avalanche grainflow on a simulated aeolian dune 1 avalanches maintain the slipface of aeolian dunes which alters their airflow characteristics and sediment dynamics and results in the development of grainflow cross bedding we report on a series of experiments in which avalanches were observed on a 1 1 replica of a small 1 2 m brink height transverse dune in the dune simulation wind tunnel under wind velocities of 8 11 m s 1 changes in slipface topography were observed photographically and measured utilizing a 3 d laser scanner with 1 mm2 spatial resolution avalanches in noncohesive sands were observed to progress through scarp recession from the point of initiation and continue until the slope angle is reduced changes in local slope confirm that the steep pre avalanche mean slope relaxes to a uniform value equal to the angle of repose of the test sand 32 over all involved portions of the slipface avalanche volumes are measured and demonstrate that avalanche magnitude is independent of wind speed over the range of velocities observed this independence provides the potential to significantly simplify the modeling of grainflow as a function of only the total cross brink sediment transport

measurement of eolian sand ripple cross sectional shapes assuming that the profiles of eolian sand ripples remain constant over a short distance it is possible to measure their shapes by casting a shadow perpendicular to the ripple crests we describe a method whereby this can be done accurately and easily tests of the method demonstrate that accuracies on the order of one sand grain diameter can be achieved

secondary flow deflection in the lee of transverse dunes with implications for dune morphodynamics and migration measurements of lee side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee side separation vortex over closely spaced transverse dune ridges a differential deflection mechanism is presented that explains the three dimensional pattern of lee side airflow structure for a variety of incident flow angles these flow patterns produce reversed along dune and deflected surface flow vectors in the lee that are inferred to result in net lateral diversion of sand transport over one dune wavelength for incident angles as small as 10 from crest transverse i e 80 from the crest line this lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees reversed and multi directional flow occurs for incident angles between 90 and 50 these results document the three dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model copyright 2013 john wiley sons ltd

size segregation during aeolian saltation on sand dunes recent monte carlo computer simulations have modelled the local geometric mechanism by which mixed populations of differently sized particles order themselves when shaken such that larger particles rise at the expense of the smaller this process called size segregation is distinct from the mechanism of sifting where smaller particles fall between the interstices of larger particles size segregation is applied here to the sand grain populations found on the surfaces of dunes which are involved in a sorting process driven by the wind we present data from four barchan dunes located in the eastern province of saudi arabia a simple model is used to demonstrate that size segregation driven by saltation impacts is energetically plausible at the wind velocities typically encountered on dune surfaces laboratory tests which support this model are also discussed

reservoir quality controls on rotliegend fluvio aeolian wells in germany and the netherlands southern permian basin impact of grain coatings and cements abstract reservoir quality in sandstones can be a function of temperature pressure and chemical alterations over time some intrinsic properties of sandstones can additionally impact the evolution of reservoir quality the detrital composition depositional environment and properties controlled thereby dictate some of the diagenetic changes extrinsic processes like external fluids e g from hydrothermal events or dewatering of clay minerals can additionally result in changes of reservoir quality one of these properties are grain coatings on quartz grains which can strongly affect pore cementation by quartz overgrowths to predict and infer the quality of undrilled reservoirs constraints are needed to predict their occurrence we analyse two reservoir settings in the southern permian basin and delineate the impact of depositional systems lithologies detrital and authigenic composition on reservoir quality and with regard to grain coatings results highlight the importance of diagenetic alterations and the source area in regards to burial development in different parts of the southern permian basin samples from the rotliegend bahnsen sandstone member from germany and the slochteren sandstone formation from the netherlands have been petrographically and petrophysically analysed and linked to depositional environments our data shows that the emplacement and presence of grain coating minerals cannot be simply linked to the depositional environment or processes controlled thereby e g grain size and sorting neither on a reservoir nor on a basin scale the dissolution of alumosilicates additionally has no consistent impact on the authigenesis of chloritic grain coatings on the reservoir or basin scale in this study this implies the necessity of sample specific analyses and importance of the assessment of diagenetic alterations when analysing modelling or predicting reservoir quality in similar settings worldwide

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

contribution of aeolian sand to backbarrier marsh sedimentation abstract the frequency of island overwash is commonly assumed to be the main factor regulating sand flux to backbarrier marshes however aeolian transport of sand across a barrier has received little attention in saltmarsh research the contribution of aeolian sand to backbarrier marshes is examined here with marsh cores and measurements of wind blown sediment transport across transgressive and regressive parts of a barrier island backbarrier marshes fronted by a wide washover fan or a high elevation dune ridge were supplied with aeolian sand that significantly contributed to vertical accretion especially near the marsh barrier island edge the percent volume of the marsh originating from aeolian sand based on the upper 1 cm of marsh sediment decreased by an order of magnitude only 20 m from the dune marsh boundary sandy beds sampled in every marsh core at depth are composed of aeolian sand and were likely emplaced over time after storms deposited a sandy washover fan near the marsh and or reduced vegetation cover across the island sand beds preserved within backbarrier marsh deposits are commonly thought to have been emplaced rapidly during a storm however post storm aeolian transport should also be considered as an important mechanism for forming sand beds over a longer period of time within saltmarsh strata the contribution of aeolian sand to backbarrier marsh accretion should increase as sea level continues to rise causing many barrier islands to narrow thereby reducing transport distances between the dune and marsh along developed coastlines however anthropogenic factors associated with increased development largely decrease the connectivity between the dunes and backbarrier marsh

