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

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

measuring flow velocity and sediment transport with an acoustic doppler current profiler an acoustic doppler current profiler adcp measures three dimensional velocity profiles within the water column using the doppler shift principle whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration the adcp offers many advantages over traditional single point current meters and sediment samplers including deployment from a moving launch a single instrument for both velocity and sediment transport measurements profiles of three dimensional velocity and suspended sediment and the ability to map an entire flow field limitations of adcps include a large sampling diameter close to the bed coarse measurement of vertical velocity fragmented bottom track records a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport and sensitivity of the acoustic backscatter to particle size

instruments for particle size and settling velocity observations in sediment transport in this paper we describe two sensors for measurement of particle size distribution and settling velocity distribution these measurements are critical to the correct estimation of the true sediment concentration in the field as well as to evaluating models for transport rates of sediments a multi angle measurement of laser scattering is made and inverted to obtain the particle size distribution since small angle scattering is relatively insensitive to particle composition the size distribution measurements are robust and do not require particle refractive index it is shown that with a knowledge of the size distribution true particulate volume concentration can be obtained unaffected in calibration by changes in particle size distribution the data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer the importance of these observations lies in the implication that historical data acquired with single parameter optical or other sensors needs to be revisited in the second instrument analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates in this case the history of concentration of each size class is examined to determine the settling velocity without invoking any assumptions of settling regime settling velocity data from a field experiment off the new jersey coast fit the model wf na 0 45 10 23 a 1 2 n

length of flow separation over dunes dimensional analysis was used to define the length of the flow saparation for the case of two dimensional uniform flow over dunes which was expressed in terms of bed form geometry and the sand size of the bed material experiments were conducted in a rectangular flume using artificial dunes and a flow visualization method to measure the flow separation length when dunes are flat length of the flow separation is affected by the sand grain roughness when dunes are steep the length of the flow separation is due mainly to the bed form geometry and is relatively constant the reverse rectangular steps can be considered as dunes of infinite length and the length of the flow separation for such steps will always be greater than that obtained for dunes of finite length

mean and turbulent flow structure during the amalgamation process in fluvial bed forms 1 fluvial channels present bed forms such as dunes and ripples that alter instantaneous hydrodynamics parameters such as flow velocities water surface profiles bed shear stresses and reynolds stresses and create turbulent coherent structures that are significantly different from those presented in flat bed conditions it is known that les based models are more suitable than rans models to reproduce the complex hydrodynamics around bed forms herein a les model is applied to describe the mean and turbulent flow structure under superimposed bed forms three cases were simulated run i train of ripples run ii superimposed bed forms and run iii amalgamated bed forms the les modeling was performed using a free surface condition to allow the model to develop undulations and boils on the water surface caused by effect of the bed forms some important conclusions from this study are the division of high and low shear stresses on the stoss side of the dune the progression of the flow field topology from run i and run iii and the type of turbulent coherent structures found in each stage the region of high shear stresses was related to turbulence production in which the streamwise velocity fluctuations where strips structures are related to streaks were associated to the modification of the bed morphology the turbulence horseshoes vortices thv were more frequent in run i than in the other two cases where streamwise rolls were more frequent finally the frequency of the bursting events increased from run i to run ii and decreased from run ii to run iii implications of detailed hydrodynamics into bed forms processes are also presented and discussed

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

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

a two dimensional morphological model based on next generation circulation solver ii application to caofeidian bohai bay china abstract this paper is a companion of a two part papers on the development of a two dimensional morphological model curwac2d sed based on next generation circulation solver in this paper the model developed in paper i is applied to the large scale construction of near harbor industrial zone with deep water harbor by reclamation in caofeidian bohai bay which is a typical sandbar lagoon bay type tidal inlet system characteristics of tides currents waves and sediments in caofeidian are firstly analyzed with field data from comprehensive field observations carried out in 2008 these data are then used for model calibration and validation showing a good agreement between simulated results and field measurements for tides currents and suspended sediments the verified model is applied to study the effects of the construction of caofeidian embankment and the on going engineering scheme on hydrodynamics and sediment transports as compared with no constructions under three representative tide conditions and their combinations with normal and strong waves as well as on morphology evolution the simulated results indicate that the present scheme leads to morphological changes in caofeidian with acceptable rates of erosion and sedimentation however some attentions should be paid to erosion at offshore wharf in front of caofedian foreland which may lead to potentially increasing landslide risks due to steep slope and sedimentation in laogonggou creek

