turbulence structure over two dimensional bed forms implications for sediment transport turbulence measurements from the flow over two dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms the spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near bed flow field the measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components unlike measurements in uniform boundary layers these distributions and the higher moments of the velocity do not scale with the local shear velocity indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux this conclusion is supported by observations of sediment flux over a dune

the modelling of the free surface flow of water over topography abstract a numerical model is developed for the flow of water over a bottom topography that is characteristic of sandwaves found in the coastal and river estuary environment turbulence closure is achieved at the level of the turbulence energy equation and the reynolds stresses are parameterised in terms of a gradient transfer law and an assumed partitioning of the turbulence energy between different components of the flow the pressure derives from both hydrostatic and hydrodynamic contributions an experiment is performed for steady state flow over a sequence of asymmetrical sandwaves and it is shown that there is flow separation over the lee slope of a sandwave in this case it is shown further that flow separation is a consequence of the inclusion of the hydrodynamic pressure contribution a comparison is also made between non hydrostatically and hydrostatically determined values of other mean and turbulent flow quantities

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

influence of hydraulic conditions over dunes on the distribution of the benthic macroinvertebrates in a large sand bed river 1 this study aims to relate the flow structure over mobile dunes recorded on the channel bed of the parana river argentina with the spatial distribution of the benthic macroinvertebrates the following main conclusions have been obtained 1 the dunes found in the active channel could be considered as hydraulic biotopes at a mesohabitat scale the bed forms in the thalweg region are subjected to higher shear stresses with low benthic densities 2 differences in benthic densities were also recorded at within dunes microhabitat scales the largest densities were found in the dune troughs where small bed shear stresses occur and minimum densities on the low stoss side of dunes where turbulent agitation near the bottom strongly disturb the bed particles 3 superimposed dunes on larger dunes may be considered as another microhabitat of still smaller dimensions summarizing multiscale approaches are needed if a comprehensive understanding linking hydrodynamics and morphodynamics processes with benthic ecology is intended

predicting boundary shear stress and sediment transport over bed forms to estimate bed load sediment transport rates in flows over bed forms such as ripples and dunes spatially averaged velocity profiles are frequently used to predict mean boundary shear stress however such averaging obscures the complex nonlinear interaction of wake decay boundary layer development and topographically induced acceleration downstream of flow separation and often leads to inaccurate estimates of boundary stress particularly skin friction which is critically important in predicting skin friction over 2d bed forms the approach is based on combining the equations describing the mechanics of the internal boundary layer with semi empirical structure functions to predict the velocity at the crest of a bed form where the flow is most similar to a uniform boundary layer significantly the methodology is directed toward making specific predictions only at the bed form crest and as a result it avoids the difficulty and questionable validity of spatial averaging the model provides an accurate estimate of the skin friction at the crest where transport rates are highest simple geometric constraints can be used to derive the mean transport rates as long as bedload is dominant

dune migration on macrotidal flats under symmetrical tidal flows garolim bay korea abstract intertidal dunes provide crucial information on sand movements and hence the present state of growth or decay of the particular tidal flats however evaluation of sand transport based merely on the dune configuration emerging on subaerially exposed intertidal flats or ancient deposits could be misleading without considering the degree of tidal asymmetry given the literature focused mostly on dune migration by predominant tidal currents quantitative relationships between dunes and overlying flows need to be established for tidal flats governed by strong symmetrically reversing tidal currents to fulfill such a requirement a field experiment site on macrotidal and symmetrically tidal flats in garolim bay korea was chosen and investigated for the mechanical processes of dune migration an extensive field of 2d simple dunes occurs on the lower tidal flat of garolim bay all year round with spacing and height averaging 5 m and 0 4 m respectively to unravel movements of dunes during tidal cycles a self contained instrument tisdos was deployed recording a variety of parameters such as water depth current velocity wave height suspended sediment concentrations and bed level fluctuations about 0 5 m above the dune surface the 18 day measurements in april 2003 show that the dune being monitored continued to move back and forth during tidal cycles at varying speeds controlled by a neap spring tidal regime the net displacement of the dune at the end of the measurements was almost nil under nearly equal reversing currents however the dune moved approximately half its wavelength at flood and ebb each during peak spring in strong contrast with partial modifications limited to the upper cap of the dune during the rest period of time although waves up to 0 2 m high were recorded during the measurements these had negligible effects on both the pattern and intensity of the dune migration the garolim dunes may be hydrodynamically related to the representative values of depth averaged flow velocity and water depth 0 8 m s and 2 5 m respectively derived from the peak spring measurements

tracing high pressure metamorphism in marbles phase relations in high grade aluminous calcite dolomite marbles from the greek rhodope massif in the system cao mgo al2o3 sio2 co2 and indications of prior aragonite abstract four different types of parageneses of the minerals calcite dolomite diopside forsterite spinel amphibole pargasite ti clinohumite and phlogopite were observed in calcite dolomite marbles collected in the kimi complex of the rhodope metamorphic province rmp the presence of former aragonite can be inferred from carbonate inclusions which in combination with an analysis of phase relations in the simplified system cao mgo al 2 o 3 sio 2 co 2 cmas co 2 show that the mineral assemblages preserved in these marbles most likely equilibrated at the aragonite calcite transition slightly below the coesite stability field at ca 720 c 25 kbar and a co 2 0 01 the thermodynamic model predicts that no matter what activity of co 2 garnet has to be present in aluminous calcite dolomite marble at uhp conditions