relations of sand trapping efficiency and migration speed of transverse dunes to wind velocity a self consistent model which describes transverse dune migration in equilibrium is introduced it shows that an equilibrium expression for dune migration speed cd must take into account sand trapping efficiency te and that te is strongly related to the wind speedup over the windward surface an expression for sand trapping efficiency te is analytically derived from a microscale analysis of sand grain deposition on the slip face sand trapping efficiency te is mainly determined by shear velocity on a level surface u and rapidly decreases as u increases for each dune height h dune migration speed cd first increases and then decreases monotonically after reaching the maximum as the shear velocity on a level surface u increases dune migration speed cd is not inversely proportional to dune height h for low dunes small sand trapping efficiency te suppresses cd whereas for high dunes wind speedup and large te resist the decrease of cd some field data show the same tendency the dune to plane bed transition observed in subaqueous and venusian bedforms could be associated with the decrease of sand trapping efficiency te copyright 2000 john wiley sons ltd

sedimentology of a proterozoic erg the venkatpur sandstone pranhita godavari valley south india reappraisal of the late proterozoic venkatpur sandstone indicates that the bulk of the sandstone is aeolian in origin aeolian stratification types namely i inverse graded translatent strata ii adhesion laminae iii grainflow strata and iv grainfall strata are present throughout the outcrop belt nine facies have been identified that represent both aeolian and related aqueous environments within a well developed erg cosets of large cross beds at the bellampalli section in the nw of the study area record dune fields in the interior of the sand sea to the se at the godavari river and ramgundam sections a progressive increase in the relative proportion of the flat bedded to cross bedded facies and intercalated non aeolian facies delineates the transition from the dune field to sand sheet environment an alternating sequence of aeolian and marine sediments at laknavaram in the extreme se marks the termination of the sand sea palaeocurrent data suggest that the nw se trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction abundant horizontally stratified units in the vankatpur sandstone do not always represent the interdune sediments on the basis of the thickness and geometry of the units nature of bounding surfaces and associated facies sequence the facies is variously interpreted to represent interdune inland sabkha sand sheet and coastal sand flat deposits

eolian ripples as examples of self organization in geomorphological systems abstract ripples arise spontaneoulsy from sandy beds subjected to winds sufficient to cause the saltation of surficial grains while the initially fastest growing wavelengths are 4 6 times the mean hop length of grains impelled downwind by long trajectory high energy saltating grains the ripple wavelengths developed over long periods can be several times this numerical simulations based upon techniques pioneered by p k haff and coworkers reveal that a complex process of ripple coalescence is responsible for this evolution of the ripple field small fast moving ripples catch up with and are absorbed by larger slower forms each such merger resulting in a growth of the mean wavelength and a decline in the relative dispersion of the wavelengths an heuristic argument is presented which accounts for the fast initial growth and subsequent slowing of growth of the mean wavelength as the ripple field evolves ripple cross sectional shape appears to be controlled in large part by a distrianment function which represents the declining likelihood of a grain stopping on a surface as it slopes more steeply downwind that the resulting asymptotic wavelength in the simulations depends strongly upon impact angle supports sharp s 1963 suggestion that the length of the shadow zone plays an important role in controlling the final ripple wavelength

morphometric analysis of slipface processes of an aeolian dune implications for grain flow dynamics grain flows are an integral part of sand dune migration they are a direct response to the local windregime and reflect complex interactions between localized over steepening of a dune slipface andcomplex turbulent airflow on the lee slope grain flows are primarily responsible for delivering sediment to the base of a dune thus driving slipface advancement yet there are few constraints on their morphological and spatial characteristics or the amount of sediment that is redistributed bythese flows using a combination of high resolution terrestrial laser scanning and video recordings four distinct grain flow types are identified based on morphology and area on a dune slipface grainflow morphologies range from small superficial flows to larger flows that affect greater portions of the slipface moving significant amounts of sediment detailed field observations are presented of the dynamics of lee slopes including measurements of the initiation location thickness magnitude and frequency statistics of grain flows as well as volume estimates of redistributed sediment for each grain flow observed high resolution laser scans enable accurate quantification of bulk sediment transfer from individual grain flows and can be used to study grain flows in a variety of environments a categorization of grain flow morphologies is presented that links styles of flows with wind strength and direction turbulent airflow sediment deposition and environment

sediment transport dis continuity across a beach dune profile during an offshore wind event abstract flow dynamics and sediment transport responses over a large vegetated foredune at prince edward island canada during an offshore wind event are examined data were collected along an instrumented transect that extended from the dune crest down the lee side seaward slope of the dune across a wave cut scarp and on to the back beach when the wind direction at the dune crest was approximately crest normal less than about 15 deviation the mean near surface flow directions along the dune slope and on the back beach were generally onshore indicating reversed onshore flow relative to the regional offshore wind direction although flow patterns were consistent with a lee side recirculation eddy large excursions in flow direction were also prevalent suggesting that the eddy was unstable and alternated with highly turbulent wake flow as wind direction at the crest veered to greater than 20 from crest normal lee side winds shifted toward strongly alongshore flow with minimal directionally variability on the dune slope the wind vectors were slightly offshore whereas on the back beach they were slightly onshore wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer mean particle counts averaged over a 15 min interval derived from laser sensors positioned at the crest were large 7 76 per second in comparison to those measured in the immediate lee of the crest 0 52 per second and farther down the dune slope these results demonstrate that sediment transport across the beach dune system was spatially discontinuous during this offshore wind event rebuilding of the dune ramp at the toe of the scarp occurred quite independently of and with a different sediment source than the broadening of the dune crest which was fed with sediment from the landward side of the foredune such process decoupling is an example of the complexity by which foredunes evolve or are maintained and as with previous research reinforces the importance of offshore and alongshore wind events to beach dune morphodynamics

