reynolds stress and sand transport over a foredune reynolds shear stress rs u w and sand transport patterns over a vegetated foredune are explored using three dimensional velocity data from ultrasonic anemometers at 0 2 and 1 2 m and sand transport intensity from laser particle counters at 0 014 m a mid latitude cyclone on 3 4 may 2010 generated storm force winds exceeding 20 m s 1 that shifted from offshore to obliquely alongshore quadrant analysis was used to characterize the spatial variation of rs quadrant components q1 through q4 and their relative contributions were parameterized using the flow exuberance relation exfl q1 q3 q2 q4 the magnitudes of rs and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics a flow exuberance effect was evident such that q2 ejection like and q4 sweep like quadrants that contribute positively to rs dominated on the beach dune toe and lower stoss whereas q1 and q3 that contribute negatively to rs dominated near the crest this exuberance effect was not expressed however in sand transport patterns instead q1 and q4 with above average streamwise velocity fluctuations u were most frequently associated with sand transport q4 activity corresponded with most sand transport at the beach toe and stoss locations 52 60 100 at the crest 25 to 86 of transport was associated with q1 while q4 corresponded with most of the remaining transport 13 to 59 thus the relationship between sand transport and rs is not as straightforward as in traditional equations that relate flux to stress in increasing fashion generally rs was poorly associated with sand transport partly because q1 and q4 contributions offset each other in rs calculations thus large amounts of transport can occur with small rs turbulent kinetic energy or reynolds normal stresses u 2 w 2 may provide stronger associations with sand transport over dunes although challenges exist on how to normalize and compare these quantities copyright 2013 john wiley sons ltd

on the necessity of applying a rotation to instantaneous velocity measurements in river flows in studies on river channel flow turbulence it is often the case that the measured mean vertical velocity is different from zero indicating that the frame of reference of the current meter is not parallel to the flow streamline this situation affects the estimate of reynolds shear stress in the streamwise and vertical planes and consequently the analysis of the flow turbulent structure one way to solve this problem is to correct data by applying a rotation and this is reviewed in the first part of the paper however in fluvial geomorphology the studied flow is often complex and streamlines may exhibit significant changes from one point of measurement to the other in this context applying a rotation complicates the situation more than it simplifies it the second part of this paper examines the question of velocity data correction in complex flows using a field example of the turbulent boundary layer over a very rough gravel bed and a laboratory example taken from flow at a river channel confluence in both cases velocity vectors are spatially variable in the first case errors in the reynolds shear stress estimates are relatively low ranging from 13 to 7 per cent deg while in the second case they are much larger 200 to 164 per cent deg the significance of these errors on the interpretation of turbulence statistics in river channel flows is discussed we propose that corrections should be applied in all clear cases of sensor misalignment and when the frame of reference changes spatially and temporally however no corrections should be used where different flow velocity vector orientations not sensor misalignment are responsible for the mean vertical velocity differing from zero

wind tunnel evaluation of a wedge shaped aeolian sediment trap abstract a detailed set of tests in a wind tunnel investigated the characteristics of flow and sampling efficiency of a low cost sediment trap that can be used in wind tunnels and field applications the trap is constructed of thin sheet aluminum and has a wedge shape 32 internal inlet angle with a 20 500 mm rectangular inlet orifice that extends 40 mm beyond the wedge form relatively unimpeded airflow through the trap is achieved by the large exit area 130 500 mm which is covered with fine stainless steel mesh 62 5 m openings 60 porosity and collected all but the finer silt sediment sizes despite its angular shape the trap has good characteristics of flow and a very high sampling efficiency 90 over a wide range of wind velocities because of its near isokinetic characteristics of flow results also indicate that this trap and others that have wide side profiles are very sensitive to incident wind angle incident angles 5 significantly reduce the sampling efficiency and promote scour around the trap inlet

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

simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel topographic interactions generate multidirectional and unsteady air ow that limits the application of velocity pro le approaches for estimating sediment transport over dunes results are presented from a series of wind tunnel simulations using irwin type surface mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp crested model dunes findings complement existing theories on secondary air ow effects on stoss transport dynamics and provide new information on the in uence of lee side air ow patterns on dune morphodynamics for all speeds investigated turbulent unsteadiness at the dune toe indicates a greater more variable surface shear despite a signi cant drop in time averaged measurements of streamwise shear stress at this location this effect is believed suf cient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region on the stoss slope streamline compression and ow acceleration cause an increase in ow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60 70 per cent greater than at ow reattachment on the lower stoss of closely spaced dunes streamwise ow accelerations rather than turbulence have greater in uence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed reversed ow within the separation cell generates signi cant surface shear 30 40 per cent of maximum values for both spacings this supports eld studies that suggest reversed ow is competent enough to return sediment to the dune directly or in a de ected direction high variability in shear at reattachment indicates impact of a turbulent shear layer that despite low values of time averaged streamwise stress in this region would inhibit sediment accumulation downwind of reattachment shear stress and ow steadiness increase within 6 h h dune height of reattachment and approach upwind values by 25 h a distance of at least 30 h is suggested for full boundary layer recovery which is comparable to uvial estimates the irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust simple and cost effective design shows promise for validating these ndings in natural dune settings copyright 2003 john wiley sons ltd