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

role of near bed turbulence structure in bed load transport and bed form mechanics the interactions between turbulence events and sediment motions during bed load transport were studied by means of laser doppler velocimetry and high speed cinematography sweeps u 0 w 0 w 0 which contribute negatively to the bed shear stress and are relatively rare individually move as much sediment as sweeps of comparable magnitude and duration however and much more than bursts u 0 and inward interactions u 0 w 0 when the magnitude of the outward interactions increases relative to the other events therefore the sediment flux increases even though the bed shear stress decreases thus although bed shear stress can be used to estimate bed load transport by flows with well developed boundary layers in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity it is not accurate for flows with developing boundary layers such as those over sufficiently nonuniform topography or roughness in which significant spatial variations in the magnitudes and durations of the sweeps bursts outward interactions and inward interactions occur these variations produce significant peaks in bed load transport downstream of separation points thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms

a physically based model to predict hydraulic erosion of fine grained riverbanks the role of form roughness in limiting erosion hydraulic erosion of bank toe materials is the dominant factor controlling the long term rate of riverbank retreat in principle hydraulic bank erosion can be quantified using an excess shear stress model but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the lower mekong river boundary shear stress is estimated using a model kean and smith 2006a 2006b for flow over the irregular bank topography that is characteristic of fine grained riverbanks bank erodibility parameters were obtained using a cohesive strength meter tolhurst et al 1999 the new model was used to estimate annual bank erosion rates via integration across the mekong s annual flow regime importantly the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration thereby providing a stronger physical basis for the simulation of bank erosion model predictions as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery indicate a tendency to overpredict erosion root mean square error equals 0 53 m yr form roughness induced by bank topographic features is shown to be a major component 61 85 of the spatially averaged total shear stress and as such it can be viewed as an important factor that self limits bank erosion

numerical computation of sand dune migration with free surface flow 1 this paper presents a morphodynamic model for free surface flows over bed forms the model reproduces bed form evolution and migration in a physically based manner the flow model component of the coupled morphodynamic model which is two dimensional in the streamwise and vertical directions explicitly treats unsteadiness and nonhydrostatic effects comparison with measurements shows that the flow model accurately simulates both mean flow and turbulence structure induced by bed forms under free surface flow the flow model performance is enhanced by the use of a high order godunov scheme known as the cubic interpolated pseudoparticle cip technique to compute the advection phase of the navier stokes equation in addition an enhanced k turbulence closure with nonlinear terms is incorporated in the present model in order to allow better predictions of reynolds stress anisotropy in regions with flow separation performance of the nonlinear k model is compared with standard k closure in the context of the flow as well as morphodynamic simulation nonequilibrium bed load sediment transport is treated using a eulerian stochastic formulation of the sediment exchange process in terms of pickup and deposition functions the proposed approach for sediment transport explicitly considers the local flow variability during morphodynamic computation model results show that the vertical grid distribution pattern affects the simulation result particularly for bed form morphodymics the model successfully simulates some important physical features of bed form evolution such as amalgamation and asymmetric geometry the simulation results are in agreement with laboratory experiments as well as some commonly used prediction methods for geometric characteristics of bed forms

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

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

on the calculation of suspended load for noncohesive sediments calculation of suspended sediment transport rates requires integration of the product of sediment concentration times velocity over the depth of the flow hence accurate profiles of these quantities are required especially near the sediment bed although this would appear to be a straightforward task many effects can cause significant deviation of these quantities from those values expected from simple known relationships for example density stratification by suspended particulates can dampen turbulence fluctuations and significantly reduce the effective viscosity and diffusivity of the flow the momentum and suspended sediment equations are solved herein including the effects of stratification by suspended sediment and the effects due to size distribution and bedforms the results show that stratification and bedforms generally reduce the transport capacity of the flow whereas treating the sediment as an assemblage of different sizes rather than a single size d50 increases the expected transport

bed and flow dynamics leading to sediment wave initiation 1 new piv based experiments show that the nascent seed waves from which both ripples and dunes develop are generated on planar mobile sediment beds in a two stage process the first stage comprises the motion of random sediment patches that reflect the passage of sediment transport events caused by attached eddies these eddy transport events propagate at speeds that are proportional to their size and less than overhead eddy convection velocities but potentially larger than local average fluid and sediment velocities in the second stage interactions of the moving patches result in a bed disturbance that exceeds a critical height and interrupts the bed load layer quasi regular seed waves are then generated successively downstream of this stabilised growing disturbance via a scour deposition wave that arises from the requirement of sediment mass conservation and the sediment transport and bed stress distributions downstream of a bed perturbation seed waves are thereby of preferred lengths that scale with the grain size i e length o 130 grain diameters agreeing with compiled measurements this two stage generation mechanism is valid for fully turbulent hydraulically smooth and rough bed flows of small to large sediment transport rates it is furthermore valid for laminar flows although the critical disturbances leading to seed wave generation arise through bed discontinuities and not eddy based sediment transport events the identified generation mechanism which accounts for turbulence effects explains the observed similar scaling of alluvial closed conduit and lightweight sediment seed waves the present measurements highlight further aspects of the flow dynamics preceding seed wave generation including decreases in von karman s constant due to bed mobility near bed eddy convection speeds in excess of local double averaged in time and space streamwise velocities and the validity of the four range spectral scaling model for open channel flows proposed earlier by nikora

