modelling controls on aeolian dune field pattern evolution a second generation source to sink cellular automaton based model presented here captures and quantifies many of the factors controlling the evolution of aeolian dune field patterns by varying only a small number of parameters the role of sediment supply sediment availability and transport capacity together defined as sediment state in the development and evolution of an aeolian dune field pattern over long time scales is quantified from model simulations seven dune field patterns can be classified from simulation results varying the sediment supply and transport capacity that control the type and frequency of dune interactions the sediment availability of the system and ultimately the development of dune field patterns this model allows predictions to be made about the range of sediment supply and wind strengths required to produce the dune field patterns seen in the real world a new clustered dune field pattern is identified from model results and used to propose an alternative mechanism for the formation of superimposed dunes bedforms are hypothesized to cluster together simultaneously forming two spatial scales of bedforms without first developing a large basal dune with small superimposed dunes manipulation of boundary conditions produces evolving dune fields with different spatial configurations of sediment supply trends of spacing and crest length increase with decreasing variability as the dune field matures this simple model is a valuable tool which can be used to elucidate the dominant control of aeolian sediment state on the construction and evolution of aeolian dune field patterns

electrification of sand on titan and its influence on sediment transport triboelectric or frictional charging is a ubiquitous yet poorly understood phenomenon in granular flows recognized in terrestrial volcanic plumes and sand storms such electrification mechanisms are possibly present on titan there dunes and plains of low density organic particles blanket extensive regions of the surface unlike earth titan hosts granular reservoirs whose physical and chemical properties possibly enhance the effects of charging on particle motion here we demonstrate in laboratory tumbler experiments under atmospheric conditions and using organic materials analogous to titan that titan sands can readily charge triboelectrically we suggest that the resulting electrostatic forces are strong enough to promote aggregation of granular materials and affect sediment transport on titan indeed our experiments show that electrostatic forces may increase the saltation threshold for grains by up to an order of magnitude efficient electrification may explain puzzling observations on titan such as the mismatch between dune orientations and inferred wind fields we conclude that unlike other solar system bodies nanometre scale electrostatic processes may shape the geomorphological features of titan across the moon s surface frictional charging of granular materials may readily occur on saturn s moon titan laboratory experiments under titan like conditions suggest that the resulting electrostatic forces are strong enough to affect sand transport on titan

probabilistic exner sediment continuity equation for mixtures with no active layer the exner equation of sediment continuity is the foundation of river morphodynamics generalization of this equation to mixtures of grain sizes has required the introduction of an active layer i e a buffer layer between the sediment moving in the water column and the immobile substrate below the active layer is defined to be a well mixed layer with no vertical structure that encompasses those grains available to exchange directly with the moving sediment the sediment in the substrate below exchanges with the active layer only as the bed aggrades or degrades the active layer concept is a useful one that has served the research community well for 3 decades however the division of the erodible bed into a discrete active layer and substrate must represent only an approximation of a more general formulation that contains no active layer and in which parameters pertaining to the entrainment from and deposition to the bed vary continuously with depth below the sediment water interface here the probability density function of bed elevation is used to derive a general exner equation of sediment continuity with no discrete layers the formulation is applicable to both sediment mixtures and tracers in uniform sediment although the treatment requires more information than that of the active layer approach it offers the prospect of a better understanding of how streams create a stratigraphic record of their activities through deposition

factors determining desert dune type while most observers recognize four elemental types of desert dunes longitudinal transverse barchan and star1 3 there is little agreement about which factors determine these types the angular relationships between the resultant of sand shifting winds and both the crest and principal slipfaces of the elemental types have been discussed qualitatively for many decades these relationships have been quantified but the wide range of dune types that combine the elemental forms and the juxtaposition of different dune types show that wind regime is not the only determinant for example sand availability has been considered important we have now quantified the importance of both wind regime and sand availability as determinants of dune type we show here that the four elemental types occur in areas uniquely defined by both the equivalent sand thickness of sediments held in the dunes and a measure of directional variability of sand moving winds wind strength does not appear to be important vegetation has an ambiguous role and the particle size of dune sediments is unimportant

