reste que peu de temps dans l'éjection, la particule n'a pas le temps de monter et elle est alors transportée par saltation. A partir de l'analyse de ces expériences, nous proposons un modèle conceptuel de transport par suspension dans les éjections en fonction du flux de quantité de mouvement autour des particules, de leur position dans les éjections et des trajectoires de suspension qui ont pues être reconstruites pour les expériences en turbulence contrôlée.

Le transport par charriage reste un phénomène mal compris en raison notamment de la polydispersité des particules et de la ségrégation résultante. Nous avons mené une étude expérimentale à l'échelle de la particule dans le but de comparer le mouvement de billes de verre sphériques de 4~mm et 6~mm et d'analyser la ségrégation de mélanges de deux tailles. Les particules sont entraînées dans un mouvement quasi-bidimensionnel par un écoulement turbulent et supercritique dans un canal à forte pente sur un lit mobile sous des conditions d'équilibre de charriage. Les écoulements ont été filmés sur le côté et analysés par traitement d'image afin de déterminer les trajectoires des particules, leur état de mouvement (arrêt, roulement, saltation) et la hauteur d'eau. Alors que les billes de 6~mm des cas unimodaux sont principalement en roulement, la saltation est prédominante pour les billes de 4~mm et les billes du mélange. Nos données monodisperses supportent le modèle d'érosion-dépôt de cite{Char06}. Les profils de vitesse présentent trois zones (exponentielle, linéaire, logarithmique). De même que les profils de concentration, ils ont des similitudes avec les profils en charriage côtier et en écoulement granulaire sec. Nos expériences bidisperses ont révélé un phénomène de ségrégation assimilable au processus de "kinetic sieving".