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

morphology and flow fields of three dimensional dunes rio parana argentina results from simultaneous multibeam echo sounding and acoustic doppler current profiling 1 most past studies of river dune dynamics have concentrated on two dimensional 2 d bed forms with constant heights and straight crest lines transverse to the flow and their associated turbulent flow structure this morphological simplification imposes inherent limitations on the interpretation and understanding of dune form and flow dynamics in natural channels where dune form is predominantly three dimensional for example studies over 2 d forms neglect the significant influence that lateral flows and secondary circulation may have on the flow structure and thus dune morphology this paper details a field study of a swath of 3 d dunes in the rio parana argentina a large 0 35 km wide 1 2 km long area of dunes was surveyed using a multibeam echo sounder mbes that provided high resolution 3 d detail of the river bed simultaneous with the mbes survey 3 d flow information was obtained with an acoustic doppler current profiler adcp revealing a complicated pattern of dune morphology and associated flow structure within the swath dune three dimensionality appears intimately connected to the morphology of the upstream dune with changes in crest line curvature and crest line bifurcations junctions significantly influencing the downstream dune form dunes with lobe or saddle shaped crest lines were found to have larger more structured regions of vertical velocity with smaller separation zones than more 2 d straight crested dunes these results represent the first integrated study of 3 d dune form and mean flow structure from the field and show several similarities to recent laboratory models of flow over 3 d dunes

turbulence measurements in open channel flows over artificial bed forms measurements of mean and turbulence characteristics in uniform open channel flows of constant mean depth over artificial fixed one dimensional periodic bed features were obtained using two component laser doppler velocimetry two types of bed forms based on triangular elements of the same height n 20 of the flow depth and wavelength n 12 5 times the height but of different shape were studied 1 45 triangle ridges separated by a flat region and 2 triangular elements approximating dunes with a mildly sloping upstream face instead of a flat region the role of the total shear velocity based on the mean energy slope and the mean flow depth as a possible velocity scale is studied velocity defect and the reynolds shear stress profiles were generally well approximated by the flat bed profiles for regions of flow far from the bed sufficiently far from the recirculation and reattachment zones turbulence characteristics collapsed fairly well when scaled with the total shear velocity over most of the depth but do not follow the flat bed behavior in the lower part of the flow possible implications of the clear water measurements for the description of suspended sediment concentration profiles and total suspended load computations are explored

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

form drag in rivers due to small scale natural topographic features 2 irregular sequences 1 the size shape and spacing of small scale topographic features found on the boundaries of natural streams rivers and floodplains can be quite variable consequently a procedure for determining the form drag on irregular sequences of different sized topographic features is essential for calculating near boundary flows and sediment transport a method for carrying out such calculations is developed in this paper this method builds on the work of kean and smith 2006 which describes the flow field for the simpler case of a regular sequence of identical topographic features both approaches model topographic features as two dimensional elements with gaussian shaped cross sections defined in terms of three parameters field measurements of bank topography are used to show that 1 the magnitude of these shape parameters can vary greatly between adjacent topographic features and 2 the variability of these shape parameters follows a lognormal distribution simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements in addition a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced identical topographic elements also is given the methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness

flow separation in the lee side of transverse dunes a numerical investigation we investigate flow separation in the air flow over transverse sand dunes numerical simulations of the air flow over differently shaped dunes are performed the length of the recirculation region after the brink of the dune is found to depend strongly on the shape of the dune a phenomenological expression for the separation length is presented suitably non dimensionalized it depends linearly on the angle of the dune at the slip face brink we propose that the shape of the separating streamline is well approximated by an ellipse

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

depth integrated suspended load calculations the net transport of sediment by suspension is calculated by integrating the product of the sediment concentration and velocity over the water depth for many flows accurate calculations must include effects of density stratification by suspended sediment as well as effects due to bed forms and a mixture of sediment sizes each of these effects can cause differences of up to two orders of magnitude for some cases and therefore often must be included for reasonable accuracy however even though stratification and bed forms tend to decrease the entrainment capacity of a flow and treating the sediment as a single size usually underpredicts the actual sediment concentration the interactions among these processes are complex and it is nearly impossible to generalize the net effect results are presented for the depth integrated product of sediment concentration and velocity for a wide range of flow and sediment conditions plots arc provided so that more accurate predictions of net transport of suspended

does small bodied salmon spawning activity enhance streambed mobility female salmonids bury and lay their eggs in streambeds by digging a pit which is then covered with sediment from a second pit that is dug immediately upstream the spawning process alters streambed topography winnows fine sediment and mixes sediment in the active layer the resulting egg nests redds contain coarser and looser sediments than those of unspawned streambed areas and display a dune like shape with an amplitude and length that vary with fish size substrate conditions and flow conditions redds increase local bed surface roughness 10 1 channel width w but may reduce the size of macro bedforms by eroding reach scale topography 100 101w research has suggested that spawning may increase flow resistance due to redd form drag resulting in lower grain shear stress and less particle mobility spawning also prevents streambed armoring by mixing surface and subsurface material potentially increasing particle mobility here we use two dimensional hydraulic modeling with detailed prespawning and postspawning bathymetries and field observations to test the effect of spawning by small bodied salmonids on sediment transport our results show that topographical roughness from small salmon redds has negligible effects on shear stress at the reach unit scale and limited effects at the local scale conversely results indicate sediment mixing reduces armoring and enhances sediment mobility which increases potential bed load transport by subsequent floods river restoration in fish bearing streams should take into consideration the effects of redd excavation on channel stability this is particularly important for streams that historically supported salmonids and are the focus of habitat restoration actions

sediment entrainment and deposition measurements in long island sound entrainment and deposition fluxes were measured in central long island sound data were collected with a 3 mhz acoustic transceiver range gated to sample the sediment concentration in 1 cm intervals in the bottom 70 cm of the water column horizontal and vertical velocities were sampled 68 cm above the bottom a turbulent control volume derivation was used to estimate the fluxes and their variation over one 3 5 hr deployment and a second deployment that spanned a tidal cycle time histories of three measures of the shear stress as well as the turbulent and wave kinetic energy were correlated with the entrainment fluxes turbulent and wave kinetic energy was the most reliable correlate with entrainment while the reynolds stress and drag coefficient estimates fared less well the former failing at ebb tide and the latter during periods of low average velocity