bedload transport of a bimodal sediment bed 1 despite several decades of investigations accounting for the effect of the wide range of grain sizes composing the bed of rivers on bedload transport remains a challenging problem we investigate this problem by studying experimentally the influence of grain size distribution on bedload transport in the simple configuration of a bimodal sediment bed composed of a mixture of 2 populations of quartz grains of sizes d1 0 7 0 1 mm and d2 2 2 0 4 mm respectively the experiments are carried out in a tilted rectangular flume inside which the sediment bed is sheared by a steady and spatially uniform turbulent flow using a high speed video imaging system we focus on the measurement of the average particle velocity and the surface density of moving particles defined as the number of moving particles per unit surface of the bed these two quantities are measured separately for each population of grains as a function of the dimensionless shear stress or shields number and the fraction of the bed surface covered with small grains we show that the average velocity and the surface density of moving particles obey the same equations as those reported by lajeunesse et al 2010 for a bed of homogeneous grain size once in motion the grains follow therefore similar laws whether the bed is made of uniform sediment or of a bimodal mixture this suggests that the erosion deposition model established by lajeunesse et al 2010 for a bed of uniform sediment can be generalized to the case of a bimodal one the only difference evidenced by our experiments concerns the critical shields number for incipient sediment motion above a uniform sediment bed the latter depends on the particle reynolds number through the shields curve in the case of a bimodal bed our experiments show that the critical shields numbers of both populations of grains decrease linearly with the fraction of the bed surface covered with small grains we propose a simple model to account for this observation

the pattern of grainfall deposition in the lee of aeolian dunes a simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance the deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity the velocity being weighted by its probability density the resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink beyond which deposition rates fall off roughly exponentially an important length scale emerges that characterizes this decay with distance from the brink the length increasing with wind velocity and decreasing with grain diameter it is shown that this length scale is on the order of one metre for typical grain size and wind conditions that this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes the position of a pivot point on the lee slope may be predicted separating source regions from accumulation regions for grainflow avalanche deposits the calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits but they provide the initial geometry for grainflow avalanches the initial failures should coincide with the steepest gradient in grainfall deposition slightly downslope from the grainfall maximum

large grains matter contrasting bed stability and morphodynamics during two nearly identical experiments while the stabilizing function of large grains in step pool streams has long been recognized the role they play in gravel bed streams is less clear most researchers have ignored the role of large grains in gravel bed streams and have assumed that the median bed surface size controls the erodibility of alluvial boundaries the experiments presented herein challenge this convention two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material the two distributions had the same range of particle sizes and nearly identical bulk d50 values 1 6 mm however the d90 of experiment gsd1 was slightly finer 3 7 mm than that for experiment gsd2 3 9 mm transport rates during gsd1 were nearly four times greater than during gsd2 even though the dimensionless shear stress was slightly lower and the channel developed a sinuous pattern with well developed riffles pools and bars during gsd2 the initial rectangular channel remained virtually unchanged for the duration of the experiment the relative stability of gsd2 seems to be associated with a slightly larger proportion of stable large grains on the bed surface at the beginning of gsd1 3 5 of the bed was immobile while almost twice as much of it 6 1 was immobile at the beginning of gsd2 the results demonstrate that the largest grains not the median size exert first order control on channel stability copyright 2017 john wiley sons ltd

morphology and dynamics of star dunes from numerical modelling star dunes are common in sand seas but the mechanisms driving their formation are unclear numerical modelling indicates that the morphology of the dunes is controlled by the frequency of changes in the wind regime

sorted bed forms as self organized patterns 1 model development 1 here we present a modified version of the exploratory numerical model originally presented by murray and thieler 2004 and use it to further investigate the development of sorted bed forms on the inner continental shelf the new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model the shallow water wave equations are replaced by the general equations and changes in water depth related to morphological development are accounted for this model reproduces the main results of the previous one supporting the hypothesis that sorted bed forms are a self organized pattern and develop as a result of a coupling between bed composition and sediment flux the sensitivity of the model predictions evaluated in terms of the geometry of the sorted bed forms and their growth rate with respect to external forcing variables and internal model parameterizations shows increasing decreasing sorted bed form spacing height with increasing wave height and period the opposite occurs for increasing water depths more complicated behavior is observed when the ratio fine to coarse sediment diameter or the percentage of fine and coarse material is varied the shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave generated ripples can significantly affect or even inhibit growth rates of sorted bed forms results from a sensitivity analysis show that there are deficiencies in the description of small scale processes that limit the ability to quantitatively predict sorted bed form evolution highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled

