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

radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions 1 the investigations carried out between 2002 and 2004 during six field experiments within the operational radar and optical mapping in monitoring hydrodynamic morphodynamic and environmental parameters for coastal management oroma project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters the coastal monitoring radar of the gkss research center geesthacht germany is based on a kelvin hughes rsr 1000 x band 9 42 ghz vertical vv polarized river radar and was mounted on board the research vessel ludwig prandtl during the experiments in the lister tief a tidal inlet of the german bight in the north sea the important progress realized in this investigation is the availability of calibrated x band radar data another central point of the study is to demonstrate the applicability of the quasi specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed radar data have been taken at very low grazing angles 2 6 of flood and ebb tide oriented sand wave signatures at the sea surface during ebb tidal current phases current speeds perpendicular to the sand wave crest 0 6 m s 1 have been measured at wind speeds 4 5 m s 1 and water depths 25 m the difference between the maximum measured and simulated normalized radar cross section nrcs modulation of the ebb tide oriented sand wave is 27 for the flood tide oriented sand wave a difference of 21 has been calculated the difference between the minimum measured and simulated nrcs modulation of the ebb tide oriented sand wave is 10 and for the flood tide oriented sand wave a value of 43 has been derived phases of measured and simulated nrcs modulations correspond to asymmetric sand wave slopes the results of the simulated nrcs modulation show the qualitative trend but do not always quantitatively match the measured nrcs modulation profiles because the quasi specular scattering theory at very low grazing angle is a first order theory

experimental study on transient and steady state dynamics of bedforms in supply limited configuration the purpose of the present study is to investigate experimentally the development of bedforms in a configuration where the sediment supply is limited the experimental setup is a rectangular closed duct combining an innovative system to control the rate of sediment supply qin and a digitizing system to measure in real time the 3d bedform topography we carried out different sets of experiments with two sediment sizes 100 mm and 500 mm varying both the sediment supply and the water flow rate to obtain a total of 46 different configurations after a transient phase steady sub centimeter bedforms of various shapes have been observed barchans dunes straight transverse dune linguoid transverse dunes and bedload sheets height spacing migration speed and mean bed elevation of the equilibrium bedforms were measured for a given flow rate two regimes were identified with fine sediment i a monotonic increasing regime where the equilibrium bedform height and velocity increase with the sediment supply rate qin and ii an invariant regime for which both parameters are almost independent of qin for coarse sediment only the first regime is observed we interpret the saturation of height and velocity for fine sediment bedforms as the transition from a supply limited regime to a transport limited regime in which the bedload flux has reached its maximum value under the prevailing flow conditions we also demonstrate that all experiments can be rescaled if the migration speed and height of the bedforms are respectively divided and multiplied by the cube of the shear velocity this normalization is independent of grain size and of bedform morphology these experimental results provide a new quantification of the factors controlling equilibrium height and migration speed of bedforms in supply limited conditions against which theoretical and numerical models can be tested

aeolian processes across transverse dunes i modelling the air flow this paper discusses a two dimensional second order closure model simulating air flow and turbulence across transverse dunes input parameters are upwind wind speed topography of the dune ridge and surface roughness distribution over the ridge the most important output is the distribution of the friction velocity over the surface this model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation the sand transport model is discussed in a separate paper the simulated wind speeds resemble patterns observed during field experiments despite the increased wind speed over the crest the friction velocity at the crest of a bare dune is reduced compared to the upstream value because of the effect of stream line curvature on turbulence these curvature effects explain why desert dunes can grow in height in order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model in these equations streamline curvature is an important parameter the main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex as a result the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot in the sand transport model this was overcome by defining a separation zone copyright 1999 john wiley sons ltd

sorting in grain flows at the lee side of dunes abstract sediment sorting at the lee side of ripples dunes and bars has already been recognized long ago a predictive model of the sorting is necessary but unavailable for implementation in sediment transport models for sediment mixtures relevant processes in sedimentological and physical literature are reviewed and compared to the sparse data of sediment sorting a synthesis is given of the most important variables governing the sorting processes for the benefit of future experimentation and modelling these variables are the sorting standard deviation skewness and bimodality of the sediment mixture delivered to the brink point the height of the dune or bar relative to the average grain size of the mixture the velocity of the flow above the brink point relative to the settling velocity for all grain size fractions and the frequency of the grain flows in addition the initiation mechanism and frequency of the grain flows affect the pattern and effectiveness of sorting