structure of flow over alluvial bedforms an experiment on linking field and laboratory methods this paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time space averaging approach measurements of turbulence were obtained in a small river in illinois usa over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica flow structure over the artificial bedform is similar to that observed in laboratory studies but is markedly different from the flow structure over natural bedforms these differences are most pronounced in the roughness sublayer whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity the double time space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests however vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms thus despite the absence of flow separation wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence copyright 2006 john wiley sons ltd

numerical model of turbulent flow over sand dune a two dimensional numerical model based on solution of the reynolds averaged navier stokes equations and the k turbulence model which takes into account sand grain roughness is developed to describe the flow in a fixed dune bed channel the model predicts the velocity and turbulence fields as well as the pressure and friction distributions along the dune the details of the flow in the separation eddy are calculated the model predictions are in general agreement with existing detailed experimental data in a rectangular channel with two dimensional dunes of typical but regular shape the calculated pressure and friction distributions enable determination of the resistance components of the channel without further empiricism however compared to a well known and representative semiempirical engineering formula for the prediction of resistance of natural dune bed channels the two dimensional numerical model predicts significantly different contributions of friction and pressure to the total resista

mechanics of flow over ripples and dunes a simple predictive model for velocity and boundary shear stress fields over two dimensional bedforms of finite amplitude is presented herein and the results of that model are compared with flume data the production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of mclean and smith 1986 their model is modified in order to take account of the flow response to short wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior the theory is also extended to include the region between the bedform crest and the reattachment point thereby providing a spatially complete field of velocity and shear stress results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser doppler velocimeter our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well developed natural bedforms

a self limiting bank erosion mechanism inferring temporal variations in bank form and skin drag from high resolution topographic data fluvial bank erosion rates are often quantified by assuming that the erosion rate is a function of the excess above a critical threshold boundary shear stress applied by the flow research has shown that the form roughness induced by natural topographic bank features such as slumps spurs and embayments is the dominant component of the spatially averaged total shear stress meaning that form roughness provides an important control on bank erosion rates however measuring the relative components of the total shear stress for a natural system is not straightforward in this paper we employ the method of kean and smith 2006a b to partition the form and skin drag components of river bank roughness using a time series 2005 2011 of high resolution topographic surveys of an eroding bank of the cecina river in central italy this method approximates the form drag component of the roughness along a longitudinal bank profile as a series of user defined gaussian curves the extracted metrics are used in conjunction with an estimate of the outer region flow velocity to partition the form and skin drag components of the total boundary shear stress according to the kean and smith analytical solution the relative magnitude of the form and skin shear stress at each survey date is analysed alongside dems of difference to reveal that intense episodes of erosion are followed by periods of quiescence we show that this is due to the protection offered by increased form drag roughness following erosion we conceptualise the dynamic feedbacks that exist between river discharge bank erosion processes and bank form roughness into a simple model of the self limiting nature of river bank erosion

real barchan dune collisions and ejections 1 from high resolution satellite imagery of barchan sand dunes we provide geomorphological evidence of collisions that result in the ejection of a barchan from the wake of another barchan dune previous interpretations suggest this outcome is evidence of soliton or solitary wave behaviour however the physical mechanisms for mass exchange are not fully understood resulting in debate our evidence and interpretation indicates that mass is transferred to the upwind barchan by shadowing a portion of downwind barchan s stoss slope turbulent unsaturated airflow erodes the surface between the dunes creating a smaller dune that ejects from the wake region previous observations lacked the spatial resolution required to document this process therefore our observations clarify the collision dynamics of barchans a broader implication of our observations is the role of collisions in maintaining an equilibrium size distribution in barchan swarms

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

shape and dimensions of stationary dunes in rivers the shape and dimensions length height of sand dunes in rivers are calculated by use of the measured bed shear stress distribution downstream a rearward facing step the transport of sediment is split up into bed load and suspended load which makes it possible to explain the transition to plane bed the model can predict flow resistance curves in alluvial streams

fluvial dunes initiation characterization flow structure research into fluvial dunes spans disciplines studies at grain to reach scales and methodological approaches that include theoretical experimental numerical and field investigations despite significant research efforts to date it remains difficult to provide definitive answers to some fundamental questions regarding dunes this paper reviews three notable challenges that remain regarding fluvial dunes namely scale consistent linking of bed morphologies with turbulent flow fields the intriguing question of what causes trains of highly ordered sediment waves to form in beds of river sediments and how to define the important characteristics of a dune covered bed including lengths shapes and their statistical nature in each case the particular challenge is discussed and then recent research and ways forward are presented copyright 2010 john wiley sons ltd

an empirical model of aeolian dune lee face airflow airflow data gathered over dunes ranging from 60 m tall complex crescentic dunes to 2 m tall simplecrescentic dunes were used to develop an empirical model of dune lee face airflow for straight crested dunes the nature of lee face flow varies and was found to be controlled by the interaction of at least three factors dune shape the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability three types of lee face flow separated attached and deflected along slope or attached and undeflected were found to occur separated flows characterized by a zone of low speed 0 3o of crestal speed back eddy flow typically occur leeward of steep sided dunes in transverse flow conditions unstable atmospheric thermal stability also favours flow separation attached flows characterized by higher flow speeds up to 84 of crestal speed that are a cosine function of the incidence angle typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions depending on the slope of the lee face attached flow may be either deflected along slope lee slopes greater than about 20 or have the same direction as the primary flow lee slopes less than about 20 neutral atmospheric thermal stability also favours flow attachment as each of the three types of lee face flow is defined by a range of wind speeds and directions the nature of lee face flow is intimately tied to the type of aeolian depositional process i e wind ripple or superimposed dune migration grainflow or grainfall that occurs on the lee slope and the resulting pattern of dune deposits therefore the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross strata