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

macroturbulence generated by dunes fraser river canada abstract a selectable frequency acoustic profiler and electromagnetic current meter were deployed in the lower fraser river canada to examine large turbulent structures features were generated on the lower stoss sides of dunes and appeared at the surface tens of metres downstream as strong upwellings boils they were identified with acoustic clouds in the acoustic profile event recurrence periods measured from acoustic time series and from ejection events identified in the current meter record had similar two component frequency distributions but the acoustic values lacked the highest frequencies a comparison of measured periods with predictions from the strouhal law indicate that both dune amplitude and the internal boundary layer developed over the dunes are significant length scales associated with the generation of the macroturbulence our data give no support to the notion that boils develop from microturbulent bursts

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

on the relationship between flow and suspended sediment transport over the crest of a sand dune rio parana argentina the links between large scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades with past studies illustrating the origin of such turbulence and its influence on flow resistance sediment transport and bedform morphology in this study of turbulence and sediment suspension over large sand dunes in the rio parana argentina time series of three dimensional velocity and at a point suspended sediment concentration and particle size were measured with an acoustic doppler current profiler and laser in situ scattering transmissometer respectively these time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment the analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections regions of high suspended sediment concentration are found to correlate well with such events the frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations such a finding suggests that a combination and interaction of vortex shedding and wake flapping changing length of the lee side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures including information on the topology of dune related turbulent flow structures at the flow stage investigated the turbulent flow events and their associated high suspended sediment concentrations are seen to grow with height above the bed until a threshold height ca 0 45 flow depth is reached above which they begin to decay and dissipate

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

observations of coherent flow structures over subaqueous high and low angle dunes large scale coherent flow structures cfs above dunes are the dominant source of flow resistance and constitute the principal mechanism for sediment transport and mixing in sand bed river and estuarine systems based on laboratory observations cfs formation has been previously linked to flow separation downstream of high angle dunes with lee slopes of 30 how cfs form in natural deep rivers and estuaries where dunes exhibit lower lee slopes and intermittent flow separation is not well understood here we present particle image velocimetry piv measurements from an experiment where dune lee slope was systematically varied 30 20 10 while other geometric and hydraulic parameters were held constant we show that cfs form downstream of all three dune geometries from shear layer vortices in the dune lee the mode of cfs formation undergoes a low frequency oscillation with periods of intense vortex shedding interspersed with periods of rare vortex shedding streamwise alignment of several vortices during periods of intense shedding results in wedge shaped cfs that are advected above the dune stoss side streamwise length scales of wedge shaped cfs correspond to large scale motions lsm we hypothesize that the advection of lsm over the dune crest triggers the periods of intense shedding in the dune lee lsm are weaker and smaller above low angle dunes however the low frequency oscillation in cfs formation periods persists the formation of smaller and weaker cfs results in a reduction of flow resistance over low angle dunes

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

hydraulic and morphological characteristics of middle and upper reaches of the parana river argentina and brazil the parana river basin is a 2 600 000 km2 south american fluvial system most of which is in brazil and argentina the channel is about 4000 km long the mean annual discharge is 17 000 m3 s 1 and the annual sediment load is more than 150 000 000 tons the basin of the paraguay river its main tributary covers about 38 of the parana basin this paper compares the upper and middle channels of the parana hydraulic morphological and sedimentological features of the parana river upstream and downstream of the confluence with the paraguay were analyzed in order to evaluate the changes in these variables the effect of the paraguay on water and suspended sediment discharges was examined the paraguay river contribution explains most of the variation in channel conditions upstream and downstream of the confluence the channel pattern however remains braided the high bed load of the parana river in proportion to the total sediment discharge 21 in the upper course 25 in the middle course determines the similarity in drainage

