combined visual and quantitative techniques in the study of natural turbulent flows an important difficulty in understanding turbulence in natural flows is that one does not see what is being measured by devices recording velocity fluctuations this study shows how visualization of turbulent structures can be effectively implemented in natural river flows in order to match turbulent flow structures with their velocity signatures flow visualization is combined with high frequency 20 hz velocity measurements from electromagnetic current meters two fluvial environments are examined in a river confluence where the difference in turbidity provides a natural tracer very strong negative velocity fluctuations for the streamwise component are concomitant with an upwelling motion a video recording made from a bridge of the passage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration of 1 2 s in the mixing layer in a gravel bed river we have examined patterns of eddy shedding in the lee of a pebble cluster these patterns were highlighted by dye milk injected in the separation zone downstream black and white images were obtained from an underwater lens linked to a video camera recorder we observed a complex dynamic characterized by two dominant patterns of eddy shedding one with a clear motion of eddies toward the water surface and another with a downward expansion toward the bed of the structures this pattern is related to the passage of intermittent wedges of low speed fluid separated by wedges of fast moving fluid

numerical modeling of flow processes over gravelly surfaces using structured grids and a numerical porosity treatment this article describes the development and validation of a method for representing the complex surface topography of gravel bed rivers in high resolution three dimensional computational fluid dynamic models this is based on a regular structured grid and the application of a porosity modification to the mass conservation equation in which fully blocked cells are assigned a porosity of zero fully unblocked cells are assigned a porosity of one and partly blocked cells are assigned a porosity of between 0 and 1 according to the percentage of the cell volume that is blocked the model retains an equilibrium wall function and an rng type two equation turbulence model the model is combined with a 0 002 m resolution digital elevation model of a flume based water worked gravel bed surface acquired using two media digital photogrammetry and with surface elevations that are precise to 0 001 m the model is validated by comparison with velocity data measured using a three component acoustic doppler velocimeter adv model validation demonstrates a significantly improved level of agreement than in previous studies notably in relation to shear at the bed although the resolution of model predictions was significantly higher than the adv measurements making model assessment in the presence of strong shear especially difficult a series of simulations to assess model sensitivity to bed topographic and roughness representation were undertaken these demonstrated inherent limitations in the prediction of 3 d flow fields in gravel bed rivers without high resolution topographic representation they also showed that model predictions of downstream flux were more sensitive to topographic smoothing that to changes in the roughness parameterization reflecting the importance of both mass conservation i e blockage and momentum conservation effects at the grain and bed form scale model predictions allowed visualization of the structure of form flow interactions at high resolution in particular the most protruding bed particles exerted a critical control on the turbulent kinetic energy maxima typically observed at about 20 of the flow depth above the bed

developments in monitoring and modelling small scale river bed topography recent research in fluvial geomorphology has emphasized the spatially distributed feedbacks amongst river channel topography flow hydraulics and sediment transport although understanding of the behaviour of dynamic river channels has been increased markedly through detailed within channel process studies less attention has been given to the accurate monitoring and terrain modelling of river channel form using three dimensional measurements however such information is useful in two distinct senses firstly it is one of the necessary boundary conditions for a physically based deterministic modelling approach in which three dimensional topography and river discharge drive within channel flow hydraulics and ultimately spatial patterns of erosion and deposition and therefore channel change secondly research has shown that an alternative means of estimating the medium term bedload transport rate can be based upon monitoring spatial patterns of erosion and deposition within the river channel this paper presents a detailed assessment of the distributed monitoring and terrain modelling of river bed topography using a technique that combines rigorous analytical photogrammetry with rapid ground survey the availability of increasingly sophisticated terrain modelling packages developed for civil engineering application allows the representation of topographic information as a landform surface intercomparison of landform surfaces allows visualization and quantification of spatial patterns of erosion and deposition a detailed assessment is undertaken of the quality of the morphological information acquired this allow some general comments to be made concerning the use of more traditional methods to monitor and represent small scale river channel morphology