measurements of the aeolian sand transport saturation length the wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length which is the length needed for the sand flux to adapt to a change in wind strength the influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial in this paper we present direct measurements of the saturation length performed in a wind tunnel experiment complementary to this initial dune wavelengths are measured under different wind conditions in particular after storms using the linear stability analysis of dune formation it is then possible to deduce the saturation length from field data both direct and indirect measurements agree that the saturation length is almost independent of the wind strength this result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes

development and steady states of transverse dunes a numerical analysis of dune pattern coarsening and giant dunes we investigate the development and steady states of transverse dunes for ranges of flow depths and velocities using a cellular automaton dune model subsequent to the initial bed instability dune pattern coarsening is driven by bed form interactions collisions lead to two types of coalescence associated with upstream or downstream dominant dunes in addition a single collision ejection mechanism enhances the exchange of mass between two adjacent bed forms throughpassing dunes the power law increases in wavelength and amplitude exhibit the same exponents which are independent of flow properties contrary to the wavelength dune height is limited not only by flow depth but also by the strength of the flow superimposed bed forms may propagate and continuously destabilize the largest dunes we identify three classes of steady state transverse dune fields according to the periodicity in crest to crest spacing and the mechanism of size limitation in all cases the steady state is reached and maintained through the dynamic equilibrium between flow strength and dune aspect ratio in the limit of low flow strength where it becomes the primary factor of size limitation the bed shear stress in the dune trough regions is close to its critical value for motion inception comparisons with natural dune fields suggest that many of them may have reached a steady state finally we infer that the sedimentary patterns in the model may be used to bring new constraints on the development of modern and ancient dune fields

setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bed forms 1 we present a new 3 d cellular automaton model for bed form dynamics in which individual physical processes such as erosion deposition and transport are implemented by nearest neighbor interactions and a time dependent stochastic process simultaneously a lattice gas cellular automaton model is used to compute the flow and quantify the bed shear stress on the topography local erosion rates are assumed to be proportional to the shear stress in such a way that there is a complete feedback mechanism between flow and bed form dynamics in the numerical simulations of dune fields we observe the formation and the evolution of superimposed bed forms on barchan and transverse dunes using the same model under different initial conditions we perform the linear stability analysis of a flat sand bed disturbed by a small sinusoidal perturbation comparing the most unstable wavelength in the model with the characteristic size of secondary bed forms in nature we determine the length and time scales of our cellular automaton model thus we establish a link between discrete and continuous approaches and open new perspectives for modeling and quantification of complex patterns in dune fields

the effect of wind speed and bed slope on sand transport this paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport the use of a sloping wind tunnel has enabled estimation of the friction angle to be about 40 for saltating particles in the range 170 540 m a formula relating dimensionless mass transport to friction speed and bed slope is proposed and mass transport data for five uniform sand samples and one non uniform sand sample are shown to fit the equation well in particular the relationship reveals an overshoot in mass transport slightly above threshold collisions a feature also evident when previous experimental data is re examined as the number of mid air collisions between the saltating particles increases greatly with wind speed the overshoot may occur as a result of increasing energy losses resulting from the collisions finally it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate

vertical sorting in bed forms flume experiments with a natural and a trimodal sediment mixture two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport in the two sets of experiments we used a sediment mixture from the river rhine and a trimodal mixture respectively the vertical sorting profiles showed a downward coarsening trend within the bed forms and in some experiments an essentially immobile coarse bed layer formed underneath the migrating bed forms three mechanisms contribute to the formation of such a coarse bed layer 1 the avalanching process at the lee face 2 conditions of partial transport in which a relatively large amount of coarse material does not participate in the transport process and 3 the winnowing of fines from the trough surface and subsurface the experiments show that vertical sorting fluxes not only occur through net degradation or aggradation but also through the migration of bed forms and through the variability in trough elevations this is contradictory to the way vertical sorting processes are modeled in most existing sediment continuity models for nonuniform sediment the present study is therefore also a plea for modifying existing sediment continuity models to account for vertical sorting processes other than through net aggradation or degradation

the application of high resolution 3d seismic acquisition techniques for carbonate reservoir characterization in china this paper describes major challenges to seismic acquisition in china s tarim basin figure 1 and how high resolution 3d seismic surveys can improve reservoir imaging in this area