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

on the dynamics of sediment suspension by residual reynolds stresses confirmation of bagnold s theory bagnold s dynamic theory for sediment suspension requires that the immersed weight of suspended grains over unit bed area is supported by an upward directed residual reynolds stress yy arising from asymmetrical shear turbulence the present paper presents an analysis of previously published turbulence data which confirms the existence of this residual stress and indicates its generation in the lowermost part of the buffer layer of turbulent shear flows the magnitude of yy is estimated as about 0 3 yx calculations from experimental data on suspended fine sand transport over upper phase beds reveals that yy is in approximate equilibrium with the weight stress due to the suspended load

turbulence at a roughness transition in a depth limited flow over a gravel bed turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented the bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large protuberant clasts superimposed on the bed in the downstream section an array of four electromagnetic current meters operating at a frequency of 20 hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as reynolds shear stress uv cross correlation and quadrant analysis of velocity fluctuations the results indicate that the presence of protuberant clasts modifies the turbulence structure increases turbulence intensity and appears to dominate turbulence generation distinct flow zones are observed in particular a clear distinction is made between i a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying with some vortices or rollers possibly being trapped in this near bed flow region and ii an inner layer located immediately above the major protuberant clasts identified by positive vertical velocities and hence upward directed fluid motion the dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth limited flows over coarse and irregular bed surfaces

morphology of extrusive basalts and its relationship to seismic velocities in the shallow oceanic crust recent seismic experiments have revealed that a shallow low velocity zone is a common feature within the uppermost few hundred meters of very young less than 100 kyr oceanic crust at fast spreading ridges the shape of this zone suggests that the low velocities are related to the constructional morphology of the extrusive layer 2a portion of the crust with this in mind we use simple analytical techniques to study the effects of characteristic morphologic elements of the extrusive basalts on their elastic properties we find that high compliance of the contacts between pillows and pillow fragments can account for all of the observed velocity anomaly voids between pillows are by comparison to their effects on porosity considerably less important radial cooling cracks and voids within the pillows themselves will affect the velocities in direct proportion to their effect on the individual pillows the relatively rapid increase in velocity with age in these very young extrusives can be explained by welding of pillow contacts and stiffening of the individual pillows by infilling of radial fractures both of which may take place without a large reduction in porosity if voids between pillows are filled initially by sediments the cementation of these materials can also cause large increases in velocity without a large addition of mass because of the importance of such subtle variations in morphology there can be no one to one relationship between seismic velocities and porosities within the extrusive layer of the oceanic crust therefore independent measures of crustal porosity must be found for example using sea floor gravity surveys

suspended sediment concentration and calibre in relation to surface flow structure in squamish river estuary southwestern british columbia surface grabs of suspended sediments from boils and the background flow in squamish estuary revealed strong differences between the two environments boils exhibit higher sediment concentrations and larger particle sizes than the background flow the higher sediment concentrations and larger grain sizes in boils are related to tidal phase with maximum concentrations and sediment sizes occurring in the early decelerating phase of the tidal flow sediment concentrations and grain sizes in the background flow are comparatively invariant with tidal phase boils appear to play an important if not dominant role in suspending and transporting sand sized sediments in squamish estuary

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

geometry and kinematics of dunes during steady and unsteady flows in the calamus river nebraska usa the geometry and kinematics of river dunes were studied in a reach of the calamus river nebraska during day long surveys dune height length steepness migration rate creation and destruction were measured concurrently with bedload transport rate flow depth flow velocity and bed shear stress within a survey individual dune heights lengths and migration rates were highly variable associated with their three dimensional geometry and changes in their shape through time notwithstanding this variability there were discernible changes in mean dune height length and migration rate in response to changing discharge over several days changes in mean dune height and length lagged only slightly behind changes in discharge therefore during periods of both steady and unsteady flow mean dune lengths were quite close to equilibrium values predicted by theoretical models mean dune steepnesses were also similar to predicted equilibrium values except during high falling flows when the steepness was above that predicted variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period however the calculated rates of change of height of individual dunes vary considerably from those measured rates of dune creation and destruction were unrelated to discharge variations contrary to previous results instead creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses but not at higher bed shear stresses when suspended sediment transport is significant

dune migration in a steep coarse bedded stream during 1986 and 1987 migrating bed forms composed of coarse sand and fine gravel d50 1 8 to 9 1 mm were documented in the north fork toutle river at kid valley washington at flow velocities ranging from 1 6 to 3 4 m s 1 and depths of 0 8 to 2 2 m the bed forms predominantly lower regime dunes were studied with a sonic depth sounder transducer suspended in the river at a stationary point twelve temporal depth sounding records were collected during storm runoff and nearly steady average streamflow with record durations ranging from 37 to 261 min waveform height was defined by dune front heights which ranged from 12 to 70 cm a weak correlation between flow depth and the standard deviation of bed elevation was noted dune front counts and spectral analyses of the temporal records showed that dune crests passed the observation point every 2 to 5 min dunes were often superposed on larger bed forms with wave periods between 10 and 30 min gradual changes in waveform height and periodicity occurred over several hours during storm runoff the processes of dune growth and decay were both time dependent and affected by changes in streamflow rates of migration for typical dunes were estimated to be 3 cm s 1 and dune wavelengths were estimated to be 6 to 7 m