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

high resolution numerical modelling of three dimensional flows over complex river bed topography this paper describes preliminary results from a method for the treatment of complex bed topography for the understanding of three dimensional flow processes it develops an established porosity based method and couples it to high resolution bed topographical data acquired using digital photogrammetry the results provide quantitative data on flow processes that match and perhaps exceed current measurement capabilities and so raise difficult questions for model validation however the detailed flow fields provided by the method allow us to develop current models for flow form interactions in gravel bed rivers of most importance the method will allow us to enhance significantly the application of computational fluid dynamics to complex river bed morphology for large eddy simulation roughness parameterization sediment entrainment and deposition studies and understanding interactions between flow and vegetation in relation to conveyance studies copyright 2002 john wiley sons ltd

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

skin friction behind isolated hemispheres and the formation of obstacle marks we have measured the mean magnitude and direction and the rms fluctuation intensity of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long downstream of reattachment the mean skin friction magnitude is about 20 greater than its free stream value along the centre line and comparably reduced to either side of it the horizontal divergence of the skin friction vector field is positive divergent along the centre line and negative convergent to either side of it neither of these conditions favours development of a ridge longer than the separation length along the centre line the development of ridges or tails many obstacle heights long commonly observed in nature requires considerable modification of the simple sediment free wakes we have studied

effects of a pebble cluster on the turbulent structure of a depth limited flow in a gravel bed river pebble clusters are important morphological features of gravel bed rivers and have a significant effect on the spatial and temporal response of the flow this paper presents original results from measurements of velocity taken at a high spatial and temporal resolution in a turbulent flow upstream and downstream from a pebble cluster three electromagnetic current meters 0 013 m in diameter mounted on a wading rod were deployed upstream and downstream of an isolated pebble cluster along 31 vertical profiles that contained between 7 and 13 points in a 5 m long section the high sampling resolutions in a natural environment permitted characterization of the complex dynamics of the turbulent flow in the vicinity of the cluster the description of these dynamics is based on the interpretation of contour maps of several turbulence statistics including moments of velocity distributions reynolds shear stress quadrant analysis and autocorrelation function downstream from the cluster flow is decelerated the intensity of turbulence increases and zones of strong vertical motions are present the superimposition of the maps allowed us to decompose the field of turbulent flow around a pebble cluster into distinct regions in a downstream direction from the cluster we observed zones of flow acceleration recirculation vortex shedding flow reattachment fluid upwelling and flow recovering results indicate the existence of highly dynamic boundaries between zones where strong intermittent events are generated they also highlight the difference between shedding and upwelling motions of the flow

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

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

large eddy simulation of sediment transport currents over ripples a large eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples the code solves the navier stokes equations and an advection diffusion equation for the sediment and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study the main features of interest in this flow are seen because of the high resolution of the grid especially near the ripple boundary this yields a detailed bottom shear stress distribution properly identified coherent structures and resolved sharp gradients in the sediment concentration globally the sediment is carried up into the flow from locations where the shear stress is high on the upslopes of ripple crests and is advected downstream by the current and upward by the vertical velocity component regions of positive vertical velocity are associated with gortler vortices which represent a significant sediment transport mechanism in this flow