bed load transport of mixed size sediment fractional transport rates bed forms and the development of a coarse bed surface layer fractional transport rates bed surface texture and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar the start up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration the start up and equilibrium observations together allow for the first time an unambiguous description of the mutual adjustment among the transport the bed configuration and the bed surface as the transport system moves toward equilibrium we find that a substantial interaction exists among the transport bed surface and bed configuration bed forms and a coarse surface layer coexist over a range of bed shear stress under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer at higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms if the bed surface is defined as that over which the bed forms move a coherent relation between the bed surface texture and the transport grain size distribution may be defined at equilibrium the transport rates of all fractions were not equally mobile defined as identical transport and bed grain size distributions although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well mixed start up condition to an equilibrium state development of a partial static armor wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport is suggested to explain these adjustments between the transport and bed surface grain size distributions constraints on equilibrium mixed size sediment transport are defined the special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed

flexural fold structures and active faults in the northern western weihe graben central china abstract field investigations and analyses of tectonic topography related to late pleistocene holocene activity of faults and fault related flexural folds in the northern western weihe graben central china reveal that 1 four main active faults are present in the study area the beishan piedmont fault bpf the kouzhen guanshan fault kgf the qishan mazhao fault qmf and the weihe fault wf of which the bpf kgf and wf are normal faults whereas the qmf is a left lateral strike slip fault 2 active flexural folds trending ene wsw are widely developed on the late pleistocene holocene loess tablelands and alluvial fans as well as on terrace risers 3 vertical slip rates on the bpf kgf and wf are 0 5 1 1 mm yr and the left lateral slip rate on the qmf is 1 5 mm yr and 4 a recent seismic faulting event with a magnitude m 7 occurred in the past 3000 yr our results show that active faults and fault related flexural folds are developing in the weihe graben under an ongoing extensional regime with listric faulting occurring along pre existing faults associated with the spreading of continental crust in intracontinental graben systems around the ordos block

sorted bed forms as self organized patterns 2 complex forcing scenarios 1 we employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated a priori unpredictable behavior this happens because the system responds to a change in wave characteristics by attempting to self organize into a patterned seabed of different geometry and spacing the new wavelength might not have enough time to emerge before a new change in wave characteristics occurs leading to less regular seabed configurations the new seabed configuration is also highly dependent on the preexisting morphology which further limits the possibility of predicting future behavior for the same reasons variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms spatial variability in grain size distribution and different types of net sediment aggradation degradation can also result in the development of sorted bed forms characterized by a less regular shape numerical simulations qualitatively agree with observed geometry spacing and height of sorted bed forms also in agreement with observations is that at shallower depths sorted bed forms are more likely to be affected by changes in the forcing conditions which might also explain why in shallow waters sorted bed forms are described as ephemeral features finally simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms instead variations in sorted bed form characteristics may result from variations in local hydrodynamic and or sedimentary conditions

the effects of kinetic sorting on sediment mobility on steep slopes in poorly mobile static armour sorting is usually considered the result of hiding exposure effects we called this effect static sorting in opposition to very efficient grain to grain mechanisms produced by a mobile mixture called kinetic sorting we hypothesized that kinetic sorting can be an important contributor to the morphodynamics of mountain streams and attempted to demonstrate this with new flume experiments two long runs were produced with natural poorly sorted sediments and with transport stages of the coarse fraction defined by the ratio between the shear stress and the critical shear stress for transport smaller and higher than 1 respectively both runs produced an efficient transfer downstream of the injected material but with a major difference the first run no kinetic sorting produced permanent armour figuring clusters akin to what has already been observed in similar experiments the second run with kinetic sorting also produced bed armouring but this armour was periodically totally destroyed leading to substantial bed erosion this phenomenon was explained by kinetic sorting the effects of which are to produce an efficient downward migration of fine materials and bed surface armouring the consequence is that fine materials are hidden to the flow during aggradation allowing the slope to attain values much steeper than would have been expected at equilibrium for the mixture however whereas the surface armouring tends to stabilize the bed construction of a layer of fine sediments at the subsurface also contributes to making it very unstable these two contradictory effects explain the complex bed behaviours and the existence of very large bedload and slope fluctuations copyright 2014 john wiley sons ltd