morphology of submarine barchans and sediment transport in barchans fields off the dongfang coast in beibu gulf submarine barchans in three new regions were described using new high resolution multibeam bathymetric data off the dongfang coast in beibu gulf currents seismic profiles of horns surface sediments and comparative surveys in 2007 and 2009 were obtained to study barchan mobility and sand transport in the barchan fields the submarine barchans occur at depths of 15 25 m reaching up to 551 9 m in horn length and 8 6 m in height most barchans have asymmetrical horns and display wider plane shapes than subaerial barchans for the same length and height the morphology of barchans exhibits regional variations that are mainly controlled by finite water depth daily reverse currents sand supply changes and interaction of dunes barchan asymmetries internal bedding inclinations and grain size changes are in agreement with the dominant current directions results show that submarine barchans migrated 5 43 m in two years 2007 2009 through comparison of bathymetric data because of the marginally cohesive fine sediment composition and long distance from contemporary sediment sources it is believed that barchans in region c are not of modern origin however these bedforms can be also affected by high energy flow regimes collisions limit barchan widths and provide efficient transport paths between barchans on their horns large sand supply through the horns increases the barchan heights and also changes their shapes some small dunes are identified that are moving away from the barchans field in 2007 and have departed from the host barchans in 2009 these small barchans were probably caused by the separation of the faster moving horn tails from the host barchans when extreme situations happen the mechanism gives a new explanation for sand loss and size determination of submarine barchans c 2014 elsevier b v all rights reserved

early stage aeolian protodunes bedform development and sand transport dynamics early stage aeolian bedforms or protodunes are elemental in the continuum of dune development and act as essential precursors to mature dunes despite this we know very little about the processes and feedback mechanisms that shape these nascent bedforms whilst theory and conceptual models have offered some explanation for protodune existence and development until now we have lacked the technical capability to measure such small bedforms in aeolian settings here we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour on a 0 06 m high protodune we observe vertical growth of the crest by 0 005 m in two hours our direct measurements of sand transport on the protodune account for such growth with a reduction in time averaged sediment flux of 18 observed over the crestal region detailed measurements of form also establish key points of morphological change on the protodune the position on the stoss slope where erosion switches to deposition is found at a point 0 07 m upwind of the crest this finding supports recent models that explain vertical dune growth through an upwind shift of this switching point observations also show characteristic changes in the asymmetric cross section of the protodune flow form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness by 3 constituting a reshaping of protodune form towards more mature dune morphology the approaches and findings applied here a demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early stage dune evolution b highlight the crucial role of form flow feedbacks in enabling early stage bedform growth alluding to a fluctuation in feedbacks that require better representation in dune models and c provide a new stimulus for advancing understanding of aeolian bedforms

a field study of mean and turbulent flow characteristics upwind over and downwind of barchan dunes field measured patterns of mean velocity and turbulent airflow are reported for isolated barchan dunes turbulence was sampled using a high frequency sonic anemometer deriving near surface reynolds shear and normal stresses measurements upwind of and over a crest brink separated barchan indicated that shear stress was sustained despite a velocity reduction at the dune toe the mapped streamline angles and enhanced turbulent intensities suggest the effects of positive streamline curvature are responsible for this maintenance of shear stress this field evidence supports an existing model for dune morphodynamics based on wind tunnel turbulence measurements downwind the effect of different dune profiles on flow re attachment and recovery was apparent with transverse incident flow a re attachment length between 2 3 and 5 0h h is dune brink height existed for a crest brink separated dune and 6 5 to 8 6h for a crest brink coincident dune the lee side shear layer produced elevated turbulent stresses immediately downwind of both dunes and a decrease in turbulence with distance characterized flow recovery recovery of mean velocity for the crest brink separated dune occurred over a distance 6 5h shorter than the crest brink coincident form as the application of sonic anemometers in aeolian geomorphology is relatively new there is debate concerning the suitability of processing their data in relation to dune surface and streamline angle this paper demonstrates the effect on reynolds stresses of mathematically correcting data to the local streamline over varying dune slope where the streamline angle was closely related to the surface windward slope time averaged shear stress agreed best with previous wind tunnel findings when data were rotated along streamlines in the close lee however the angle of downwardly projected separated flow was not aligned with the flat ground surface here shear stress appeared to be underestimated by streamline correction and corrected shear stress values were less than half of those uncorrected

airflow up the stoss slope of sand dunes limitations of current understanding the windward stoss side of a sand dune acts as a streamlined obstacle in the path of the wind continuity principles necessitate compression of the flow field up the stoss slope of a dune and shear stress must progressively increase as the flow accelerates measurements in transverse flow over thirteen dunes at padre island texas the algodones california and white sands new mexico confirm that velocity profiles on the stoss slope are not log linear and that flow acceleration occurs very close to the surface within an internal boundary layer as a consequence in the overlying flow where measurements have historically been made an overall decrease in shear stress occurs up the slope the actual shape of the velocity profiles and the identification of the appropriate segment of the profile from which to derive the shear stress that drives saltation represent major problems not approachable by traditional means

a study on velocity of aeolian sand particles over leeward slope by particle image velocimetry abstract current studies on the movement of saltation sand grains in aeolian flow over a complex surface are not satisfactory this is mainly because the landform conditions of most aeolian sand transport models are based on flat sand beds without considering the effect of complex surface and slope on the structure of aeolian sand flow in this paper we used particle image velocimetry piv to measure the state of motion of aeolian sand particles over the leeward slope area analyzed the characteristics of these sands particles with height h above the surface on the leeward slope including the relationship of their mean speed to the slope height and the incoming wind speed and the relationship of particle size with height finally we proposed a formula to describe the linear relationship of the dust emission rate to the incoming wind speed on the bed surface in the leeward slope area our conclusions are of significance for the study of wind driven sand flow in the leeward slope