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

investigation of controls on secondary circulation in a simple confluence geometry using a three dimensional numerical model recent research into river channel confluences has identified confluence geometry and particularly bed discordance as a control on confluence flow structures and mixing processes and this has been illustrated using both field measurements in natural confluences and laboratory measurements of simplified confluences generalization of the results obtained from these experiments is limited by the number of confluence geometries that can be examined in a reasonable amount of time this limitation may be overcome by numerical models in which confluence geometry is more readily varied and data acquired more rapidly this paper aims to i validate the application of a three dimensional numerical model to a simple confluence geometry ii simulate the effects of different boundary condition values upon flow structures and iii interpret the implications of these simulations for river channel confluence dynamics the model used in this research solves the three dimensional form of the navier stokes equations and is used to simulate the flow in a parallel confluence of unequal depth channels and to investigate the effect of different combinations of velocity and depth ratio between the two tributaries the results generally agree with empirical evidence that secondary circulation is generated in the absence of streamline curvature but only for specific combinations of depth and velocity ratio this research shows how understanding of the interaction of these controls is enhanced if pressure gradients are considered the velocity ratio is the prime determinant of the cross stream pressure gradient that initiates cross stream velocities however for significant secondary circulation to form cross stream velocities must lead to significant transfer of fluid in the cross stream direction this depends on the vertical extent of the cross stream pressure gradient which is controlled by the depth ratio in this study strong secondary circulation occurred for a depth differential of 25 or more as long as the velocity in the shallower tributary was at least as great as that in the deeper channel this provides an important context for interpretation of previous work and for the design of new experiments in both the field and the laboratory 1998 john wiley sons ltd

sediment pick up functions experimental research has been performed to determine the sediment pick up by a uniform flow without initial sediment load a sediment lift was designed by which the sediment particles were moved upwards at a constant rate through a circular opening in the flume bottom five types of bed material sand have been used with d d5 d0 values in the range 130 1500 m the mean flow velocities were in the range 9 5 1 0 m s the experimental results have been used to derive an empirical pick up function the results have also been used to evaluate several existing pick up functions finally the proposed empirical pick up function has been applied to predict the bed load and suspended load transport in the latter application the proposed function has been used as a bed boundary condition in a mathematical model for suspended sediment transport

on large scale flow structures in a gravel bed river abstract previous studies have suggested the presence of large scale flow structures in gravel bed rivers these structures are pictured as intermittent high speed wedges separated by regions of lower velocity however the characteristics of these structures have not been examined in detail through either visualisation techniques or detailed field measurements this paper confirms the presence of large scale flow structures in gravel bed rivers pictures their sequence and patterns and characterises their mean and individual properties the analysis relies on a new technique for displaying velocity fluctuations in a space time matrix that allows one to see the structures as they pass an array of current meters streamwise and vertical velocities were measured simultaneously with an array of three electromagnetic current meters the sampling frequency was 20 hz five velocity profiles of up to 13 1 min series of measurements and one profile of three 20 min measurements were sampled these data suggest the presence of large wedges of faster fluid joined by regions of slower fluid space time correlation analysis confirmed the presence of vertical coherence of the flow the average angle of the front of the wedges is 36 although individual structures are variable in size and shape a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s the high speed wedges display a complex organisation and do not show a preferred sequence of events as was postulated by previous studies because of their duration and size these high speed wedges are likely to play a major role in bedload sediment transport

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

numerical modelling of separated flow in river bends model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank the occurrence of separated flow in sharply curving river bends and the associated deposition of fine grained sediment is widely recognized but the factors controlling the existence and extent of separated flow have not been clearly identified a three dimensional fixed lid computational fluid dynamics program was used to simulate time averaged flow in idealized channel bends in order to investigate geometric controls on flow separation the code was applied first to a field site it successfully modelled the qualitative features of the flow including a separation zone and was insensitive to choice of turbulence closure or exclusion of lateral variation in water surface topography the simulations of idealized bends demonstrate that the existence and extent of concave bank flow separation can be significantly influenced by changes in bend planform point bar topography and upstream planform 1998 john wiley sons ltd