dynamical processes on desert pavements and the healing of surficial disturbances abstract mature desert pavements are traditionally regarded as hallmarks of stability but their stability is dynamic not static in a study aimed at documenting this dynamic stability and its role in healing surface disturbances experiments were performed over a 5 yr period on small cleared patches or plats on pavement surfaces in panamint valley california these experiments show that stones from plat edges begin to resurface the clearing at rates of about 1 per year on 40 cm square plats and 10 per year on 10 cm square plats stones contributing to the regenerated pavement have smaller average diameters than stones on the surrounding pavement cavities 5 10 cm deep formed in mature pavement by removal of embedded boulders fill by ravel and slope failure after five years cavity depth has been reduced by as much as 60 forty year old boulder cavities are nearly completely refilled and have been repaved by smaller than average pavement stones gaps caused by removal of small stones 2 3 cm have completely healed in 5 yr displacement of surface stones by small animals is a major component of the healing process

vertical sorting and the morphodynamics of bed form dominated rivers a sorting evolution model existing sediment continuity models for nonuniform sediment suffer from a number of shortcomings as they fail to describe vertical sorting fluxes other than through net aggradation or degradation of the bed and are based on a discrete representation of the bed material interacting with the flow we present a new type of sediment continuity model that is based on a stochastic description of the bed surface rather than discrete bed layers the model is aimed at conditions dominated by bed load transport wherein the river bed is fully covered by river dunes application of the model should be limited to spatial scales covering a significant number of bed forms the resulting model i e the sorting evolution model is suitable for unsteady conditions as it takes into account the time evolution of for instance vertical sorting in modeling net aggradation or degradation of the river bed the present paper lists the various submodels of a morphodynamic model system to which the sorting evolution model is applied we compare the results of the morphodynamic model system to measured data from two flume experiments the new model shows reasonable results for the predicted time evolution of the vertical sorting profile as well as the time evolution of the grain size distribution of the bed load transport yet the model does not properly include sorting mechanisms associated with partial transport and the winnowing of fines from the trough surface and subsurface the model serves as a basis for a future simplification of the model into a new stochastics based bed layer type sediment continuity model in which vertical sediment fluxes are included in a parameterized way

texture and composition of titan s equatorial sand seas inferred from cassini sar data implications for aeolian transport and dune morphodynamics the texture composition and morphology of dunes observed in the equatorial regions of titan may reflect present and or past climatic conditions determining the physio chemical properties and the morphodynamics of titan s dunes is therefore essential to understanding of the climatic and geological history of the largest moon of saturn we quantitatively derived average surface properties of dune and interdune areas texture composition from modeling of the microwave backscattered signal and monte carlo inversion of the despeckled cassini sar data over titan s three largest sand seas belet shangri la and fensal we present the first analysis of the backscatter functions extracted from despeckled sar images that cover such a large range in incidence angles including data from the beginning of the cassini mission up to its grand finale we show that dunes and interdunes have significantly different physical properties dunes are found to be more microwave absorbent than interdunes additionally potential secondary bedforms such as ripples and avalanches may have been detected providing potential evidence for currently active dunes and sediment transport our modelling shows that the interdunes have multi scale roughnesses with higher dielectric constants than the dunes which have a low dielectric constant consistent with organic sand the radar brightness of the interdunes can be explained by the presence of a shallow layer of significantly larger organic grains possibly non mobilized by the winds together our findings suggest that titan s sand seas evolve under the current multi directional wind regimes with dunes that elongate with their crests aligned in the residual drift direction

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

pseudo feathery dunes in the kumtagh desert abstract the kumtagh desert is the last explored desert in china the unique patterns on aerial photographs and satellite images have led previous researchers to the conclusion that the kumtagh desert is the only place over the world where typical feathery dunes are developed and that some unique wind regimes are at work recent field investigation reveals that the feathery patterns in fact reflect the albedo contrasts between the seif dunes bright dune like drifts and the dark inter dune lag sediments the wind regime in the area containing the so called feathery dunes is typical of the regime that produces seif dunes that were considered to be shafts of the feathery dunes and the vanes of the feathery dunes are in fact bright dune like drifts with indistinct height from the surroundings rather than true transverse dunes further analysis indicates that the sediments of seif dunes dune like drifts and dark inter dune flats differ in grain size distribution mineralogy reflectance spectrum and particle micro morphology sediments that constitute seif dunes and dune like drifts are finer and poorly rounded contain relatively more quartz and hence have higher albedo while the sediments that constitute the dark inter dune flats are coarser and better rounded contain far less quartz but relatively more andesite and dacite and hence have lower albedo as a result what was originally perceived as unique feathery dunes are in fact only pseudo feathery dunes composed primarily of seif dunes this discovery also demonstrates that blowing sand drifts can form dune like patterns that remain visible for long periods of time it is important to distinguish between the patterns of true dunes and those of dune like drifts pseudo dunes that cause albedo contrasts in interpreting remote sensing images