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

analysis and classification of bedload transport events with variable process characteristics abstract knowledge about the magnitude of bedload fluxes at given hydraulic conditions in natural streams is essential for improved process understanding for the application calibration and validation of bedload transport formulas and for numerical sediment transport models nonetheless extensive field measurements of bedload transport are challenging and therefore data from such efforts are rare bedload transport has been measured comprehensively at the downstream section of the urslau torrent in salzburg austria since 2011 we used an integrative monitoring system that combines direct mobile basket sampler slot sampler and indirect measuring devices geophone plates continuous information about the intensity and distribution of bedload transport within the channel cross section is available in high spatial and temporal resolution seven geophone plates at a stream width of 8 m are part of a measurement system that delivers data in 1 min intervals these geophone data are calibrated using results of direct bedload measurements providing an opportunity to calculate bedload rates and bedload yields in selected time periods continuous data on the bedload transport process over three years enabled assessing several bedload transport events the investigation of bedload transport rate discharge relationships reveals order of magnitude changes for individual events we observed shifts in the data reflecting different bedload rates at comparable hydraulic conditions this study reveals that variable sediment supply conditions affect the prevailing bedload transport rates at the urslau stream calculating the bedload transport efficiency enables comparing bedload transport events that exhibit similar process characteristics finally we provide a conceptual model of bedload transport process types as a function of bedload transport efficiency and dimensionless stream power

on the modelling of two dimensional aeolian dunes in this study the modelling of aeolian dunes is treated from the background of the physical processes that form them the paper concentrates on two dimensional modelling which can be applied to transverse and barchan dunes the basic assumptions and equations are discussed and three models are presented a a kinematic model which is based on a linear variation of the sediment transport with the topographic height b an analytical model based on a boundary layer model of the wind velocity and bagnold s linearized sediment transport formula c a generalization of this analytical model in a computer program which includes a simple routine for simulating the redistribution of sediment through avalanching the relative importance of the simplifications is considered and indications are given for the practical applications of these three models in field studies

changes in grain size of sand in transport over a foredune suspended sand is sampled at several heights and positions on a beach and foredune providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport grain size analysis is used to study the changes in grain size distribution during landward transport mean grain size and sorting decrease during transport changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune sediment trapped on the seaward slope at a height of 30 50 cm above the surface closely resembles the sediment trapped landward which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles the movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune leading to a change from saltation on the beach to modified saltation and suspension on the foredune small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward resulting in a gradual decrease in grain size of the landward deposited sediment

experimental verification of aeolian saltation and lee side deposition models we report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes in steady sea breeze conditions on a 3 m tall dune at point ano nuevo california we measured simultaneously the near brink wind speed and the deposition on both horizontal and lee face collector platforms we then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds in addition the total vertical number flux of grains is constrained to roughly 107 108 grains m 2 s 1 at shear velocities of 0 33 0 40 m s 1 the lee side deposition pattern which shows the expected maximum in deposition rate at a distance of several decimetres from the brink is also well fit by the model in addition simultaneous collection of horizontal and lee deposition patterns along with the numerical simulation of these patterns strongly implies that the windfield in the lee of this particular dune is best described as a non recirculating wake grainflows on the lee face are caused by failure of grainfall depositional bumps our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows rather than to increase their size implying that very large thick grainflows require a different mechanism

grainfall processes in the lee of transverse dunes silver peak nevada grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross beds in the geological record and provide a key to the interpretation of the aeolian rock record despite their recognized importance there have been very few field laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes as part of an ongoing study the relationships among grainfall wind speed and direction stoss sand transport rates and dune morphometry height and aspect ratio were investigated on four relatively small straightcrested transverse dunes at silver peak nevada between 55 and 95 of the total grainfall was found to be deposited within 1 m of the crest and 84 99 within 2 m depending primarily on dune size and shape grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent with a weak positive dependence on wind speed for small dunes with low aspect ratios grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest whereas at increased distance from the dune crest it eventually approached the smaller decay rates observed on the large dunes no dependence of grainfall on wind speed was observed for these small dunes comparison of field data with predictions from anderson s 1988 saltation model of grainfall based on the computation of saltation path lengths indicates lack of agreement in the following areas 1 deposition rate magnitude 2 variation in decay rate with wind speed and 3 the magnitude and location of the localized lee slope depositional maxima the silver peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall this work confirms earlier speculation by mcdonald anderson 1995 that temporary turbulent suspension or modified saltation of relatively large grains does occur within the dune wake so that transport distances generally are larger than predicted by numerical simulations of true saltation

reconstruction of linear dunes from ancient aeolian successions using subsurface data permian auk formation central north sea uk abstract a series of well logs and cores penetrating the predominantly aeolian auk formation permian rotliegend group central north sea uk have been evaluated to determine the morphology and style of migratory behaviour of the original dune bedforms the overall depositional environment and to assess implications for reservoir heterogeneity this has been achieved by detailed facies analysis of subsurface datasets and by comparison of the observed sedimentary styles of accumulation to analogous modern aeolian dune fields aeolian bedform type morphology detailed migratory behaviour and the nature of the accumulation surface have been interpreted analysis of the facies architecture of preserved cross bedded sets and cosets indicates accumulation on a dry substrate via the migration and climb of large linear bedforms that possessed low angle inclined lower plinths up to 15 m thick dune plinth elements are dominated by wind ripple and reworked wind ripple strata and were preferentially preserved as successive bedforms migrated over one another at low angles packages of grainflow dominated strata representative of accumulation on the higher part of the bedform lee slope are less common and tend to be preserved mostly in the upper parts of large cosets of strata 30 m thick large linear bedforms were separated by dry interdune areas although the primary direction of sand transport was along the elongated crests of the bedforms a secondary component of transverse motion enabled the lateral migration and preferential preservation of lee slope deposits that arose from a minor oblique component of bedform migration in places the architecture records the preservation of small barchanoid dune deposits either within interdune depressions or superimposed on the lower flanks of the large linear bedforms the preserved aeolian facies types exert a primary control on reservoir quality few previous studies have documented linear dunes in ancient successions these findings represent a valuable case example