three dimensional flow structure around small scale bedforms in a simulated gravel bed environment pebble clusters are common small scale morphological features in gravel bed rivers occupying as much as 10 per cent of the bed surface important links exist between the presence of pebble clusters and the development of flow structures these links are poorly understood at the three dimensional level particularly neglected has been the effect of clusters on the lateral flow characteristics a laboratory study was conducted using a hydraulic flume within which simulated pebble clusters were superimposed onto a plane bed of gravel material high resolution three dimensional flow data were collected above the bed at two different flow depths using an acoustic doppler velocimeter the results present evidence of the importance of lateral flow in the development of turbulent flow structure narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three dimensional separation zone in their lee this may indicate the presence of horseshoe type vortical structures analogous to those identified in less hydraulically rough environments however it is likely that these structures are more complicated given the mutual interference of the surrounding medium the lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters the results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters flow acceleration up the stoss side of the cluster recirculation behind the cluster in the wake region vortex shedding from the pebble crest and shear layer flow reattachment downstream of the cluster upwelling of flow downstream of the point of reattachment and recovery of flow copyright 2001 john wiley sons ltd

direct numerical simulation of bedload transport using a local dynamic boundary condition temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages the primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation pocket friction angle of a bed grain to the grains that it rests upon variability of the near bed turbulent velocity field and the local modification of this velocity field by upstream protruding grains a model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain size sediment bed experimental data from the velocity field downstream and below the tops of upstream protruding grains are presented from these data an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model the temporal variability of near bed turbulence is provided by a measured near bed time series of velocity over a gravel bed the distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies

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

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers a reply clifford et al 1992 argue that differences in hydraulic conditions at the moment of sediment entrainment in turkey brook flood by flood are attributable to variations in the form drag that arises from cluster microforms while not denying the possibility of variable cluster concentration we show that the time dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold clifford et al further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event this merits further examination however we point out that clusters are not only observed at low flow and therefore already present and able to contribute significant form drag as the next bedload generating flood waxes but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events the revelation by clifford et al that flow structure in the river quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements perhaps clusters is not surprising there is however some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing nevertheless the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport

the theoretical foundations and potential for large eddy simulation les in fluvial geomorphic and sedimentological research abstract large eddy simulation les is a method for resolving the time dependent structure of high reynolds number turbulent flows with les it is possible to model and track the behaviour of coherent turbulent structures and study their effect on the flow field hence les is potentially an important research tool in the fluvial sciences where flow mixing sediment entrainment and sediment transport are all affected by the presence of coherent vortices and their interactions with channel boundaries and other flow structures this paper introduces the les methodology discusses a variety of ways for representing small scale processes within les the subgrid scale modelling problem and provides some examples of early work into the use of les in a fluvial context a number of advances in computational power and numerical methods are required before les can be effectively applied at the river reach scale this paper considers some recent developments and their potential for providing validated large eddy simulations of river flow at the channel scale

stream flow characterization and feature detection using a discrete wavelet transform an exploration of the wavelet transform as applied to daily river discharge records demonstrates its strong potential for quantifying stream flow variability both periodic and non periodic features are detected equally and their locations in time preserved wavelet scalograms often reveal structures that are obscure in raw discharge data integration of transform magnitude vectors over time yields wavelet spectra that reflect the characteristic time scales of a river s flow which in turn are controlled by the hydroclimatic regime for example snowmelt rivers in colorado possess maximum wavelet spectral energy at time scales on the order of 4 months owing to sustained high summer flows hawaiian streams display high energies at time scales of a few days reflecting the domination of brief rainstorm events wavelet spectral analyses of daily discharge records for 91 rivers in the us and on tropical islands indicate that this is a simple and robust way to characterize stream flow variability wavelet spectral shape is controlled by the distribution of event time scales which in turn reflects the timing variability and often the mechanism of water delivery to the river five hydroclimatic regions listed here in order of decreasing seasonality and increasing pulsatory nature are described from the wavelet spectral analysis a western snowmelt b north eastern snowmelt c mid central humid d south western arid and e rainstorm island spectral shape is qualitatively diagnostic for three of these regions while more work is needed to establish the use of wavelets for hydrograph analysis our results suggest that river flows may be effectively classified into distinct hydroclimatic categories using this approach 1998 john wiley sons ltd