bedform alignment in directionally varying flows many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport for this reason the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand covered board in steady winds transverse oblique and longitudinal bedforms were created by changing only two parameters the angle between the two winds and the proportions of sand transported in the two directions regardless of whether the experimental bedforms were transverse oblique or longitudinal as defined by the bedform trend relative to the resultant transport direction they all had trends that yielded the maximum gross transport across the bedforms the fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

two modes for dune orientation earth s sand seas dune fields experience winds that blow with different strengths and from different directions in line with the seasons in response dune fields show a rich variety of shapes from crescentic barchans to star and linear dunes these dunes commonly exhibit complex and compound patterns with a range of length scales and various orientations which up to now have remained difficult to relate to wind cycles here we develop a model for dune orientation that explains the coexistence of bedforms with different alignments in multidirectional wind regimes this model derives from subaqueous experiments which show that a single bidirectional flow regime can lead to two different dune orientations depending on sediment availability i e the erodibility of the bed sediment availability selects the overriding mechanism for the formation of dunes increasing in height from the destabilization of a sand bed with no restriction in sediment availability or elongating in a finger on a non erodible surface from a localized sand source these mechanisms drive the dune orientation therefore dune alignment maximizes dune orthogonality to sand fluxes in the bed instability mode while dunes are aligned with the mean sand transport direction in the fingering mode applied to earth s deserts the model quantitatively predicts the orientation of rectilinear dunes and their superimposed patterns this field study suggests that many linear dunes on earth elongate from sources and are simply aligned with the mean sand transport direction

a theoretical model for aeolian polydisperse sand ripples abstract a continuum model is constructed to investigate the effects of sediment heterogeneity on the formation of aeolian sand ripples and the grain size sorting over the bed forms the evolutions of small perturbations of both bed elevation and bed composition are studied analytically by a linear stability approach in this model the transport of polydisperse sediment is considered as the integrated processes of saltation reptation caused by impact driven and creep caused by gravity driven and drag driven theoretical analysis indicates that size selective sand transport leads to grain sorting phenomena which appreciably affect the dynamics of the formation of aeolian sand ripples the sediment heterogeneity is beneficial to the stability of sand bed and that the wavelength is lengthened compared with the case of uniform sand longer ripples appear when the sediment heterogeneity is characterized by a poorly sorted on the other hand the relative volume fraction of the different grain size in the sediment heterogeneity appreciably affects the characteristics of sand ripples moreover it is found that the theoretical model successfully reproduces the observed tendency of coarser grains to accumulate towards the crests of sand ripples while finer grains accumulating towards the troughs lastly a physical interpretation of the theoretical model results which qualitatively agree with field observations and wind tunnel experiments is given

grain size characteristics of linear dunes in the southwestern kalahari abstract linear dunes in the southwestern kalahari sand sea exhibit characteristic grain size and sorting patterns in which sands become coarser better sorted and more finely skewed from interdune to crest in addition at a regional scale sands tend to become finer and better sorted in the direction of transport these patterns can best be explained in terms of the differential movement of sand in creep and saltation populations whereby the saltating population moves more rapidly and is concentrated downwind and in dunes slower moving creep populations remain close to source areas and in interdunes the fines will tend to be concentrated in interdunes because they can penetrate between coarse grains where they are protected from further movement this type of model probably explains most grain size and sorting relationships in desert sand seas