aeolian sand transport at the lanphere dunes northern california aeolian sand transport was studied at the lanphere dunes a coastal dune complex in northern california by comparing slipface advance rates with transport predicted based on local wind data the slipfaces of a 2 5 m high transverse ridge and 10 m high parabolic dune were monitored over a period of three months to estimate sand discharge the study was performed during the dry season which has the maximum sand driving potential over the three month study period average sand discharge was 12 5 m3 per m width per year at the transverse ridge and 8 8 m3 per m width per year at the parabolic dune a method was developed for modelling slipfaces that are sinuous and where sediment transport rates are not constant across the width of the slipface field measurements were used to generate three dimensional representations of dune slipfaces periodic measurements over the course of three months were used to compute the volume of displaced sediment theoretical sand transport was computed from local wind data using the bagnold model and compared with the observed transport rates predicted rates were substantially lower than observed rates wind velocities rarely exceeded the threshold velocity discrepancies between the observed and predicted values appear to be caused by a combination of wind data recording procedures and differences between wind velocities at the anemometer location and the site where sand transport was measured wind data collected by weather bureaux have been utilized in numerous studies for modelling sediment transport such data typically have sample intervals of one hour or greater and are often averaged prior to reporting the effect of averaging was investigated by comparing sand transport estimates based on daily average wind velocities with those based on the original hourly observations the daily average data were depleted of high velocity winds and sand transport estimates were accordingly much lower than those based on the hourly data copyright 2000 john wiley sons ltd

a model for flow over two dimensional bed forms when non cohesive sediment is set in motion by a unidirectional flow waves of sand often result moreover these waves typically are asymmetrical with steep lee faces that produce flow separation behind each wave a wake region forms which grows in height and decays in strength with distance downstream producing a near bottom acceleration the no slip condition at the bed however requires an internal boundary layer to form beneath the wake region which retards the flow there the interaction of these flow processes produces a local maximum in the boundary shear stress downstream of a bump even over an otherwise flat bed because the erosion rate is proportional to the stress divergence for bed load and weak suspended load transport erosion will occur upstream of this point and desposition will occur downstream of it thus influencing the bottom shape herein the equations of motion are solved for the accelerating internal boundary layer beneath a wake the resulting velocity and boundary shear str

eolianite grain size distributions as a proxy for large changes in planetary atmospheric density a digitized version of the ahlbrandt 1979 global dune dataset and data for distributions in figure are available at https github com erikgoosmann aeoliandata

grain size dependence of eolian saltation lengths during snow drifting eolian saltation a primary process in the transport of fine granular material by wind produces a variety of geophysical effects on earth and other planetary surfaces wind tunnel experiments were carried out to investigate the dependence of saltation on grain size the saltation length of snow particles was estimated at size intervals of 0 05 mm in diameter by measuring local vertical mass fluxes in 17 snow collectors arrayed at the lee end of the snow surface the measured mean saltation length of snow particles of 0 01 1 mm in diameter ranged from 0 1 to 1 0 m at wind velocities of 5 10 m s mean saltation length decreased with increasing diameter and decreasing wind speed we suggest that the probability of the saltation length of a particle at each diameter is described by a monotonically decreasing distribution function that is the shorter the saltation length the higher the frequency of its occurrence one ramification of this distribution is that the mean saltation length does not imply the dominance of saltating particles of this length

lee slope sediment processes leading to avalanche initiation on an aeolian dune 1 in order to detail the governing conditions through which a slipface matures to the point of failure dry sand avalanches were observed in the dune simulation wind tunnel on a 1 1 replica transverse dune with a crest height of approximately 1 2 m areal distributions of grainfall and reptation were measured using traps changes in the slipface elevation were observed using 3 d laser scanning with a vertical accuracy of 0 096 mm for approximately every 1 mm2 of surface area grainfall decayed exponentially from the brink with a constant rate across all wind velocities reptation removed sediment from areas close to the brink and deposited it downslope creating low amplitude cross slope ripples on the slipface a critical length scale separating grainscale and bulk sediment behavior is identified and it defines the lower limit to the validity of angle of repose measurements avalanche initiation occurred in an area of steep surface slope below a sediment bulge with distance from the brink independent of wind velocity the time between avalanches was found to be constant for constant wind velocity

sediment transport by wind toward a general model a general model of eolian sediment transport by both saltation and suspension is developed saltation is analyzed through calculations of single trajectories given initial conditions of liftoff speed angle and spin profiles of particle concentration and mass flux generated by assuming that all trajectories are identical display distinct maxima at the top of the trajectory by incorporating the realistic distribution of trajectories that arises from the stochastic nature of grain impacts with a granular bed the model yields monotonic decreases in particle concentration and mass flux with height in accordance with published empirical data the analysis of suspension as the balance between downward advective flux as a result of the settling of grains and their upward diffusive flux as a result of turbulence results in concentration profiles that fall off as power laws with height also in accordance with empirical data a relation for reference level concentrations is presented for both blowing snow and dust profiles the solutions for saltation and suspension are combined to yield mass flux profiles for the entire range of grain sizes in the bed the saltation layer emerges naturally as that region where the total flux of saltating grains exceeds that of suspended grains the model points clearly to the need for further research on the stochastic grain bed interactions which control both the probability distribution of liftoff velocities in saltation and the production of fine particles in suspension

the propagation of an error in long and sharp s 1964 data on barchan dune movement a misprint in a classic study of desert dunes long and sharp geological society of america bulletin 1964 vol 75 pp 149 156 is reported its influences on subsequent literature are examined the true data of dune migration speed collected by long and sharp are found to show better correlation with dune height than seen in two recent textbooks and a paper by the authors copyright 2002 john wiley sons ltd