turbulent stresses and secondary currents in a tidal forced channel with significant curvature and asymmetric bed forms acoustic doppler current profilers are deployed to measure both the mean flow and turbulent properties in a channel with significant curvature direct measurements of the reynolds stress show a significant asymmetry over the tidal cycle where stresses are enhanced during the flood tide and less prominent over the ebb tide this asymmetry is corroborated by logarithmic fits using 10 min averaged velocity data a smaller yet similar tendency asymmetry in drag coefficient is inferred by fitting the velocity and estimated large scale pressure gradient to a one dimensional along channel momentum balance this smaller asymmetry is consistent with recent modeling work simulating regional flows in the vicinity of the study site the asymmetry in drag suggests the importance of previously reported bed forms for this channel and demonstrates spatial and temporarily variations in bed stress secondary circulation patterns observed in a relatively straight section of channel appear driven by local curvature rather th

investigation of three dimensional wind flow behaviour over coastal dune morphology under offshore winds using computational fluid dynamics cfd and ultrasonic anemometry the behaviour of offshore directed winds over coastal dune and beach morphology was examined using a combination of modelling 3 d computational fluid dynamics cfd and field measurement both model simulations and field measurements showed reversal of offshore flows at the back beach and creation of an onshore sediment transport potential the influence of flow reversals on the beach dune transport system and foredune growth patterns has previously received little attention detailed wind flow measurements were made using an extensive array of mast mounted 3 d ultrasonic anemometers 50 hz arranged parallel to the dominant incident wind direction large eddy simulation les of the offshore wind flow over the dune was conducted using the open source cfd tool openfoam the computational domain included a terrain model obtained by airborne lidar and detailed ground dgps measurements the computational grid 22 million cells included localized mesh refinement near the complex foredune terrain to capture finer details of the dune morphology that might affect wind flows on the adjacent beach measured and simulated wind flow are presented and discussed the cfd simulations offer new insights into the flow mechanics associated with offshore winds and how the terrain steering of wind flow impacts on the geomorphological behaviour of the dune system simulation of 3 d wind flows over complex terrain such as dune systems presents a valuable new tool for geomorphological research as it enables new insights into the relationship between the wind field and the underlying topography the results show that offshore and obliquely offshore winds result in flow reversal and onshore directed winds at distances of up to 20m from the embryo dune toe the potential geomorphological significance of the findings are discussed and simple calculations show that incoming offshore and obliquely offshore winds with mean velocities over 13m s 1 and 7m s 1 respectively have the potential to create onshore directed winds at the back beach with mean velocities above 3 3m s 1 these are above the threshold of movement for dry sand and support previous conclusions about the significance of offshore winds in dune and beach budget calculations

the pole tide and its effect on grace time variable gravity measurements implications for estimates of surface mass variations gravity recovery and climate experiment grace provides monthly solutions for the earth s gravity field in the form of spherical harmonic coefficients these can be used to infer changes in mass at the earth s surface the pole tide the response of the earth and oceans to polar motion causes gravity signals dominated by harmonics of degree 2 order 1 if the pole tide is not removed from grace data it affects the coefficients of those harmonics c21 s21 and introduces errors when using those coefficients to determine surface mass variations the pole tide is partially removed by grace processing centers before solving for the gravity field but long period pole tide signals are not usually included in the grace pole tide correction and so those signals are still present in the grace coefficients we discuss this issue from the standpoint of somebody who uses the grace gravity fields to infer changes in surface mass we arrive at a recommendation for an optimal grace pole tide correction we describe how to modify the c21 s21 coefficients provided by the processing centers so that they conform with our recommendation we discuss the size of the pole tide contributions to c21 s21 compared to those of the direct load induced contributions as an example we show how an incompletely removed pole tide can impact grace results for the trend in ocean mass we consider the impact of mantle anelasticity on long period pole tide corrections and conclude that it is unlikely to affect those corrections by more than 20

sediment volume in the north polar sand seas of mars data from studies of the cross sectional area of terrestrial transverse dunes have been combined with maps of dune morphometry derived from viking orbiter images to generate new estimates of sediment thickness and dune sediment volume in the north polar sand seas of mars a relationship between dune spacing and equivalent sediment thickness est was developed from field data on namibian and north american dunes and was applied to data on dune spacing and dune cover measured on viking orbiter images to generate maps of dune sediment thickness for martian north polar sand seas there are four major sand seas in the north polar region of mars covering an area of 6 8 105 km2 equivalent sediment thickness ranges between 0 5 and 6 1 m with a mean of 1 8 m the sand seas contain a total of 1158 km3 of dune sediment which may have been derived by erosion of polar layered deposits and concentrated in its present location by winds that change direction seasonally

algorithm for resistance to flow and transport in sand bed channels an algorithm is developed that relates depth to discharge and determines bed and suspended load transport for the entire range of bed forms found in sand bed channels equilibrium state geometry of lower flow regime bedforms is also predicted grain shear stress and form resistance are differentiated using a drag coefficient closure and a two segment logarithmic velocity profile a meyer peter type formulation is used to compute sand transport in the bed load layer and for computing suspended sand transport mclean s procedure of 1991 and 1992 is adopted a bed form classification scheme is developed that uses a particle size parameter and the transport strength to predict bed form type it correctly identifies 73 of the 1 192 mostly flume scale calibration data sets for 194 sets of dune transition bed form calibration data a modified version of van rijn s 1984 bed form geometry predictor yields a geometric average predicted to observed height ratio of 1 00 after calibration the algorithm produces overall geometric averages of predicted to observed depth and predicted to observed transport of 1 00 for a verification data set of 855 observations mostly from rivers and canals the overall geometric averages of predicted to observed depth and transport are 0 87 and 1 14