formation of parabolic dunes from barchan and transverse dunes along israel s mediterranean coast the mediterranean coastal dunes of israel underwent a land use change during the second half of the 20th century due to intense agricultural and pastoral activity the coastal dunes were stripped of natural vegetation until the end of the first half of the 20th century the barchan and transverse dunes were shaped by strong southwesterly winter winds a decrease in human activity during the second half of the 20th century brought about a renewal of natural vegetation on the dune crest the only area with neither erosion nor deposition the establishment of vegetation on the crest changes the dynamics of these barchan and transverse dunes so that not all of the sand eroded from the windward side is carried to the lee slip face some is trapped by plants consequently there is a change in the shape of the windward slope from convex to concave and the dune gradually becomes parabolic in this study we trace the morphodynamics of the dunes by analysing 12 sets of aerial photographs which were taken from 1944 to 1995 the average rate of advance of 15 dunes has decreased from 3 4 m a 1 to 1 9 m a 1 while the vegetation cover has increased from 4 3 to 17 per cent during this period copyright 2002 john wiley sons ltd

relations between dune morphology air flow and sediment flux on reversing dunes silver peak nevada relations between wind speed sediment flux and dune morphology were measured for two reversing dunes situated in the south western part of the silver peak dunefield in clayton valley west central nevada the larger dune was 120 m in length with a height of 12ae5 m and the smaller dune 80 m long and 6 m high both dunes were sharp crested aligned approximately e w perpendicular to the dominant wind direction and had slightly concave stoss profiles twenty seven rotating cup anemometers were placed 0ae3 m elevation along n s transects on each of the dunes at each anemometer site a passive wedge shaped sediment trap was used to measure sediment flux amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed up factors ucrest ubase ranging from 1ae50 3ae19 with a corresponding increase in sediment flux by 1 2 orders of magnitude in general the ratio of crest flux to base flux qc qb increased with increasing incident basal wind speed on both dunes direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with owen s transport model friction speed u was approximated from near surface 0ae3 m point wind speed although not all assumptions of the owen model are upheld the modified model performance is sufficiently robust to predict short term variation in stoss sediment flux on the study dunes improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes in this regard further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions

large eddy simulation of interacting barchan dunes in a steady unidirectional flow we have performed large eddy simulations of turbulent flow 4 over barchan dunes in a channel with different interdune spacings in the downstream direction at reynolds number re 26000 based on the free 6 stream velocity and channel height simulations are validated against ex perimental data at re 55460 the largest interdune spacing 2 38 where is the length of the barchan model presents similar characteristics to the isolated dune in the experiment indicating that at this distance the sheltering effect of the upstream dune is rather weak we examine 3d realizations of the mean and instantaneous flow to explain features of the flow field relevant to sediment transport barchan dunes induce two counter rotating streamwise vortices along each of the horns which direct high momentum fluid toward the symmetry plane and low momentum fluid near the bed away from the centerline the flow near the bed upstream of the dune diverges from the centerline plane decelerates and then rises on the stoss side of the dune while accelerating the flow close to the centerline plane separates at the crest and reattaches on the bed away from the centerline plane and along the horns flow separation occurs intermittently the flow in the separation bubble is routed towards the horns and leaves the dune at their tips the separated flow at the crest reattaches on the bed except on the centerline symmetry plane of the dune where a weak saddle point of separation ap pears at the bed the distribution of the bed shear stress characteristics of the separation and reattachment regions and instantaneous wall turbulence are discussed characteristics of the internal boundary layer developing on the bed after the reattachment region are studied the interdune spacing isfound to affect significantly the turbulent flow over the stoss side of the downstream dunes at smaller interdune spacings coherent high and low speed streaks are shorter but stronger and the spanwise normal reynolds stress is larger the turbulent kinetic energy budgets show the importance of the pressure transport and mean flow advection in transporting energy from the overlying wake layer to the internal boundary layer over the stoss side of the closely spaced dunes the characteristics of the separated shear layer are altered slightly at smaller interdune spacing the separation bubble is smaller the separated shear layer is stronger and the bed shear stress is larger away from the dunes typical wall turbulence structures are observed but coher ent eddies generated in the separated shear layer due to the kelvin helmholtz instability are dominant near the dune coherent structures are generated more frequently at smaller interdune spacing they move farther away from the bed towards the free surface and remain in between the horns at larger interdune spacings these coherent structures are advected in the spanwise direction with the mean streamwise vortices and can be observed outside of the dunes