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

a field study of turbulence and sediment dynamics over subaqueous dunes with flow separation this study examines flow turbulence and sand suspension over large dunes in canoe pass a distributary channel of the fraser river delta canada dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30 velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter obs probes the general patterns of time averaged velocity and sand suspension are consistent with previous studies including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near bed velocity and sand concentration along the stoss side of the dune frequency spectra of near bed velocity and obs records from leeside separation zones are composed of two distinct frequencies providing field confirmation of scale relations based on flume experiments the low frequency spectral signal probably results from wake flapping and the high frequency signal from vortex shedding the wake flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand predictions from a depth scale strouhal law show good agreement with measured wake flapping frequencies cross correlations of obs records reveal that turbulent sand suspension structures advect downstream at 23 25 above the horizontal these advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

an experimental study of turbulent flow over a low angle dune 1 many large sand bed alluvial channels are dominated by dunes that possess low angle lee sides often 10 which play a critical role in the transportation of sediment and generation of significant bed form roughness despite the fact that these low angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee study of flow associated with low angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms results from a detailed scale model experimental study of flow over a low angle dune which is based on a prototype dune from the fraser river canada present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms two dimensional laser doppler anemometer measurements over a low angle dune maximum lower lee side slope 14 show that dune morphology exerts a dominant control on the turbulent flow causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above the scale model confirms that permanent flow separation does not occur over low angle dunes but instead is replaced by a small region here 7 of the dune wavelength in length of intermittent flow reversal which may be present for up to 4 of the time shear layers generated along this small zone of decelerated and or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes turbulence production associated with low angle dunes is dominated by eddies generated along this shear layer which produce highly variable horizontal and vertical velocities and large reynolds stresses in this region these results show that macroturbulence associated with low angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low angle dunes in natural sand bed rivers these results highlight the need to consider the temporal evolution and intermittency of shear layer behavior often very near the bed when interpreting the generation of macroturbulence and dispersal of sediment associated with low angle dunes

burst like sediment suspension events in a sand bed river in a much quoted paper jackson 1976 hypothesized that turbulent bursting motions such as those documented in laboratory boundary layers play a major role in alluvial sediment suspension to date the hypothesis remained largely untested due to difficulties in monitoring turbulent suspension in rivers this study provides field data documenting burst like turbulent motions over a sandy bed channel and quantifying the role of these motions in sand suspension the data were collected in a 10 m deep channel of the fraser river near mission british columbia canada turbulent fluctuations of both flow components downstream and normal to the bed along with the output of an optical suspended sediment sensor were monitored 1 m above the river bed typical flow velocities averaged 0 9 ms 1 at the sensors where mean suspended sediment concentrations were 500 mgl 1 decimetre height small dunes on the backs of larger half metre amplitude dunes covered the channel bed in the area brief but intense burst like ejection and inrush events were identified in the flow records where they are responsible for a high degree of intermittency in shear stress over the dunes 80 per cent of the turbulent momentum exchange across the 1 m level can be ascribed to such brief 3 8 s duration events active under 12 per cent of the time in addition the record of fluctuating sediment concentrations reveals these burst like motions to be highly effective in vertically mixing suspended sediment and thus ultimately in maintaining suspended sediment transport above the dune bed the bulk 60 and 90 per cent in two deployments of the vertical sediment mixing was accomplished by intense events active some 10 per cent of the time no discrete recurrence timescale for these burst like mixing events is evident however rather a continuous variation of return periods was observed as a function of the magnitude of vertical mixing event considered to that extent conceptual models of sediment transport in terms of burst events with a predictable recurrence such as proposed by jackson 1976 may be misleading

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

spectral analysis of turbulent flow and suspended sediment transport over fixed dunes laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two dimensional dunes in a sediment starved flow contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures spectral analysis reveals that peak spectral energies generally occur at 1 2 hz for the stream wise velocity component and 2 4 hz for the cross stream and vertical velocity components spectra show larger and better defined energy peaks near the shear layer peak spectral energies for suspended sediment concentration occur near 1 hz throughout the flow squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant integral timescales for velocity range from 0 20 s for the streamwise component to 0 06 s for the cross stream and vertical components integral length scales for velocity range from 0 065 to 0 135 m for the streamwise component which is comparable to flow depth and from 0 020 to 0 030 m for the crossstream and vertical components which is comparable to dune height for suspended sediment concentration integral timescales and length scales are similar to the streamwise velocity component