direction of arrival estimation for radar ice sounding surface clutter suppression ice sounding radars are able to measure ice sheets by profiling their glaciological features from the surface to the bedrock the current airborne and in particular future space based systems are suffering from off nadir surface clutter which can mask the depth signal of interest the most recent surface clutter suppression techniques are based on multi phase center systems combined with sophisticated coherent postprocessing the performance of the techniques can be improved by accurate direction of arrival doa estimates of the surface clutter this paper deals with data driven doa estimation for surface clutter signals which includes a formulation of the mathematical foundation of spatial aliasing doa estimation is applied to data acquired with the p band polarimetric airborne radar ice sounder at the jutulstraumen glacier antarctica the effects of spatial aliasing related to a large phase center spacing are analyzed and an unwrapping procedure is presented and applied to the data finally doa estimation of full scene data is analyzed and used to show an along track and incidence off nadir angle dependent variation of the effective scattering center of the surface return which is caused by a varying penetration depth

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

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

the geometry and dynamics of low relief bed forms in heterogeneous sediment in a laboratory channel and their relationship to water flow and sediment transport abstract the geometry and dynamics of low relief bed forms pebble clusters bed load sheets and bars developed in a mixed size and mixed density sediment sandy gravel were studied in a laboratory channel in most runs equilibrium uniform flow was established with discharge 0 05 m3 s mean flow depth 0 1 m mean flow velocity 0 82 0 87 m s mean bed shear stress 1 6 2 2 pa dimensionless bed shear stress 0 04 0 07 and mean bed load transport rate 0 009 0 013 kg m s low relief bed forms were observed directly and in bed height records and bed profiles measured with a high resolution ultrasonic bed profiler pebble clusters are groups of relatively large temporarily immobile clasts with lee side and stoss side accumulations of finer sediment and with adjacent scoured zones w th heavy mineral concentrations pebble clusters are concentrated in relatively coarse areas of the bed such as in the troughs of bed load sheets and bars asymmetric sinuous crested bed load sheets ranged from one to two grain diameters up to 12 mm in height and 0 6 1 2 m in length migration rates determined from bed form spacings and periods were typically 1 1 5 mm s these bed forms had relatively fine grained crests and coarse grained troughs and a well mixed grain texture along the stoss side bed load sheets had little associated form drag low relief bars were 10 17 mm high and 1 7 2 4 m long and typical migration rates were 0 4 mm s the migration of these bars was associated with fluctuations in the water surface slope and bed load transport rate with periods of about two hou s low relief bed forms in heterogeneous sediments appear to be ubiquitous features of lower stage plane beds and those transitional to dune covered beds where the bed is not completely mobile

bed shear stress over subaqueous dunes and the transition to upper stage plane beds plane beds that replace small bedforms e g ripples on the backs of subaqueous dunes are dynamically similar to upper stage plane beds local bed shear stresses at the position on dunes where plane beds develop may be two to three times larger than on a completely plane bed under the same general hydraulic conditions these observations help explain the transition from dunes to a wholly upper stage plane bed

direct numerical simulation of an iron rain in the magma ocean core formation in terrestrial planets is a complex process possibly involving several mechanisms this paper presents a direct numerical simulation of one of these the separation of an emulsion of metal in a magma ocean the model using a fully lagrangian approach called the moving particle semi implicit method solves the equations of fluid dynamics including a proper treatment of surface tension it allows investigation of the balances controlling the distribution of drop size and velocity in both two and three dimensional situations a scaling analysis where buoyancy is balanced by both surface tension and inertia correctly predicts the average values in these quantities the full calculation gives an average drop radius of 1 5 cm falling at a velocity of about 30 cm s 1 analysis of the full distribution remains interesting and shows that a significant part of the smallest droplets is entrained upward by the return flow in molten silicate and might be entrained by succeeding thermal convection in addition we investigate the conversion of gravitational energy into viscous heating and the thermal equilibration between both phases we find that viscous heating is essentially produced at the surface of iron drops and that thermal equilibration is dominated by advection scaling thermal diffusion to chemical diffusion leads to the estimation that the latter would happen in less than 100 m in the magma ocean