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

sediment flux from the morphodynamics of elongating linear dunes although dunes are very common bedforms in terrestrial sand seas the description of linear dune growth either by extension or lateral accretion is still hindered by our limited understanding of the underlying mechanisms therefore sand flux estimates from remote imagery rely essentially on the migration speed of barchan dunes but not on the dynamics of linear dunes here we use 50 yr of high resolution aerial and satellite imagery of the tenere desert niger the world s largest source of mineral aerosols to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration as they elongate from topographic obstacles in a zone of low sediment availability with multimodal winds these elongating lee dunes are ideal to isolate and quantify linear dune growth only by extension using similar conditions in a numerical model we show how deposition downstream of low hills may result in nucleation and development of bedforms from elongation we derive the local sand flux parallel to the linear dune crests this study shows that the morphodynamics of linear dunes under complex wind regimes can also be used for assessing sediment flux and wind conditions comparably to the more established method of using sand flux estimates perpendicular to the barchan dune crests in zones of unidirectional wind

grain size segregation and stratigraphy in aeolian ripples modelled with a cellular automaton aeolian ripples are distinguished by coarse grained crests finegrained troughs and thin veneers of coarse grains on the upwind stoss slopes migration of these sorted bedforms during periods of net deposition results in a characteristic inversely graded stratigraphy that distinguishes aeolian sandstones from those of fluvial origin1 here we investigate the formation and evolution of aeolian ripples using a cellular automaton model of a sandbed which incorporates two grain sizes and is subject to the impacts of episodically hopping saltating grains using this model we identify the physical processes responsible for both the spatial sorting and the stratigraphic signature of the ripples high energy saltating grains are found to eject small grains preferentially from the impact site leading to a coarsening of the heavily bombarded stoss slopes coarse grains do not in general leap far enough to escape the shadow zones on the downwind lee slopes and therefore tend to accumulate and cycle around the ripple crests small grains on the other hand are ejected at higher velocities enabling them to hop further incorporating net deposition into the model results in a stratigraphy closely resembling thin planar laminae pinstriping found in aeolian sandstones the inverse grading of grain sizes is a direct consequence of their different hop lengths

emergence of oblique dunes in a landscape scale experiment the dynamics of dune evolution under bimodal wind regimes are poorly understood owing to a lack of long term wind records and the limitations of most experimental set ups a 4 year landscape scale experiment in the tengger desert mongolia demonstrates that the orientation of oblique dune crests is controlled by the wind regime

methane storms as a driver of titan s dune orientation titan s equatorial dunes propagate eastwards whereas titan s surface winds blow towards the west atmospheric simulations suggest that tropical methane storms generate strong eastward gusts that may dominate sand transport on titan s surface

mean sediment residence time in barchan dunes when a barchan dune migrates the sediment trapped on its lee side is later mobilized when exposed on the stoss side then sand grains may undergo many dune turnover cycles before their ejection along the horns but the amount of time a sand grain contributes to the dune morphodynamics remains unknown to estimate such a residence time we analyze sediment particle motions in steady state barchans by tracking individual cells of a 3 d cellular automaton dune model the overall sediment flux may be decomposed into advective and dispersive fluxes to estimate the relative contribution of the underlying physical processes to the barchan shape the net lateral sediment transport from the center to the horns indicates that dispersion on the stoss slope is more efficient than the convergent sediment fluxes associated with avalanches on the lee slope the combined effect of these two antagonistic dispersive processes restricts the lateral mixing of sediment particles in the central region of barchans then for different flow strengths and dune sizes we find that the mean residence time of sediment particles in barchans is equal to the surface of the central longitudinal dune slices divided by the input sand flux we infer that this central slice contains most of the relevant information about barchan morphodynamics finally we initiate a discussion about sediment transport and memory in the presence of bed forms using the advantages of the particle tracking technique

growth mechanisms and dune orientation on titan dune fields on titan cover more than 17 of the moon s surface constituting the largest known surface reservoir of organics their confinement to the equatorial belt shape and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply herein we present a comprehensive analysis of titan s dune orientations using automated detection techniques on nonlocal denoised radar images by coupling a new dune growth mechanism with wind fields generated by climate modeling we find that titan s dunes grow by sediment transport on a nonmobile substratum to be fully consistent with both the local crestline orientations and the eastward propagation of titan s dunes the sediment should be predominantly transported by strong eastward winds most likely generated by equinoctial storms or occasional fast westerly gusts additionally convergence of the meridional transport predicted in models can explain why titan s dunes are confined within 30 latitudes where sediment fluxes converge