sand transportation and reverse patterns over leeward face of sand dune abstract sand saltation has complex interactions with turbulent flow and dune form most models of wind blown sand consider ideal circumstances such as steady wind velocity and a flat surface and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter dune space on the wind flow neglecting the effect of morphology on sand saltation particularly airflow and sand transportation over the leeward slope wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes in order to understand sand dune dynamics lee face airflow and sediment transportation should be paid more attention previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field in addition the reverse sand particles over leeward face could not be collected by sand trap in field numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field in addition the reverse transport of sand particles over leeward face could not be collected by sand traps in field therefore this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope a numerical model of sand saltation over slope terrain is constructed which also considers the coupling effects between air flow and sand particles the large eddy simulation method is used to model turbulent flow sand transport is simulated by tracking the trajectory of each sand particle the results show that terrain significantly alters the turbulent air flow structure and wind blown sand movement especially over the leeward slope here mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow and the mass flux decreases with height in the reversed direction the height of 0 5 h is the height of vortex core in the reversed flow region the vortex core is a critical point in the flow region where few particles are transited in the reversed region the reversed mass flux of sand particles is 25 of the mass flux in the flow direction this research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control

aeolian processes across transverse dunes ii modelling the sediment transport and profile development this paper discusses a model which simulates dune development resulting from aeolian saltation transport the model was developed for application to coastal foredunes but is also applicable to sandy deserts with transverse dunes sediment transport is calculated using published deterministic and empirical relationships describing the influence of meteorological conditions topography sediment characteristics and vegetation a so called adaptation length is incorporated to calculate the development of transport equilibrium along the profile changes in topography are derived from the predicted transport using the continuity equation vegetation height is incorporated in the model as a dynamic variable vegetation can be buried during transport events which results in important changes in the sediment transport rates the sediment transport model is dynamically linked to a second order closure air flow model which predicts friction velocities over the profile influenced by topography and surface roughness modelling results are shown for a the growth and migration of bare initially sine shaped dunes and b dune building on a partly vegetated and initially flat surface results show that the bare symmetrical dunes change into asymmetric shapes with a slipface on the lee side this result could only be achieved in combination with the secondorder closure model for the calculation of air flow the simulations with the partly vegetated surfaces reveal that the resulting dune morphology strongly depends on the value of the adaptation length parameter and on the vegetation height the latter result implies that the dynamical interaction between aeolian activity and vegetation reaction to burial growth rates is highly relevant in dune geomorphology and deserves much attention in future studies copyright 1999 john wiley sons ltd

dune migration and slip face advancement in the rabe crater dune field mars 1 eight overlapping images of a dune slip face in rabe crater 35 e 44 s from the mars global surveyor mars orbiter camera show changes interpreted to be multiple grainflow events that would indicate present day sand saltation and dune migration new occurrences of these features appear sporadically throughout late southern summer and early fall and then no further changes occur throughout winter by the following summer the pattern of old streaks had been almost completely covered by new dark streaks assuming that this activity is typical from year to year migration rates are estimated to be on the order of 1 2 cm per martian year produced by south to southeasterly winds that blow mostly during the southern spring and early summer this slow migration rate is consistent with a present day sediment state that is either transport or availability limited

erosion profiles due to particles entrained by wind application of an eolian sediment transport model erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains a recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry the modeling of vertical erosion profiles in turn provides an exacting test of the transport model realistic distributions of liftoff velocities for saltating grains give rise to kinetic energy flux profiles characterized by a strong maximum as much as 1 4 m above the bed during strong winds kinetic energy flux due to suspended grains also peaks above the bed the height and strength of the maxima depend very strongly on the grain size distribution as grain size diminishes increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface for both saltating and suspended grains kinetic energy flux scales with the fifth power of the wind shear velocity erosion profiles in man made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces lending considerable support for the model the modeling also elucidates several aspects of ventifaction as small ventifacts 1 2 m in diameter and the lower portions of larger ones are predominantly eroded by saltating grains upwind surface irregularities are quickly damped leading to facets that dip upwind and sharply truncate lee surfaces sandblasting by suspended grains dominates along the upper portions of large ventifacts where grain paths are partially deflected by the obstacle and may lead to radially symmetrical flutes and grooves

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

diffusion of sediment on the lee of dune like bedforms theoretical and numerical analysis theoretical and empirical analyses of flow structure sediment transport and sediment size characteristics at the crest of dune like bedforms indicate that it is possible to describe at least semi quantitatively the diffusion and deposition of sediment on the leeside of such structures a numerical program based on this analysis simulates the grain size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching sorting of the bed material in particular appears to exert a strong control on both the grain size and the deposition rate gradients

seasonally active slipface avalanches in the north polar sand sea of mars evidence for a wind related origin 1 meter scale mro hirise camera images of dune slipfaces in the north polar sand sea of mars reveal the presence of deep alcoves above depositional fans these features are apparently active under current climatic conditions because they form between observations taken in subsequent mars years recently other workers have hypothesized that the alcoves form due to destabilization and mass wasting during sublimation of co2 frost in the spring while there is evidence for springtime modification of these features our analysis of early springtime images reveals that over 80 of the new alcoves are visible underneath the co2 frost thus we present an alternative hypothesis that formation of new alcoves and fans occurs prior to co2deposition we propose that fans and alcoves form primarily by aeolian processes in the mid to late summer through a sequence of aeolian deposition on the slipface over steepening failure and dry granular flow an aeolian origin is supported by the orientations of the alcoves which are consistent with recent wind directions furthermore morphologically similar but much smaller alcoves form on terrestrial dune slipfaces and the size differences between the terrestrial and martian features may reflect cohesion in the near subsurface of the martian features the size and preservation of the largest alcoves on the martian slipfaces also support the presence of an indurated surface layer thus new alcoves might be sites of early spring co2sublimation and secondary mass wasting because they act as a window to looser less indurated materials that warm up more quickly in the spring

magnitude and frequency of grain flows on a desert sand dune preliminary studies of the magnitude and frequency of lee side avalanches grain flows on a namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates these studies indicate the potential of using a simple digital video camera technique to document the magnitude frequency and geometry of grain flows on desert sand dunes

coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics avalanche grainflow processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations in sedimentary systems however the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes here for the first time terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee slip face of a barchan sand dune we determine grainfall zone extent and flux and show their variability under differing wind speeds the increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope these findings highlight important interactions between wind speed grainfall and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

spatial and temporal patterns of aeolian sediment transport on an inland parabolic dune bigstick sand hills saskatchewan canada abstract topographic changes from erosion pins and on site meteorological data document the spatial and temporal patterns of aeolian sediment transport at monthly to annual timescales across an active parabolic dune within a vegetation stabilized inland prairie dune field over two years the sediment budget calculated from digital elevation models shows that the total volume of erosion 9890 m 3 is greater than the amount of deposition 6990 m 3 indicating a net loss of 2900 m 3 of sediment or 29 of eroded sediment from the dune sediment erosion occurred mainly on the stoss slope 3600 m 3 36 of eroded sediment but also on the south 2100 m 3 21 and north sides of the dune head 1700 m 3 17 the blowouts along the arms 1740 m 3 18 and the crest 650 m 3 7 erosion from the deflation basin is limited by surface roughness and armoring effects of a gravel lag deposit 100 m 3 1 thus the blowouts currently contribute to maintaining dune mobility because no other sediment input occurs from upwind sediment deposition onto the dune occurred primarily beyond the brink on the south and southeast lee slopes 5500 m 3 80 coinciding with the southeasterly resultant transport direction for november 2004 05 the net loss of about 2900 m 3 29 may be attributed to sediment carried in suspension over and beyond the dune correlation analysis between sediment transport and meteorological variables suggests that monthly to seasonal changes of surface conditions e g vegetation cover ground freezing moisture buffer the relative importance of temperature and precipitation on rates of sediment transport conversely wind correlates well on a monthly to seasonal basis because it is a driver of transport under all types of surface conditions seasonal effects produce a complex interaction between wind climate and surface conditions this leads to a dynamic range of threshold velocities which in turn causes spatial and temporal variations in transport limiting and supply limiting conditions collectively these findings have implications for modeling parabolic dune morphodynamics and sediment transport in mid to high latitude inland settings

pilot study on provenance and depositional controls on clay mineral coatings in active fluvio eolian systems western usa abstract understanding the distribution of grain coating minerals in sandstones may be crucial in evaluating the reservoir quality of deeply buried lithologies since they can inhibit syntaxial overgrowth cementation and thus preserve intergranular pore space in unconsolidated sediments the presence of clay mineral grain coatings and their coverage are often related to the depositional subenvironment grain size sorting and skewness these properties are interpreted to reflect the amount of clay minerals present to form coatings and relate to sediment transport and reworking samples from three active fluvio eolian depositional systems in the usa mesquite flat sand dunes near stovepipe wells ca algodones dunes near brawley ca and coral pink sand dunes near kanab ut were studied using petrography transmitted light and sem and xrd analyses to test and apply the available correlations to fluvio eolian sediment samples the frequently used correlations to the depositional subenvironment grain size sorting and skewness only show poor fits r2

estimating the windward slope profile of a barchan dune a new theoretical scheme is presented to model the shape of a sand dune at equilibrium that does not require iterative calculations of the interaction between the wind flow and topography the model is constructed by incorporating theory based on aerodynamics into a grain scale model that estimates the shear velocity at the dune crest through the calculation of the sand trapping efficiency of the slipface in the lee of the dune published field data collected in southern peru california and southern morocco show that as a dune becomes higher the windward slope becomes steeper for the model proposed the wind flow over a dune was first assumed to be similar to that over a gaussian hill by further assuming a fixed shear velocity on the level surface the windward slope angle and migration speed of dunes in southern peru can be explained to comply with all available data some aspects are still open to further investigation however the theoretical insight presented herein implies that the upper limit of dune height may be greater in windier environments

a kinematic model for the structure of lee side deposits a kinematic model for the structure of the lee side deposit of a dune like bedform gilbert type delta or similar step is developed based on the assumptions that initial deposition is entirely by grainfall that the rate of deposition decreases as a power function of distance downflow from the brink of the slipface and that the resulting steepening of the slipface is periodically interrupted by avalanching the parameters used in the model are 1 the deposition rate at a given distance from the brink 2 the exponent in the equation relating the deposition rate to distance from the brink 3 the bedform migration rate 4 the bedform height 5 the avalanche speed 6 the angle of initial yield and 7 the residual angle after avalanching from these parameters can be calculated structural characteristics such as the proportions of bottomset and foreset deposits the proportions of avalanche and grainfall deposits in the foreset deposit and the spacing of avalanche grainfall couplets the model correctly predicts the trends of changing avalanche activity and changing structural character with changes in flow character grain size and bedform height in both air and water moreover the model correctly predicts certain consistent structural differences between aeolian and subaqueous lee side deposits quantitative evaluation of the model requires more accurate data on the values of the input parameters than are presently available

estimation of some aeolian saltation transport parameters a re analysis of williams data a new method for analysing observed aeolian sand transport rate profiles of the kind obtained by williams is presented the method involves a mathematical model of aeolian saltation detailed information about the saltation process can be calculated from the transport rate profile by means of this model the method is used to perform a re analysis of williams trap data among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by bagnold owen in their theories of aeolian saltation other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established and that the vertical component of the mean launch velocity decreases with the grain size it is approximately inversely proportional to the grain diameter our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling the influence of grain shape on the transport characteristics is mainly due to changes in the grains ability to jump when hitting the bed it is found that angular grains have a lower mean jump height than spherical grains