size shape and dynamics of large scale turbulent flow structures in a gravel bed river in this paper we present a detailed investigation of the size scale and dynamics of macro turbulent flow structures in gravel bed rivers we used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel bed river the array was deployed successively in various configurations in order to quantify the vertical lateral and longitudinal extent of the flow structures and to estimate their advecting velocities to depict the spatial and temporal properties of the flow structures we used space time velocity matrices space time correlation analysis and coherent structure detection schemes the results show that the large scale turbulent flow structures in a gravel bed river occupy the entire depth of the flow and that they are elongated and narrow the length of the structures is 3 to 5 times the flow depth while the width is between 0 5 and 1 times flow depth in spite of the high roughness of the bed these values are similar to those reported in the literature for laboratory experiments on large scale turbulent flow structures the dynamics of the large scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near bed region our evidence suggests that large scale flow incursions trigger ejections in the near bed region that can develop into megabursts that can reach the water surface

large eddy simulation of the velocity intermittency structure for flow over a field of symmetric dunes owing to their frequent occurrence in the natural environment there has been significant interest in refining our understanding of flow over dunes and other bedforms recent work in this area has focused in particular on their shear layer characteristics and the manner by which flow structures are generated however field based studies are reliant on single or multi point measurements rather than delimiting flow structures from the velocity gradient tensor as is possible in numerical work here we extract pointwise time series from a well resolved large eddy simulation as a means to connect these two approaches the at a point analysis technique is termed the velocity intermittency quadrant method and relates the fluctuating longitudinal velocity u 1 t to its fluctuating pointwise holder regularity despite the difference in boundary conditions our results agree very well with previous experiments that show the importance in the region above the dunes of a quadrant 3 u 1 0 dominant near the wall and quadrant 2 dominant close to the lower part of the separated shear layer these results are consistent with a near wall advection of vorticity into a region downstream of a temporarily foreshortened reattachment region and the entrainment of slow moving and quiescent fluid into a faster more turbulent shear layer a comparison of instantaneous vorticity fields to the velocity intermittency analysis shows how the pointwise results reflect larger scale organisation of the flow we illustrate this using results from two instantaneous datasets in the former extreme velocity intermittency events corresponding to a foreshortened recirculation region and high pressures on the stoss slope of the dune immediately downstream arise and the development of intense flow structures occurs as a consequence in the other case development of a skimming flow with relatively little exchange between the inner and outer regions results in exceedances because of the coherence associated with this high velocity high turbulence outer region thus our results shed further light on the characteristics of dune flow in the near wall region and importantly for field based research show that useful information on flow structure can be obtained from single point single velocity component measurements

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

measuring velocity and shear stress over dunes with acoustic doppler profiler this study evaluates the use of an acoustic doppler profiler adp for measurement of velocity profiles over dunes in an estuary the adp can be deployed from a moving launch and provides three dimensional velocity profiles that can be used to describe flow structure and to provide rough estimates of shear stress quadratic stress models are recommended for shear stress because they estimate skin friction and form stress whereas the law of the wall only reliably estimates total stress over dunes the most serious limitation of the adp is a beam geometry that results in a large sampling diameter close to the bed that does not adequately measure velocity in dune troughs

turbulent flow over a dune green river colorado detailed echo sounder and acoustic doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide low gradient alluvial reach of the green river based on the geometric position of the sensor over the bedforms measurements were taken in the wake in transitional flow at the bedform crest and in the internal boundary layer spatial distributions of reynolds shear stress turbulent kinetic energy turbulence intensity and correlation coefficient are qualitatively consistent with those over fixed two dimensional bedforms in laboratory flows spectral and cospectral analysis demonstrates that energy levels in the lee of the crest i e wake are two to four times greater than over the crest itself with minima over the stoss slope within the developing internal boundary layer the frequency structure in the wake is sharply defined with single dominant peaks peak and total spectral and cross spectral energies vary over the bedform in a manner consistent with wave like perturbations that break or roll up into vortices that amalgamate grow in size and eventually diffuse as they are advected downstream fluid oscillations in the lee of the dune demonstrate strouhal similarity between laboratory and field environments and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field copyright 2005 john wiley 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 morphodynamics of fluvial sand dunes in the river rhine near mainz germany ii hydrodynamics and sediment transport the dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply limited reach of the river rhine during daylong surveys suspended sediment concentration bedload transport rate water depth flow velocity turbulence intensity near bed shear stress and water temperature were recorded over individual isolated dunes this paper considers the hydrodynamic environment and sediment transport over the dunes a companion paper details the sedimentology and morphology of the dunes flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes and gravels are rarely entrained by in bank discharges unsteady and non logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes and the near bed flow demonstrates evidence of large scale coherent low frequency flow structures these may reflect stacked sequences of separated boundary layers generated by secondary dunes however the low amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides bed roughness and near bed shear stress commonly increase steadily over the stoss of dunes but may decrease near the crest especially where a crestal platform exists that is devoid of secondary bedforms bed roughness scales with the physical size of bed roughness elements however variability in roughness lengths is large owing to the composite nature of bed roughness bedload transport over stoss slopes is spatially variable but often shows an increase with increasing bed shear stress over the stoss well formed wakes only develop downstream of lee slopes which are close to the angle of repose otherwise separation is weak and suspension and settling of fine sediments is of little consequence to dune evolution wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side which may be related to vortex shedding from the dune however wake current speeds are extremely low and variable in direction reverse flow is not sustained and no retrogressive bedforms occur in the lee of large dunes near bed shear stress and bed roughness are usually low within the wake reflecting low current speeds weak wake flow reattachment occurs at a variable distance downstream up to several times the duneform height a two dimensional numerical model for flow over dune topography calibrated using average parameter values provided a reasonable description of the flow upstream and over the backs of large dunes as far as the crestal region wake flow could not be modelled however the temporal and spatial complexity of natural three dimensional flow over the dunes resulted in variance in parameter estimates this variance precluded modelling of flow and bedload transport consequently it was not possible to model dune evolution in a deterministic sense