sonic anemometers in aeolian sediment transport research abstract fast response wind and turbulence instruments including sonic anemometers are used more and more in aeolian sediment transport research these instruments give information on mean wind but also on fluctuations and turbulent statistics such as the uw covariance which is a direct measure of reynolds stress rs and friction velocity this paper discusses the interpretation of sonic anemometer data the transformations needed to get proper results and turbulence spectra and how they are influenced by instrument size sampling frequency and measurement height turbulence spectra characterize how much the different frequencies in the turbulent signals contribute to the variance of wind speed or to the covariance of horizontal and vertical wind speed they are important in determining the measurement strategy when working with fast response instruments such as sonic anemometers and are useful for interpreting the measurement results choices on the type of sonic anemometer observation height sampling period sampling frequency and filtering can be made on the basis of expected high and low frequency losses in turbulent signals which are affected by those variables as well as wind speed and atmospheric stability friction velocity and rs important variables in aeolian sediment transport research are very sensitive to tilt or slope errors during a field experiment the slope sensitivity of the rs was established as 9 per degree of slope which is 1 5 times the value reported in literature on the basis of theoretical considerations an important reason for the difference probably is the large influence of streamline curvature on turbulence statistics and thereby on the slope sensitivity of the rs an error of 9 per degree of slope in the rs will translate into an error of approximately 4 per degree of slope in the calculated friction velocity space time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous rs this largely explains why in previous studies a poor correlation was found between instantaneous rs measured at 3 m height and saltation flux near the surface whereas the correlation between wind speed at some height and saltation flux was much better therefore the poor correlation between rs away from the surface and saltation flux does not contradict that saltation flux is caused by rs

in situ acoustic measurements of marine gravel threshold and transport measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles the turbulence in the bottom boundary layer was measured using two electromagnetic current meters and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material visual estimates of bedload were also obtained with an underwater tv camera the acoustic technique unlike a conventional bedload sampler has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected this has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions the results of the study show that of the turbulent bursting events which contribute towards the reynolds stress only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material the measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process

the influence of topography and approach angles on local deflections of airflow within a coastal blowout the spatial variability of air flow through complex topography is an important but not fully understood component of dune development and dynamics this study examines the spatial variability of the wind field in a linear blowout in coastal dunes at jockey s ridge state park on the outer banks of north carolina a spatial array of single height anemometers and wind vanes were placed within the blowout topography exerted a significant steering effect when onshore winds approached from directions within 50 of the blowout axis under those conditions wind flow in the blowout aligned to the axis regardless of approach angle maximizing the potential for erosion and transport in the trough in other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind when prevailing winds approached from directions more oblique than 50 to the blowout axis topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent during those approach angles wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow minimizing transport potential the changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions copyright 2013 john wiley sons ltd

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

the effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer the effect of turbulent flow structures on saltation sand transport was studied during two convective storms in niger west africa continuous synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 hz the shear stress production was determined from the vertical and streamwise velocity fluctuations the greatest stress bearing events were classified as turbulent structures with sweep ejection inward interaction and outward interaction described according to the quadrant technique the classified turbulent structures accounted for 63 5 per cent of the average shear stress during the first storm and 56 0 per cent during the second storm the percentage of active time was only 20 6 per cent and 15 8 per cent respectively high saltation fluxes were associated with sweeps and outward interactions these two structures contribute positively sweeps and negatively outward interactions to the shear stress but have in common that the streamwise velocity component is higher than average therefore the horizontal drag force seems primarily responsible for saltation sand transport and not the shear stress this was also reflected by the low correlation coefficients r between shear stress and saltation flux 0 12 and 0 14 respectively while the correlation coefficients between the streamwise velocity component and saltation flux were much higher 0 65 and 0 57 respectively

wind forces and related saltation transport the effect of several wind characteristics on sand transport was studied in three experiments in north burkina faso west africa the first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height the second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms registered during the rainy seasons of 2002 and 2003 the effect of sampling time is studied for two of these convective storms the third experiment relates the turbulent structures of four convective storms to saltation transport wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones the sampling frequency was either 8 or 16 hz the sonic frame of reference was rotated according to a triple rotation horizontal fluctuations showed a fairly good correlation with height because the wind speed at both sensors was affected by the same vortices the correlation coefficients ranged from 0 42 when the distance between the sensors was 1 75 m to 0 92 when the distance was 0 25 m the instantaneous reynolds stress had the weakest correlation correlation coefficient of 0 05 at 1 75 m between the sensors and 0 56 at 0 25 m between the sensors because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground this also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous reynolds stress and saltation an increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s however this sampling interval would be too coarse to describe the vertical wind component adequately the classification of the moments of shear stress into the turbulent structures sweeps ejections inward and outward interactions showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions also saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions

turbulent reynolds stress and quadrant event activity in wind flow over a coastal foredune recent research on quasi instantaneous turbulent kinematic reynolds stresses rs u w and decomposed quadrant event activity e g ejections and sweeps over dunes in fluvial settings and in wind tunnels has shown that turbulent stresses at the toe of a dune often exceed time averaged streamwise shear stress u 2 estimates it is believed that semi coherent turbulent structures are conveyed toward the bed along concave streamlines in this region and that impact of these structures cause fluctuations in local surface stresses that assist in grain entrainment this has been hypothesized to explain how sand is supplied to the windward slope through a region of flow stagnation toward the crest surface stress increases and becomes dominated by streamwise accelerations resulting from streamline compression and convexity that suppress vertical motions high frequency 32 hz measurements of turbulent wind flow from 3 d ultrasonic anemometers are analyzed for oblique onshore flow over a vegetated coastal foredune in prince edward island canada reynolds stress and quadrant activity distributions varied with height 0 60 m and 1 66 m and location over the dune in general quadrant 2 ejection u 0 and quadrant 4 sweep activity u 0 w 0 w 0 and quadrant 3 inward interaction u 0 w 0 activity on the lower stoss slope significant ejection and sweep event activity was most frequent 85 to 92 ejections plus sweeps whereas at the upper crest significant ejection and sweep activity became less frequent while significant outward and inward interactions increased in frequency 25 to 36 an exuberance effect i e changing shape of quadrant frequency distribution skewed toward ejection and sweep activity is observed whereby streamline compression and convexity effects inhibit vertical fluctuations in flow and thus reduce the frequency of ejections and sweep activity toward the crest in separated flow in the lee of the crest quadrant distributions were more symmetrical as a result of more mixed multi directional flow these trends in turbulent event distributions and reynolds stress have implications for sediment transport dynamics across the dune and may help to explain sand transport potential and dune maintenance for example areas with a high frequency of ejection and sweep activity may have higher rates of sediment entrainment and transport whereas areas with lower ejection and sweep activity and an increase in outward and inward interactions which contribute negatively to reynolds stress generation may experience a greater potential for deposition further research on associations between quadrant event activity and coincident sand transport is required to confirm this hypothesis and the resultant significance of the flow exuberance effect in aeolian dune morphodynamics