aeolian architecture bedform climbing and preservation space in the cretaceous etjo formation nw namibia sets of aeolian cross strata within the cretaceous etjo formation of nw namibia are bounded by a hierarchy of surfaces the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour the base of the main aeolian succession is characterized by a basin wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5 in an upwind direction sw perpendicular to the palaeowind interdune surfaces form 500 m wide troughs signifying crestline sinuosity within the original bedforms superimposition surfaces are inclined at 5 10 in a downwind direction and indicate the migration of crescentic oblique dunes over larger slipfaceless transverse draa reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross strata lie parallel to them rather than downlap onto them analysis of the geometry of these bounding surfaces together with the orientation of the cross strata within the sets that they bound has enabled the detailed morphology of the original bedforms to be reconstructed the maximum preserved thickness of individual aeolian sets varies systematically across the basin from 52 m in the basin depocentre to only 8 m at the basin margin the set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin rather than the presence of smaller bedforms similarly a temporal reduction in the angle of climb rather than a reduction in bedform size is considered to be responsible for an upward decrease in preserved set thickness reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin

simulation of the effect of wind speedup in the formation of transverse dune fields a computer simulation model for transverse dune field dynamics corresponding to a uni directional wind regime is developed in a previous formulation two distinct problems were found regarding the cross sectional dune shape namely theerosionintheleeofdunesandthesteepnessofthewindwardslopes thefirstproblemissolvedbyintroducingnoerosion inshadowzones thesecondissueisovercomebyintroducingawindspeedup shearvelocityincrease factor whichcanbe accountedforbyaddingatermtotheoriginaltransportlength whichisproportionaltothesurfaceheight byincorporating these features we are able to model dunes whose individual shape and collective patterns are similar to those observed in nature moreoverweshowhowtheintroductionofanon linearshear velocity increasetermleadstothereductionofdune height andthismayresultinanequilibriumdunefieldconfiguration thisisthoughttobebecausethenon linearincreaseof thetransportlengthmakesthesandtrappingefficiencylowerthanunity evenforhigherdunes sothattheincomingandthe outgoingsandfluxareinbalance tofullydescribetheinter dunemorphologymoreprecisedynamicsintheleeofthedune must be incorporated copyright 2000 john wiley sons ltd

airflow and sand transport variations within a backshore parabolic dune plain complex ne graham island british columbia canada onshore aeolian sand transport beyond the beach and foredune is often overlooked in the morphodynamics and sediment budgets of sandy coastal systems this study provides detailed measurements of airflow sand transport via saltation and modified suspension vegetation density and surface elevation changes over an extensive 325 30 m swath of a backshore foredune parabolic dune plain complex near surface 30 cm wind speeds on the backshore ranged from 4 3 to 7 3 m s 1 gusting to 14 0 m s 1 oblique onshore flow is steered alongshore near the incipient foredune then landward into a trough blowout where streamline compression flow acceleration to 1 8 times the incident speed and increasing steadiness occur highest saltation rates occur in steady topographically accelerated flow within the blowout as such the blowout acts as a conduit to channel flow and sand through the foredune into the foredune plain beyond the blowout flow expands vegetation roughness increases and flow decelerates over the foredune plain localized flow steering and acceleration to 1 6 times the incident speed occurs followed by a drop to 40 of incident flow speed in a densely vegetated zone upwind of an active parabolic dune at 250 m from the foredune sediment properties reflect variations in near surface flow and transport processes well sorted fine skewed backshore sands become more poorly sorted and coarse skewed in the blowout due to winnowing of fines sorting improves and sands become fine skewed over the foredune plain toward the parabolic dune due to grainfall of finer sands winnowed from the beach and foredune during the fall winter season significant amounts of sand up to 110 kg m 2 are transported via modified suspension and deposited as grainfall up to 300 m landward of the foredune no distinct trend in grainfall was found although most fell on the depositional lobe of the blowout and at 200 m near an isolated active parabolic dune grainfall amounts may reflect several transporting events over the measurement period and the transport process is likely via localized modified suspension from the crest of the foredune and other compound dune features in the foredune plain this evidence suggests that the process of grainfall delivery though often overlooked in coastal research may be a key process in maintaining active dunes hundreds of metres from the shoreline in a densely vegetated foredune plain the effectiveness of this process is controlled by seasonal changes in vegetation cover and wind strength as well as shorter term e g tidally controlled variations in sand availability from the beach

aeolian slipface dynamics and grainflow morphologies on earth and mars abstract in 2015 an active dune field on mars was visited up close by the curiosity rover in gale crater providing the first high resolution ground images of fine scale windblown features not previously resolved from orbital based imagery for the first time these images allow for direct comparison with terrestrial aeolian dynamics and provide critical ground truth data to bridge the gap between model predictions and satellite observations the image data from the slipface on the namib dune within the bagnold dune field shows grainflow morphologies that are similar to dunes on earth quantitative estimates of flow thickness based on shadow length are presented for the grainflows on the namib dune slipface and compared to grainflow characteristics measured by terrestrial laser scans from the maspalomas dune field located in gran canaria spain using observations from maspalomas to support interpretations of martian slipface dynamics we discuss implications for the local wind regime style of grainflow seasonal activity and dune migration the presence of multiple large magnitude grainflows on the namib slipface suggests an active aeolian environment capable of delivering enough sediment to the slipface to initiate these flows and transport sediment to the bottom of the lee slope however the thinness of grainflows on the namib dune the formation of smaller grainflows directly below the dune brink and limited grainfall suggest a lower wind energy environment at least for the most recent slipface activity large actively migrating stoss ripples obliquely oriented to the dune crest regularly deposit sediment on to the upper slipface and may be a mechanism in which larger grainflow occur during seemingly low energy wind events this mechanism of sediment delivery may also explain the existence of a variety of slipface morphologies both young and old which are otherwise quickly erased on earth due to sediment redistribution and grainfall