predicting oil quality from sidewall cores using pfid tec and nir analytical techniques in sandstone reservoirs rio del rey basin cameroon abstract the expense and uncertainty of obtaining fluid samples from mdt tools provide a strong incentive for an alternative method for assessing oil character sidewall cores which are inexpensive and easy to obtain contain sufficient oil for microanalyses that can provide indications of oil quality degree of degradation api gravity and numerous additional characteristics including sulfur content acid number and viscosity the present study is concerned with the application of the micro techniques pfid pyrolysis flame ionization detection tec thermal extraction chromatography and nir near infra red spectroscopy for the characterization of oil from sidewall cores taken from two exploration wells in the rio del rey basin of cameroon in 1993

field measurements of flow fields and sediment transport above mobile bed forms 1 field measurements of hydrodynamic conditions bed forms and suspended sediments obtained using a suite of approximately colocated sensors above a mobile bed composed of coarse sand d50 1 28 mm in a strong tidal current with superimposed waves are used to investigate the mean and turbulent flow fields bed forms and the resuspension and transport of sediments in suspension and as bed load while there is no evidence for flow separation properties of the local turbulent flow field are found to vary spatially in response to the bed morphology time averaged suspended sediment concentration profiles c profiles are found to be spatially variable with higher concentrations observed in the trough regions between bed form crests well known expressions used to predict c profiles are found to agree well with measured time averaged c profiles the instantaneous suspended sediment concentration field sscf is found to comprise a series of semiperiodic resuspension events interspersed by relatively quiescent periods using a modified quadrant analysis scheme these events are shown to be correlated closely with times when current and wave induced flows combine and exceed the threshold for suspension the effectiveness of a given combined wave current flow event in resuspending bed sediments is found to depend upon its location relative to bed morphology it is considered that local bed response may be influenced by spatial differences in grain size and compaction brought about by the processes of bed load transport over the bed forms the total mass of sediment in suspension is found to be less than the potential transport capacity of the flow suggesting strongly that the supply of fine sediments is source limited while measured bed form geometry agrees well with predicted dimensions for ripples grain size alone suggests only dunes should be present these were not detected bed load transport qb calculated from measured bed form migration speeds and the measured suspended sediment flux qs was found to agree favorably with empirical predictions

response of low angle dunes to variable flow current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments there are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides as a consequence the ability to predict bedform response to variable flow is rudimentary using high resolution multibeam bathymetric data this study explores the dynamics of a dune field developed by tidally modulated fluvially dominated flow in the fraser river estuary british columbia canada the dunes were dominantly low lee angle features characteristic of large deep river channels data were collected over a field ca 1 0 km long and 0 5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet yielding the most comprehensive characterization of low angle dunes ever reported the data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs then increasing as the tide rises and the flow velocities decrease bedform lengths do not appear to respond to the changes in velocity caused by the tides changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity indicating that changes in the bedform geometry lag changes in the flow the data reveal that lee angle slope responds directly to suspended sediment concentration supporting previous speculation that low angle dune morphology is maintained by erosion of the dune stoss and crest at high flow and deposition of that material in the dune trough