a wind tunnel simulation of the effects of stoss slope on the lee airflow pattern over a two dimensional transverse dune 1 secondary airflow plays an important role in dune formation and development the lee airflow pattern over transverse dunes is important in determining the shape alignment and spacing of dunes and is influenced significantly by the lee slope angle in this paper we present the results of scaled wind tunnel simulations of the effects of stoss slope on the mean lee airflow patterns of transverse dunes the leeward velocity field was measured nonintrusively using particle image velocimetry to assess the airflow pattern we found that the flow separated over the dunes with a leeward angle close to the slip face angle of natural transverse dunes and was insensitive to stoss angle separation cells were characterized by a back eddy reattachment distance height area and aspect ratio were used to characterize the separation cell the first three parameters increased with increasing stoss angle for angles less than 15 but remained constant or decreased slightly with angles greater than 15 on the basis of the leeward velocity profiles six flow regions corresponding to those in previous models were recognized these regions are initially bounded by kinks but gradually transition downwind and merge to recover a uniform velocity profile after a significant distance we suggest that the separation cell is most developed when the angle of stoss slope reaches 15 but the significance of this angle in the evolution of transverse dunes needs further exploration

grain size sorting in grainflows at the lee side of deltas the sorting of sediment mixtures at the lee slope of deltas at the angle of repose is studied with experiments in a narrow deep flume with subaqueous gilbert type deltas using varied flow conditions and different sediment mixtures sediment deposition and sorting on the lee slope of the delta is the result of i grains falling from suspension that is initiated at the top of the delta ii kinematic sieving on the lee slope iii grainflows in which protruding large grains are dragged downslope by subsequent grainflows the result is a fining upward vertical sorting in the delta systematic variations in the trend depend on the delta height the migration celerity of the delta front and the flow conditions above the delta top the dependence on delta height and migration celerity is explained by the sorting processes in the grainflows and the dependence on flow conditions above the delta top is explained by suspension of fine sediment and settling on the lee side and toe of the delta large differences in sorting trends were found between various sediment mixtures the relevance of these results with respect to sorting in dunes and bars in rivers and laboratory flumes is discussed and the elements for a future vertical sorting model are suggested

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

dune field pattern formation and recent transporting winds in the olympia undae dune field north polar region of mars high resolution imaging science experiment hirise imagery of the central olympia undae dune field in the north polar region of mars shows a reticulate dune pattern consisting of two sets of nearly orthogonal dune crestlines with apparent slipfaces on the primary crests ubiquitous wind ripples areas of coarse grained wind ripples and deflated interdune areas geomorphic evidence and dune field pattern analysis of dune crest length spacing defect density and orientation indicates that the pattern is complex representing two constructional generations of dunes the oldest and best organized generation forms the primary crestlines and is transverse to circumpolar easterly winds gross bed form normal analysis of the younger pattern of crestlines indicates that it emerged with both circumpolar easterly winds and ne winds and is reworking the older pattern mapping of secondary flow fields over the dunes indicates that the most recent transporting winds were from the ne the younger pattern appears to represent an influx of sediment to the dune field associated with the development of the olympia cavi reentrant with ne katabatic winds channeling through the reentrant a model of the pattern reformation based upon the reconstructed primary winds and resulting secondary flow fields shows that the development of the secondary pattern is controlled by the boundary condition of the older dune topography

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

effects of bed forms and large protruding grains on near bed flow hydraulics in low relative submergence conditions in mountain rivers bedforms large relatively immobile grains and bed texture and topographic variability can significantly alter local and reach averaged flow characteristics the low relative submergence of large immobile grains causes highly three dimensional flow fields that may not be represented by traditional shear stress flow velocity and turbulence intensity equations to explore the influence of large protruding grains and bedforms on flow properties we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bedforms respectively detailed information on the flow was obtained by combining our measurements with a 3d numerical model commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered however at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near bed shear stresses are likely to be inaccurate zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow the reach averaged shear stress did not correlate to depth or slope as commonly assumed but instead was controlled by the relative boulder submergence and degree of plunging flow for accurate flow predictions in mountain rivers the effects of bedforms and large boulders must be considered

turbulent and macro turbulent structures developed in the benthic boundary layer downstream of topographic features the characteristics and effects of large scale flow structures developed in the benthic boundary layer downstream from large topographic features were analysed throughout a tidal cycle the observed signature of the macro turbulent features consisted of streamwise modules of low horizontal velocity and high suspended sediment concentration ssc alternating with modules of high horizontal velocity and low ssc these modules extended 10 to 20 m streamwise and exceeded 1 m vertically and are believed to be related to flow separation effects over large bedforms upstream of the deployment site the macroscale flow modules intensified the burst like turbulent events and favoured sediment transport ejection like events were magnified during modules of decreasing horizontal velocity and increasing turbidity whereas sweep like events were magnified during modules of increasing horizontal velocity and decreasing ssc the enhanced turbidity of the macroscale modules may be the result of enhanced upward diffusion of sediment by ejection events whereas the low turbidity modules may be induced by increased downward transport of suspended sediment by sweep events these hypotheses were supported by cross spectral analysis performed on velocity and suspended sediment concentration time series recorded at the site an enhanced negative contribution of outward and inward interaction events to the reynolds stress compared to those reported in uniform bbls resulted in abnormally low stress values