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

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

timing of the last deglaciation in the sierra nevada of the merida andes venezuela abstract in the tropical merida andes northwestern venezuela glacial landforms were found at altitudes between 2600 and 5000 m corresponding to 600 km2 of ice cover during the maximum glacial extension however the lack of sufficient absolute age data prevents detailed reconstruction of the timing of the last deglaciation on the northwestern flank of the mucunuque massif successive moraines and striated eroded basement surfaces were sampled for cosmogenic 10be investigation their compilation with published data allows the establishment of a detailed chronology of the post lgm glacier history the oldest moraines 18 1 and 16 8 ka correspond to the oldest dryas successive moraine ridges indicate stops in the overall retreat between the lgm and the younger dryas the cold and short older dryas stadial has been identified results indicate that most of the ice withdrew during the pleistocene the dataset supports an intensification of the vertical retreat rate from 25 m ka during the late pleistocene to 310 m ka during the pleistocene holocene afterwards the glacier was confined and located in the higher altitude zones the altitude difference of the younger dryas moraines in the mucubaji la victoria and los zerpa valleys indicates a strong effect of valley orientation on the altitude of moraine development

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

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

time and the rivers flowing fluvial geomorphology since 1960 fluvial geomorphology has been the largest single subdiscipline within geomorphology for many decades fluvial geomorphic expertise is integral to understanding and managing rivers and to developing strategies for sustainable development this paper provides an overview of some of the significant advances in fluvial geomorphology between 1960 and 2010 with respect to conceptual models fluvial features and environments being studied tools used by fluvial geomorphologists geomorphic specialty groups within professional societies journals in which fluvial geomorphic research is published and textbooks of fluvial geomorphology during this half century fluvial geomorphology broadened considerably in scope from a focus primarily on physical principles underlying process and form in lower gradient channels with limited grain size range to a more integrative view of rivers as ecosystems with nonlinear behavior and great diversity of gradient substrate composition and grain size the array of tools for making basic observations analyzing data and disseminating research results also expanded considerably during this period as did the diversity of the fluvial geomorphic community

new criterion for sediment suspension and wind speed proxy in planetary atmospheres 1 sediment suspension is a common and important phenomenon in planetary atmospheres a widely used suspension criterion also a proxy boundary layer wind speed indicator was discredited by mars exploration rover mission results concerning sediment particle sizes in active surface bed forms parameterisation of a new dynamic suspension criterion based upon turbulent stress yields predicted suspension threshold conditions consistent with those inferred for earth and predicts no suspension of sand grade particles for recent martian wind velocities at 1m height estimated in the range 28 73 m s 1 results for venus imply suspension of very fine sand particles at low surface wind speeds

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

evolution of coarse gravel bed forms field measurements at flood stage field measurements to investigate the origin and growth of mesoscale gravel bed forms in deep flows were made in the north fork toutle river washington sonar observations of the gravel streambed at a stationary point were recorded during two storm flows in december 1989 and january 1990 with concurrent bed load sampling and continuous velocity measurements mean diameter of bed load was about 3 cm flow depths were 1 4 2 4 m and bed shear stresses were 2 5 times the critical stress of mean bed load diameter as computed from the depth slope product these records document the hydrodynamic conditions under which dunelike coarse gravel bed forms were observed coarse gravel dunes height 20 cm length 6 15 m evolved more than 24 hours after peak stage primarily by accretion as inferred from bed form changes revealed in dual sonar records dune heights increased to 40 cm as mean trough elevation rose about 50 cm over several hours smaller dunes wavelength 1 3 m transitional from bed load sheets migrated on the backs of the large dunes the superposed dunes finally became indistinguishable from the large dunes which diminished in height by increasing the mean level of troughs gravel deposition occurred at the observation point in conjunction with migration of gravel dunes the direct comparison of known bed form regimes and gravel bar facies provides alternative interpretations of gravelly deposits