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

mean flow and turbulence structure over fixed two dimensional dunes implications for sediment transport and bedform stability detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed two dimensional dunes in a laboratory channel laser doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength the density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters which are assessed using higher moment measures and quadrant analysis these flow field maps illustrate that 1 the time averaged downstream and vertical velocities agree well with previous studies of quasi equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest 2 the maximum root mean square rms of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell 3 the maximum vertical rms values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream extending almost to the next crest 4 positive downstream skewness values occur within the separation cell whereas positive vertical skewness values are restricted to the shear layer 5 the highest reynolds stresses are located within the zone of flow separation and along the shear layer 6 high magnitude high frequency quadrant 2 events ejections are concentrated along the shear layer kelvin helmholtz instabilities and dominate the contribution to the local reynolds stress and 7 high magnitude high frequency quadrant 4 events occur bounding the separation zone near reattachment and close to the dune crest and are significant contributors to the local reynolds stress at each location these data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation magnitude and downstream extent of the flow separation zone and resultant shear layer furthermore the origin of dune related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting the fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the kelvin helmholtz instabilities which are both greater for dunes than ripples these instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates

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

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

sediment flux and airflow on the stoss slope of a barchan dune abstract measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics the data show that at low wind incident speeds flux increases up the dune exponentially whereas at higher wind speeds the increase with distance approaches linearity wind profile measurements conducted at the same time as the flux measurements indicate that although wind speed at a given height increases by 1 2 times from dune toe to brinkline wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values this demonstrates that conventional wind profiles derived from anemometry on dunes do not measure the part of the boundary layer that is significant for sediment transport

airflow and aeolian sediment transport patterns within a coastal trough blowout during lateral wind conditions blowouts are depressions that occur on coastal dunes deserts and grasslands the absence of vegetation in blowouts permits high speed winds to entrain and remove sediment whereas much research has examined patterns of wind flow and sediment transport on the stoss slopes and lee of sand dunes no study has yet investigated the connections between secondary air flow structures and sediment transport in a blowout where zones of streamline compression expansion and steering are less clearly delineated in this study we investigated the variability of sediment flux and its relation to near surface wind speed and turbulence within a trough blowout during wind flow that was oblique to the axis of the blowout wind flow was measured using six three dimensional 3d ultrasonic anemometers while sediment flux by eight sand traps all operating at 25 hz results demonstrated that sediment flux rates were highly variable throughout the blowout deflation basin even over short distances 0 5 m where flow was steadiest flux was greatest consequently the highest rates of sediment transport were recorded on the erosional wall crest where flow was compressed and accelerated the strength of correlation between sediment flux and wind parameter improved with an increase in averaging interval from 10 seconds to 1 minute at an interval of 10 seconds however wind speed correlated best with flux at seven of eight traps whereas at an interval of one minute turbulent kinetic energy tke provided the best correlation with flux at six of the eight traps correlation between sediment flux and wind parameters was best in the centre of the blowout and poorest on the erosional wall crest the evidence from this paper suggests for the first time that tke may be a better predictor of sediment transport at minute scale averaging intervals particularly over landforms where wind flow is highly turbulent

comparison of four methods to calculate aeolian sediment transport threshold from field data implications for transport prediction and discussion of method evolution abstract aeolian sediment transport threshold is commonly defined as the minimum wind speed or shear stress required for wind driven sediment transport accurate and consistent quantification of this threshold is essential because it is an input variable in models used to predict wind erosion dune activity and dust emissions the majority of threshold quantification has been performed with analogs analytical models or wind tunnels however in the past few decades field based approaches to threshold parameterization have become more common although several methods of calculating transport threshold from field data are available their comparability is unknown to address this issue we collected high resolution sediment transport and wind measurements 1 hz on an active sand dune for 11 days and compared four different methods of calculating threshold i time fraction equivalence method tfem ii gaussian time fraction equivalence method gtfem iii instantaneous method and iv regression method time paired measurements from the two most widely used methods tfem and gtfem were strongly correlated r 0 977 however correlations between other methods varied from r 0 861 to r 0 261 to demonstrate the implications of using different threshold calculation methods we predicted mass transport which ranged from 63 6 instantaneous method to 126 6 kg per crosswind meter regression method this inconsistency suggests that the threshold calculation method could have an appreciable impact on transport predictions threshold values are similarly inconsistent when the measurement interval is modified as such we do not recommend comparing any measured threshold with another we discuss several strategies that may mitigate the impact of this issue such as clarification of semantics and method standardization we also discuss several criticisms of field based threshold measurements and re conceptualizations that could allow investigators to develop a better understanding of field based measurements overall results from this study could allow future investigators to improve threshold and transport predictions