calculation of dune morphology 1 the shape and dimensions of dunes formed in an erodible bed and exposed to flowing water are investigated by using a numerical model the flow is calculated using a curvilinear flow model where we combine a k turbulence closure scheme with a sediment transport description and the equation of continuity for the sand bed the complete flow description allows for a discussion about what determines the length of dunes from the equation of continuity for sediment it turns out that the sediment transport over dunes is proportional to the local height of the dune this means that the maximum height of the sand wave occurs at the location of maximum sediment transport we relate the dune length to the location of maximum sediment transport it is demonstrated that the effect of streamline curvature is essential for determining the location of maximum sediment transport at low bed shear stresses due to an overshoot in bed shear stress for large dimensionless bed shear stresses the downstream decay of turbulence from the former dune is demonstrated to play a major role

turbulence structure in a river reach with sand bed measurements and analysis of the three dimensional turbulence structure in a straight lowland river reach are presented accurate measurements of velocity profiles were taken with an acoustic doppier velocimeter adv and a multichannel micropropeller based system it is shown that analytical expressions derived from investigations of laboratory open channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three dimensional at the same time clear differences of empirical parameters between river and laboratory flows are detected it was established that river turbulence is isotropic for spatial scales smaller than the river depth the data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents special attention is paid to the investigation of coherent structures their spatial scales were evaluated applying the conditional average procedure based on uw quadrant analysis on average the scales of ejection and sweep events are as large as 1 5 times the flow depth

mean flow turbulence structure and bed form superimposition across the ripple dune transition 1 this paper describes a laboratory study of the dynamics of flow associated with three different stages of bed form amalgamation across the ripple dune transition measurements of flow velocity were obtained over simplified fixed bed forms designed to simulate conditions at the ripple dune transition using a 2d laser doppler anemometer this yielded information detailing the mean velocity field turbulence statistics local turbulence production local deviations from the mean pressure caused by dynamic effects turbulent kinetic energy and the contributions to the total reynolds stresses from different coherent turbulent events the results broadly confirm previous hypotheses that as bed form amalgamation proceeds across the ripple dune transition a superimposed bedstate can induce a series of critical changes to the flow structure with higher reynolds stresses being produced near and downstream of flow reattachment in particular quadrant 4 events turbulent flow structures with a downstream velocity greater than average and directed towards the bed dominate the vertical turbulent diffusion of longitudinal momentum in near bed regions close to the crest of the next downstream ripple thus providing the potential for increased erosion and sediment transport these experiments using simple fixed beds also provide support for recent measurements that document increased suspended sediment concentrations across the ripple dune transition schindler and robert 2004 2005 and that at the transition the largest bed forms are not necessarily those associated with the most intense shear layer activity schindler and robert 2005 bed form superimposition and amalgamation may thus significantly alter the mean and turbulent flow field as compared to bed forms without superimposition

aerodynamic entrainment threshold effects of boundary layer flow conditions experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions new methods are proposed for 1 the unambiguous determination of aerodynamic threshold for any grain population and 2 approximation of the probability density function pdf distributions of threshold shear velocity for aerodynamic entrainment in wind tunnel experiments the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values initial grain dislodgements and subsequent erosion from narrow strips of loose finely fractionated ballotini were recorded photographically as wind speed was increased boundary layer parameters including average threshold shear velocity u t were calculated using the momentum integral method direct observations show that sporadic oscillation of grains preceded dislodgement at slightly higher velocities most grains rolled over their neighbours before entering saltation initial entrainment in spatially semi organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold these observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process for each grain fraction values of u t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased a probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the pdf for u t required by the model and for future stochastic treatment of the threshold condition

a numerical investigation into the importance of bed permeability on determining flow structures over river dunes although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable and has generally neglected flow between the hyporheic zone and boundary layer herein we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside in the form of returning jets that generate horseshoe shaped vortices the returning flow significantly influences the leeside flow modifying the separation zone lifting the shear layer adjoining the separation zone away from the bed simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside with two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune for the permeable bed case the upwelling flow lifts the separated flow from the bed modifies the shear layer through the coalescence with vortices generated and causes the shear layer to undulate rather than be parallel to the bed these results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients

a theoretical model for aeolian impact ripples new insights into the grain bed impact process arising from both numerical and physical experiments involving single grain impacts lead to a more complete conceptual model of the aeolian saltation process that in turn allows a simple model of aeolian impact ripples to be developed the saltating population may be idealized as consisting of 1 long trajectory high impact energy constant impact angle successive saltations and 2 short trajectory low impact energy reptations it is argued that the spatial variations in mass flux due to the reptating population lead to the growth and translation of impact ripples using the sediment continuity equation an expression for the spatial variation in the ejection rate of reptating grains from a sinusoidally perturbed bed and a probability distribution for the reptation lengths a simple stability analysis demonstrates that the flat bed is unstable to small amplitude perturbations a fastest growing wavelength emerges that is roughly six times the mean reptation length and is only weakly dependent upon the detailed shape of the probability distribution of reptation lengths the results match well with the observed initial wavelengths in wind tunnel experiments

intermittent turbulent events over sandy current ripples a motion picture analysis of flume experiments abstract a series of laboratory flume experiments was conducted in order to elucidate sediment transport processes over sandy current ripples instantaneous cinematography capable of recording up to 30 frames per second was employed to trace the coherent trajectories of the sediment particles and flow analysis of channel side view images suggests that sediment behaviour over a fully developed current ripple can be categorized as distinct stages on both stoss and lee sides near the reattachment point on the stoss side sediment particles exhibit the following intermittent but coherent motions 1 impact of sediment laden fluid on the reattachment point 2 entrainment of particles 3 spiral like turbulent motion and 4 downstream dissipation of particles the sequential sediment behaviour is called micro scale kolk which is a key mechanism responsible for turbulent sediment entrainment over current ripples the salient features of boil were not observed in the crest and lee side region sequential images show the following characteristics 1 detachment of particles and fluid at the crest 2 entrainment and shielding of particles 3 avalanching and rolling of free shear layer and 4 downstream dissipation and recreation of a lee face segregation of heavy minerals was accomplished by successive avalanches along the lee slope and mixing vortices in the separation cell