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

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

jet flow over foredunes jet flows which are localized flows exhibiting a high speed maxima are relatively common in nature and in many devices they have only been occasionally observed on dunes and their dynamics are poorly known this paper examines computational fluid dynamic cfd two dimensional 2d modelling of jet flow over a foredune topography flow was simulated in 10 increments from onshore 0 to highly oblique alongshore 70 incident wind approach angles cfd modelling reveals that the formation of a jet is not dependent on a critical wind speed and an increase in incident wind velocity does not affect the magnitude of jet flow a jet is first formed at 1 0m seawards of the foredune crest on the prince edward island foredune morphology example examined here a jet is not developed when the incident wind is from an oblique approach angle greater than 50 because there is significantly less flow acceleration across a much lower slope at this incident angle the presence of a scarp does influence the structure of the crest jet in that the jet is more pronounced where a scarp is present surface roughness affects the magnitude of jet expansion and jets are better developed on bare surfaces compared to vegetated ones

on determining the geometric and kinematic characteristics of coherent flow structures over a gravel bed a new approach using combined plif piv in gravel bed rivers the microtopography of the bed is known to exert a significant influence on the generation of turbulent flow structures that owe their origin to fluid shear generated near the bed although field and laboratory measurements have indicated that flows over gravel beds contain a range of coherent flow structures the kinematic and dynamic properties of these structures are still poorly understood this paper describes a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar laser induced fluorescence and particle imaging velocimetry plif piv the results confirm that the primary generative mechanism of coherent flow structures is at the bed where merging hairpin vortices form around bed clasts and generate larger scale fluid motions that advect downstream copyright 2010 john wiley sons ltd

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

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

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

cenozoic paleo environmental evolution of the pamir tien shan convergence zone abstract the retreat of the tethys sea and the uplift of the tibetan plateau play the critical roles in driving asian climatic changes during the cenozoic in the pamir tien shan convergence zone over 3000 m of cenozoic successions consisting of marine deposits in the lower continental clay and fine sand in the middle and molasse in the upper part record the evolution of the tethys sea the asian aridification and the deformation of the pamir in this work the existing biostratigraphic subdivisions and new electronic spinning resonance dating results were used to assign ages to formations within the ulugqat section sedimentary facies analysis and multi proxy indices were used to reconstruct the paleo environmental evolution the results show 1 the pamir tien shan convergence zone has undergone progressive environmental changes from shallow marine before 34 ma to arid land at 23 ma and finally to inter mountain basin by 5 3 ma 2 the overall increase in mean size of grains decrease in redness in magnetic susceptibility and in proportion of the ultrafine component of the sediments studied revealed a long term strengthening in potential energy to transporting medium cooling and enhanced continental aridity respectively 3 the easternmost edge of the tethys sea prevailed in the western tarim basin from late cretaceous to early cenozoic and finally retreated from this region around the eocene oligocene transition which in turn strengthened the asian aridification 4 accumulation of molasse with an upper age of 1 ma suggests that the deformation front of the pamir migrated to this area at or before that time

effect on flow structure of sand deposition on a gravel bed results from a two dimensional flume experiment 1 particle imaging velocimetry piv was used to quantify the entire two dimensional flow field over a series of fixed test beds in a laboratory flume the test beds used in the experiments provide a two dimensional representation of gravel beds with contrasting roughness the two dimensional form of the beds combined with the nonintrusive nature of piv allowed the velocity field to be quantified right down to the bed surface the three beds used were designed to simulate the progressive effect of sand deposition on a gravel bed with median grain size d50 of 25 mm such a situation is common in bimodal beds but as compared with gravel and sand cases little work has been done to investigate the interaction between the variable roughness of a bimodal bed and the flow structure above it the results demonstrate that as the effective roughness decreases the reverse flow often found in the lee of gravel particles is eliminated furthermore near bed velocities increase while shear stress and turbulent kinetic energy decrease turbulent properties are also diminished higher up in the profile although this is not the case for the mean downstream velocity which remains unaffected except at the bed quadrant analysis reveals that q2 and q4 events become less frequent around high points as effective roughness decreases all these responses to the changing bed conditions are most pronounced in areas where the effective roughness height h approaches 10 mm or less h d50 0 4 such a situation is often found immediately upstream of gravel sand transitions in natural rivers it is hypothesized that the hydraulic response outlined here provides a detailed physically based explanation for the gravel sand transition the combination of reduced bed shear stress and q2 q4 events around gravel particles will result in greater mobility of the sand fraction relative to gravel these experiments thus appear to demonstrate that there is a fundamental threshold between gravel bed and sand bed states that has not previously been quantified

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

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

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

the 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

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

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

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