evaluation of saltation flux impact responders safires for measuring instantaneous aeolian sand transport intensity the measurement of aeolian sand transport rates on small scales is of interest to the development and testing of detailed models of sand movement by wind this paper reports on laboratory evaluations and preliminary field tests of a new design of a piezo electric impact responder called a safire capable of measuring saltation impacts at a frequency of 20 hz the advantages of the safire are 1 that it provides high frequency measurements 2 that it presents a minimal obstruction to the wind flow no scour observed in the field and 3 that it is of a relatively low cost laboratory calibrations were performed with a vertical gravity flume generating known sand grain fluxes using both mixed sand and specific size fractions initial tests investigated three fundamental characteristics correspondence between digital and analogue signals generated by the instrument directional response of the probe and linearity of instrument response to mass flux instrument calibration included determination of the momentum threshold required for the sensor to register a grain impact based on this lower limit and the known distribution of grain size and speed at different fall heights a prediction is made as to the sand grain flux the safire ought to measure which is then compared with the signal response the result of this comparison is an assessment of the instrument s efficiency in counting saltating grains these safires were also deployed in the field as part of a larger investigationof spatio temporal transport variability thisexperiment providedthe opportunity to compare the instrument s performancewithtraditional sandtraps andthispaperdevelopsmethodsandassumptionsrequiredtoconvertmeasurements from impact responders to traditional mass transport rates the evaluations indicate that improvements to the instrument production process are required to ensure a standard momentum threshold among individual instruments furthermore the sensor design needs to be reconsidered in order to eliminate the variation in response depending on azimuth direction so that the sensor is uniformly omni directional d 2003 elsevier b v all rights reserved

streamline correction for the analysis of boundary layer turbulence abstract improvements in the design and affordability of ultrasonic anemometers have provided significant contributions to aeolian research by facilitating high frequency monitoring of three dimensional wind velocities from these data it is possible to calculate quasi instantaneous reynolds stresses to evaluate boundary layer turbulence moving beyond time averaged measures such as shear velocity u as ultrasonic anemometry is used more frequently in aeolian geomorphology it is important to question accepted conventions concerning data processing and analysis this paper examines data processing questions associated with the application of ultrasonic anemometry to field studies in aeolian geomorphology through an investigation of three streamline correction routines the two step three step and planar fit methods on data recorded on a gently sloping beach at magilligan strand northern ireland in may 2010 the planar fit technique has not previously been used in aeolian geomorphology results are compared with data that have been corrected only for wind direction yaw the effects that these different methods have on quadrant analysis and reynolds stress calculation are discussed streamline correction is applied as a time variable procedure using a characteristic timescale of 8 s following analysis of the resultant wind speed energy spectrum it is found that reynolds shear stress is dependent on streamline correction method with run mean estimates of resultant horizontal shear stress ranging from 0 05 to 0 11 n m 2 depending on the technique the two step method consistently maximises the shear stress and when the resultant horizontal shear is calculated it produces the most robust estimate for application to aeolian research in contrast the different methods have little effect on the identification or sequencing of turbulent structures using quadrant analysis streamline correction is an essential processing step when using reynolds decomposition however the method used has an effect on data quality as each correction re distributes stress across the whole tensor altering both normal and shear stresses the total stress tensor magnitude itself is a conserved value and may provide a suitable parameter to quantify boundary layer turbulence

high frequency sediment transport responses on a vegetated foredune wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and wenglor laser particle counters the foredune had a compound seaward slope with a wave cut scarp about 0 5 m high separating the upper vegetated portion from the lower dune ramp which was bare of vegetation wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction but overall magnitudes were similar over the whole length ten minute average transport intensity correlates strongly with mean wind speed measured at the dune crest and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique but for wind angles 45 from shore perpendicular some sand is transported onto the lower slope copyright 2012 john wiley sons ltd

formation and behavior of aeolian streamers received 5 december 2004 revised 31 march 2005 accepted 9 june 2005 published 7 september 2005 1 field experiments were conducted to determine the characteristic spatial and temporal dimensions and behavior of aeolian streamers and to identify the processes responsible for their formation an instrument array that included anemometer towers piezo electric transport sensors safires and hot film probes was deployed to measure streamers and airflow dynamics on both a coastal dune and a desert sand mound in california aeolian transport occurs predominantly in the form of families of intertwining and bifurcating streamers while under intense wind forcing more complex patterns of nested streamers and clouds with embedded streamers develop the streamers display a characteristic width of approximately 0 2 m and an average lateral spacing of about 1 m these dimensions appear to be independent of mean airflow characteristics the observations and measurements of streamers under different environmental conditions suggest that bed surface control in the form of differentiation in moisture content grain size or microtopography is not a necessary condition for the formation of streamers the results show little correlation between possible streamwise vortices or burst sweep events and streamers it is proposed that streamers are generated by near surface gusts that originate from large eddies propagating to the ground from higher regions of the boundary layer these elongated and stretched eddies scrape across the surface and initiate saltation along their path this concept accounts for the characteristic size and spacing of streamers their rapid propagation and the fast response of saltation to wind speed fluctuations