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

turbulent flow over three dimensional dunes 2 fluid and bed stresses 1 dunes formed in response to fluid flow exert a total boundary stress on the fluid that is made up of form drag and skin friction the latter of which is generally considered important for predicting sediment transport and dune evolution previous research has used various methods to estimate the total stress and its subcomponents with recent work suggesting the application of a spatial average to the equations of motion a complete three dimensional 3 d form of this analysis is derived and applied to recent measurements of turbulent open channel flow over 3 d dunes spatial averages of reynolds shear stresses over 3 d dunes cannot be used to predict total boundary shear stress however estimations of total boundary shear stress from the spatially averaged equations of motion agree with not only the sum of form drag and skin friction but also the direct measurements of average free surface slope form induced stresses from secondary circulations augment the relatively low reynolds shear stresses over the 3 d dunes these low turbulence levels have ramifications for sediment transport predictions in that use of the total boundary shear stress in prediction of sediment transport over 3 d dunes must account for these low turbulence levels so as not to overpredict transport rates

the stability of ripples and dunes abstract the stability of ripples and dunes in flows of finite depth are investigated for the linear small amplitude case and also for the more realistic finite amplitude situation for the former perturbation approach is taken leading to the general result that without the inclusion of some lag between the sediment transport rate and the boundary shear stress all wavelengths are unstable this results from an upstream shift in the stress relative to the bedform leading to deposition at the crest by assuming a realistic lag based on the mechanics of bed load transport a fastest growing wave at ripple scales results similarly if suspended sediment were included longer lags result and a dune scale instability could result some investigators have invoked a gravitational effect arguing that sediment transport is enhanced downslope and retarded upslope this creates an effective downstream shift in the transport rate and some argue that both ripple and dune instabilities result however a realistic evaluation of this effect shows that it is much too small to be effective naturally occurring bedforms are of finite amplitude typically they are asymmetrical often causing flow separation this highly non linear process creates a vastly different flow environment from that assumed in the initial stability case a wake like flow structure develops downstream of the separation zone and an internal boundary layer develops beneath the complex interaction of these two different flow regimes as well as the effect induced by the topography of the bedform itself produces a shear stress field that dictates the stability of the feature itself

large eddy simulation of three dimensional dunes in a steady unidirectional flow part 1 turbulence statistics we performed large eddy simulations of flow over a series of three dimensional dunes at laboratory scale reynolds number based on the average channel depth and streamwise velocity was 18 900 using the lagrangian dynamic eddy viscosity subgrid scale model the bedform three dimensionality was imposed by shifting a standard two dimensional dune shape in the streamwise direction according to a sine wave the statistics of the flow are discussed in 10 cases with in phase and staggered crestlines different deformation amplitudes and wavelengths the results are validated qualitatively against experiments the three dimensional separation of flow at the crestline alters the distribution of wall pressure which in turn may cause secondary flow across the stream which directs low momentum fluid near the bed toward the lobe the most downstream point on the crestline and high momentum fluid near the top surface toward the saddle the most upstream point on the crestline the mean flow is characterized by a pair of counter rotating streamwise vortices with core radius of the order of the flow depth however for wavelengths smaller than the flow depth the secondary flow exists only near the bed and the mean flow away from the bed resembles the two dimensional case staggering the crestlines alters the secondary motion the fastest flow occurs between the lobe and the saddle planes and two pairs of streamwise vortices appear a strong one centred about the lobe and a weaker one coming from the previous dune centred around the saddle the distribution of the wall stress and the focal points of separation and attachment on the bed are discussed the sensitivity of the average reattachment length depends on the induced secondary flow the streamwise and spanwise components of the channel resistance the skin friction and the form drag and the contribution of the form drag to the total resistance are also studied three dimensionality of the bed increases the drag in the channel the form drag contributes more than in the two dimensional case to the resistance except for the staggered crest case turbulent kinetic energy is increased in the separated shear layer by the introduction of three dimensionality but its value normalized by the plane averaged wall stress is lower than in the corresponding two dimensional dunes the upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer respectively affecting the turbulent kinetic energy

an integrated model for dune morphology and sand fluxes on mars the transport and deposition of sand is the most prevalent agent of landscape modification on mars today with fluxes comparable to some sand dunes on earth until now the relationship between sand flux and dune field morphology has been poorly constrained by tracking dune movement over 10 km long dune fields in herschel crater and nili patera representative of many dune fields on mars we find a downwind flux decrease that correlates with a sequence of changing morphology from barchans to barchanoids and seifs longitudinal dunes to isolated dome dunes and ending with sand sheets we show empirical consistency with atmospheric internal boundary layer ibl theory which can describe these broad flux and morphology changes in martian dune fields deviations from ibl flux predictions are from wind streamline compressions up slopes leading to a speedup effect by establishing a dune field morphology type example and correlating it with measured and predicted flux changes we provide an integrated morphology and flux model that can be applied to other areas of mars and be used to infer paleo environmental conditions from preserved sandstone