relations between morphology air flow sand flux and particle size on transverse dunes taklimakan sand sea china field experiments on two isolated transverse dunes with different scales in the taklimakan sand sea show that particle size and slope control the threshold shear velocity and sand flows on the stoss of the dunes the decreased threshold shear velocity and the unsaturated sand flows are the main reasons why sands are not deposited at the toe of the dunes the particle size controls sand flux through its influence on the aerodynamic roughness elements and threshold shear velocities the sand flux is different for dune surfaces with different particle size distributions under the control of the medium to low stress of the drifting sand winds the results of the field experiments and particle size analysis showed that sand transport had little influence on the particle size distributions the mean particle size being transported on the stoss is lower than that on the dune surface it is difficult to determine whether or not the wind is fully saturated with sands when sand flux calculation is mainly dependent on the shear velocity and the mean particle size the sands within some range of the particle size are transported and developed into simple transverse dunes and then the interdune surface is coarsened and alternated with the draas in the taklimakan sand sea copyright 2002 john wiley sons ltd

patterns of wind flow and aeolian deposition on a parabolic dune on the southeastern shore of lake michigan abstract sand deposition and patterns of wind flow were studied on green mountain beach gmb dune a large lake michigan coastal dune that is transitional between a trough blowout and a fully developed parabolic dune deposition pins were used to study the amount and pattern of aeolian sand deposition on the lee slopes of gmb dune over 20 months in the late fall and winter sand supported by ice between the grains accumulated on the upper to mid slopes and became over steepened 38 during measurement periods containing two major storms with wind gusts up to 25 m s a total of 44 of the deposition on the lee slope occurred compared to an average of 9 for comparable time periods between storms sand was delivered by mass movement to the lower slopes during the spring thaw very little sand was deposited on lee slopes during the summer maximum sand deposition occurred near the dune axis oriented southwest northeast and then decreased rapidly along the limbs maximum sand transport and dune migration were oriented towards the northeast despite the dominant regional winds during the fall and winter being from the west and northwest arrays of anemometers and wind vanes were used to study airflow patterns within gmb dune on five different days these studies indicate that steering of winds in the deflation area of the dune may be responsible for the northeasterly sediment transport they also suggest however that the ability of the dune to steer winds and thus control the direction of sand transport and deposition decreases when the angle between the oncoming winds and the dune axis is too great

dynamics of sediment storage and release on aeolian dune slip faces a field study in jericoacoara brazil sediment transport on the lee sides of aeolian dunes involves a combination of grain fall deposition on the upper portion of the slip face until a critical angle is exceeded transport of a portion of those sediments down the slip face by grain flows and finally deposition at an angle of repose we measured the mean critical and repose angles and the rate of slip face avalanching using terrestrial laser scanning tls on two barchans of different size in jericoacoara brazil wind speeds and sand fluxes were measured simultaneously at the dune crests we found that the mean critical and repose angles decrease with increasing wind speed we attribute this to turbulent shear stresses the magnitude of which are predicted using 3d large eddy simulation les modeling that episodically act down the slip face i e in the direction of gravity to trigger grain flows at lower angles than with gravity stresses alone the rate of avalanching is a maximum in the morning at our study site and coincides with the maximum rate of increase in wind speed and not with the maximum rate of sediment supply to the slip face we developed and tested a new predictive model for the rate of avalanching that includes both sediment flux delivered to the slip face and the derivative of the critical angle with time our data also suggest that the mean critical angle varies inversely with slip face height these results have important implications for aeolian dune evolution interpretations of aeolian stratigraphy and granular mechanics

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

grain scale simulations of loose sedimentary beds the example of grain bed impacts in aeolian saltation sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role in order to illustrate the modelling of collections of frictional inelastic sedimentary grains with the particle dynamics method pdm we use the grain impact process in aeolian saltation as a specific example in pdm all the forces on each particle are evaluated at a sequence of small time steps and the newtonian equations of motion are integrated forward in time interparticle forces at grain contacts are treated as springs with prescribed stiffness normal force and by a coulomb friction law tangential force particle inelasticity is represented by spring damping the granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties and the integration time step we find that for the range of impact speeds and impactor masses relevant to aeolian settings grain splashes are relatively insensitive to grain stiffness grain inelasticity and grain friction and that the pattern of ejection from the bed is largely controlled by bed microtopography a large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process the splash function representing these statistics is then available for use in calculations over longer time scales such as the evolution of the saltation curtain the details given here will enable the interested reader to adapt pdm modelling to other types of clastic sedimentary systems

influence of reed stem density on foredune development vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition and therefore on profile development in a long term monitoring field experiment three plots were planted with regular grids of reed bundles in three different densities 4 2 and 1 bundles per m2 this study reports on the differences in profile development under the range of vegetation densities topographic profiles were measured between may 1996 and april 1997 results indicate important differences in profile development for the three reed bundle densities in the highest density plot a distinct steep dune developed while in the lowest density a more gradual and smooth sand ramp was deposited when the stems had been completely buried differences in profile evolution vanished after a second planting of reed stems in january 1997 the process was repeated in may 1997 all plots had gained a sand volume ranging from 11 5 to 12 3 m3 m 1 indicating that the sediment budget is relatively constant regardless of the particular profile evolution the field evidence is compared with simulations of profile development generated by the foredune development model safe the model successfully reproduces the overall profile development but in general the equations used for vegetation transport interaction overestimate the effect of vegetation this causes some deviations between field and model results several reasons for this are discussed based on the experiments reported here recommendations are given for further research copyright 2001 john wiley sons ltd

crescentic dunes at schiermonnikoog the netherlands this paper describes the appearance and maintenance of crescentic dunes in high wind speed conditions on a frozen beach at schiermonnikoog the netherlands the dunes were cresentic forms with horns they were barchanoidal in plan view but had reverse morphologies to typical barchans the highest and steepest slopes were upwind and led to long low slopes downwind slipfaces were absent it is hypothesized that such crescentic dunes may be a stable aerodynamic form under high to very high c 15 20 m s 1 flow conditions 1997 by john wiley sons ltd