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

velocity profiles measured on the windward slope of a transverse dune velocity gradients measured on the windward slope of a transverse dune show that flow acceleration strongly affects the turbulence structure of wind wind profiles measured below a height of 1 6m are nonlogarithmic and the altered nature of velocity profiles varies over the windward slope of a dune these findings suggest that calculated values of shear velocity are largely dependent upon the height and location of wind speed measurements as well as the strength of the prevailing wind moreover if bagnold type transport equations are used to calculate rates of sand transport on a dune wind speeds must be measured within 0 2 m of the dune surface if wind speeds are measured above this height the resulting errors in calculated shear velocity will manifest themselves as errors in calculated rates of sand transport if rates of sand transport are derived from wind speeds measured above 0 2 m the calculated rates may underestimate sand transport on the windward face of a dune as a result of flow acceleration in contrast if a dune has a separate crest upwind of the brink the expansion and subsequent deceleration of flow downwind from the crest may result in an overestimation of sand transport

modelling aeolian sand transport using a dynamic mass balancing approach abstract knowledge of the changing rate of sediment flux in space and time is essential for quantifying surface erosion and deposition in desert landscapes whilst many aeolian studies have relied on time averaged parameters such as wind velocity u and wind shear velocity u to determine sediment flux there is increasing field evidence that high frequency turbulence is an important driving force behind the entrainment and transport of sand at this scale of analysis inertia in the saltation system causes changes in sediment transport to lag behind de accelerations in flow however saltation inertia has yet to be incorporated into a functional sand transport model that can be used for predictive purposes in this study we present a new transport model that dynamically balances the sand mass being transported in the wind flow the dynamic mass balance dmb model we present accounts for high frequency variations in the horizontal u component of wind flow as saltation is most strongly associated with the positive u component of the wind the performance of the dmb model is tested by fitting it to two field derived namibia s skeleton coast datasets of wind velocity and sediment transport i a 10 min 10 hz measurement resolution dataset ii a 2 h 1 hz measurement resolution dataset the dmb model is shown to outperform two existing models that rely on time averaged wind velocity data e g radok 1977 dong et al 2003 when predicting sand transport over the two experiments for all measurement averaging intervals presented in this study 10 hz 10 min the dmb model predicted total saltation count to within at least 0 48 whereas the radok and dong models over or underestimated total count by up to 5 50 and 20 53 respectively the dmb model also produced more realistic less peaky time series of sand flux than the other two models and a more accurate distribution of sand flux data the best predictions of total sand transport are achieved using our dmb model at a temporal resolution of 4 s in cases where the temporal scale of investigation is relatively short on the order of minutes and at a resolution of 1 min for longer wind and transport datasets on the order of hours the proposed new sand transport model could prove to be significant for integrating turbulence scale transport processes into longer term macro scale landscape modelling of drylands

quantifying the spatio temporal patterns of dune migration near minqin oasis in northwestern china with time series of landsat 8 and sentinel 2 observations abstract investigation of the spatio temporal patterns of dune migration in a large scale area with optical remote sensing techniques can help us to better understand aeolian phenomena and mitigate sand dust disasters with the rapid growth in data volume extracting more accurate dune displacement time series and rates from optical observations has become possible however the method is not yet fully fledged to address this issue we propose an extended algorithm for the mature optical imagery cross correlation oicc technique based on landsat 8 l8 and sentinel 2 s2 acquisitions the main innovative points of this algorithm are 1 the proposed pairing strategy for the oicc processing 2 the modularized post processing procedures for noise removal and 3 the introduction of singular value decomposition svd time series inversion of the redundant optical observations to quantify the dune migration to test the effectiveness of this algorithm it was applied in the study of dune migration near minqin oasis in northwestern china using enriched l8 and s2 images collected between april 2013 and april 2018 compared with the original oicc results in stable areas the post processing and inversion of the proposed algorithm reduce the uncertainty by around 22 35 and 3 5 for l8 29 48 and 5 12 for s2 respectively the cross comparison between the l8 and s2 derived displacement time series shows high consistency and presents a lower uncertainty than the result of the traditional no inversion method furthermore the derived displacement rates show spatial patterns that are similar to those of the manually digitized results obtained with historical google earth ge images these comparisons show the advantage of the proposed algorithm in automatically and accurately quantifying dune migration taking into account these measurements the spatio temporal evolution patterns of dune migration in the study area were analyzed from the spatial perspective the sand dunes move along a northwest southeast axis with four detected transport pathways our research also shows that around 1087 7 km2 of dune fields present an active status the active sand dunes are currently encroaching on around 155 5 km2 and 4 4 km2 of land each year outside and inside the oasis respectively representing a problem of rapid desertification temporally the displacement time series along the dominant migration direction appears as seasonal variations that are seemly consistent with the changes in local atmospheric conditions the proposed algorithm provides a new perspective to investigate the spatio temporal evolution of dune migration with medium resolution l8 and s2 optical datasets

turbulent flow structures and aeolian sediment transport over a barchan sand dune 1 the turbulent structure of airflow over a barchan sand dune is determined using quadrant analysis of wind velocity data derived from sonic anemometers results indicate an increased frequency of ejection and sweep events in the toe region of the dune characteristic of the turbulent bursting process in contrast at the crest there was a significant increase in the occurrence of outward interactions combined with high frequency saltation data our analyses show that turbulent structures characterised by a positive streamwise fluctuating velocity u sweeps at the toe and outward interactions at the crest have a dominant influence on sand transport on the dune together accounting for up to 83 and 95 of transporting events at the toe and crest respectively

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research abstract recently ultrasonic anemometers uas have become available for precise high frequency measurement of three dimensional velocity and turbulence properties except for a few wind tunnel and computational fluid dynamics cfd simulations advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence e g cup anemometers or unable to withstand sediment laden airflow e g hotfilms in contrast extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time this paper provides a pragmatic discussion on using uas in aeolian research recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near surface shear stress using uas over complex terrain are discussed physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non logarithmic velocity profiles resulting from roughness and topographically induced effects and the inability of instrumentation to measure within the thin constant stress region these constraints hinder accurate shear velocity u shear stress and sand transport estimation uas allow measurement of turbulent reynolds stress rs that in theory should equal profile derived shear stress discrepancies often exist between these quantities however due to three dimensional spanwise flow components and rapid distortion effects i e unbalanced production and dissipation of turbulence common in flow over complex terrain while the rs approach yields information on turbulent contributions to near surface stress generation little evidence exists showing that rs is a better measure of forces responsible for sediment transport consequently predictive equations for sediment transport using rs do not exist there is also a need to identify the role of micro turbulent events e g burst sweep cycles and macro turbulent structures e g separation cells shear layers in aeolian dynamics in field settings to validate recent wind tunnel and cfd simulations a conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects in unsteady non uniform flow over complex terrain streamline angles vary spatially and temporally with height and location thus determination of and correction to true streamline coordinates is difficult caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour e g lift at a dune crest and may complicate interpretations of rs instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines logistical considerations include sensor design advantages and operational limitations data communication formats and field deployment strategies each can affect measurement accuracy and are easily overlooked sampling range frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings with proper considerations of their limitations uas may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics

wavelet power spectra of aeolian sand transport by boundary layer turbulence 1 this letter reports on wavelet spectral analysis of collocated time series of streamwise wind speed and aeolian sand transport collected at 0 04 m above the bed in a field experiment on top of a large sand mound in a desert environment results suggest three distinct spatio temporal scales of interaction between boundary layer turbulence and sediment transport a long term scale associated with the regional mixing layer an integral time scale associated with the internal boundary layer and a streamer scale associated with the inertial subrange eddy cascade and saltation inertia

turbulence fields in the lee of two dimensional transverse dunes simulated in a wind tunnel the turbulence field of airflow in the lee of a dune has significant impacts on dune dynamics and related processes we used particle image velocimetry in a wind tunnel simulation to obtain detailed velocity measurements in the lee of two dimensional transverse dune models then used the results to analyse their turbulence fields the dune models used in this study had a single lee angle of 30 and a total of six stoss angles 3 5 10 15 20 and 25 we used vorticity turbulence intensity reynolds stress and turbulent kinetic energy to characterize the turbulence fields these parameters were functions of stoss angle wind velocity distance from the dune crest and height above the ground surface the stoss angles could generally be divided into two groups based on the profiles of mean velocity turbulence and reynolds stress stoss angles of 3 and 5 usually had similar profiles and angles of 15 20 and 25 formed a second group with similar profiles the profiles for the stoss angle of 10 were usually transitional and were intermediate between the two groups vorticity reynolds stress and turbulent kinetic energy increased monotonically with increasing free stream wind velocity but their variations with respect to the stoss angle were complex the stoss angles of 15 and 20 had the maximum values of these three parameters thus these angles may have special significance in dune development given the characteristics of the mean velocity fields and turbulence fields they produce within the lee airflow it is the streamwise velocity component and its turbulence that determine the surface shear stress copyright 2008 john wiley and sons ltd

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

the effect of unsteady winds on sediment transport on the stoss slope of a transverse dune silver peak nv usa rapid 10 s measurements of sediment transport and wind speed on the stoss slope of a transverse dune indicate that the majority of sand transported is associated with fluctuations in wind speed with a periodicity of 5 20 min duration increases in the sediment transport rate towards the dune crest are associated with a small degree of flow acceleration the increase in wind speed is sufficient however to greatly increase values of the intermittency index so that the duration of saltation is extended in crestal regions of the dune the pattern of sediment transport on the stoss slope and therefore the locus of areas of erosion and deposition change with the regional wind speed erosion of the crest occurs during wind speed events just above transport threshold whereas periods of higher magnitude winds result in deposition of sand upwind of the crest thereby increasing dune height although short term temporal and spatial relations between sand transport and wind speed on the stoss slope are well understood it is not clear how these relations affect dune morphology over longer periods of time

early diagenetic dolomite cements examples from the permian lower magnesian limestone of england and the pleistocene carbonates of the bahamas abstract early diagenetic dolomite cements are found in the permian lower magnesian limestone of eastern england and the pleistocene carbonates of great abaco island bahamas in the lower magnesian limestone the dolospar occurs as isopachous rinds that have grown both centripetally and centrifugally on the dolomitized micrite envelopes of hollow centered allochems because compactional fractures penetrate both the allochem walls and the cement rinds dolomite cementation is inferred to have predated any significant sediment accumulation in the great abaco carbonates limpid dolomite forms isopachous rinds around inter and intragranular porosity and is postdated by meteoric calcite cement both examples of early diagenetic dolomite cement textures developed as a consequence of changes in pore fluid chemistry from marine to mixed meteoric marine to meteoric waters therefore the presence of this cement type may point toward aliagenetic environments resulting from marine regressions

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field