modelling three dimensional flow structures and patterns of boundary shear stress in a natural pool riffle sequence fluid sediment interactions control river channel forms and processes analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour in this paper the hydraulic processes active in a natural pool riffle sequence are simulated using a three dimensional computational fluid dynamics cfd model methods employed for the prescription of model boundary conditions are outlined model calculations are assessed using comparisons with field observations acquired over a range of flows simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near bankfull and an intermediate flow event results are used to assess existing theories that seek to explain the development and maintenance of pool riffle sequences simulated results suggest that near bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases model simulations indicate that secondary flow acts to route near bed flow over the downstream side of riffles and into the pool head away from the centre of pools implications for sediment transport and pool maintenance are discussed

discussion of three dimensional numerical study of flows in open channel junctions by jianchun huang larry j weber and yong g lai

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

effect of bed morphology on flow mixing length at river confluences mixing processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems but remain poorly understood previous studies1 11 have indicated that flow mixing downstream of a confluence is a slow process typically completed at distances greater than 100 times the channel width bed morphology is now emerging as a critical factor flume experiments have shown that the width to depth ratios12 and height discordance13 14 between the confluent channels should influence mixing rates12 14 but the magnitude of these effects in real rivers is not known here we report measure ments from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates for the rivers studied mixing is complete at distances five to ten times shorter than those reported previously

scalings for large turbulent flow structures in gravel bed rivers 1 the relations between frequency and duration of large scale turbulent flow structures in gravel bed rivers and the distributions of the time intervals between turbulent events are investigated using flow visualisation and velocity measurements the time of occurrence and the duration and amplitude of large bursting motions were estimated independently from a five minute underwater video and from high resolution velocity time series sampled at 20 hz during 38 minutes at three heights above the bed results from both experiments show that the frequency duration relations of the large scale turbulent flow structures follow a power function with exponents ranging between 1 4 and 1 95 values that are typical of bursting systems e g solar flares the frequency distributions of time intervals between two successive ejections display an exponential tail these findings provide models for simulating sequences of large scale turbulent events in a gravel bed river an important step in linking flow turbulence and sediment transport

numerical simulation of three dimensional flow hydraulics in a braided channel results are presented from a numerical simulation of three dimensional flow hydraulics around a mid channel bar carried out using the fluent uns computational fluid dynamics cfd software package fluent uns solves the three dimensional reynolds averaged form of the navier stokes equations turbulence closure is achieved using a rng k model simulated flow velocities are compared with measured two dimensional velocities downstream and cross stream obtained using an electromagnetic current meter ecm the results of the simulation are qualitatively consistent with the flow structures observed in the field quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two r 0 88 and a mean difference of 0 09 m s 1 deviations between simulated and measured velocities may be identified that are both random and systematic the former may reflect a number of factors including subgrid scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ecm orientation model mesh configuration roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities these results illustrate the potential for using cfd software to simulate flow hydraulics in natural channels with complex configurations they also highlight the need for detailed spatially distributed datasets of three dimensional flow variables to establish the accuracy and applicability of cfd software copyright 1999 john wiley sons ltd

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

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

an erosion criterion for gravel bed rivers a theoretically based erosion criterion is developed for gravel bed rivers which incorporates the effect of both grain geometry and turbulent velocity fluctuations it is derived from a balance of instantaneous drag lift and gravity forces operating on individual grains and is calculated for spherical grains arranged in three distinct geometries to accommodate the temporal variation in bed shear stress the model includes a stochastic element based on the characteristics of turbulence derived from the flume evidence of mcquivey 1973a b in terms of the shields parameter results show reasonable agreement with the range of observations quoted from the field and with the experimental data of fenton and abbott 1977 finally the argument is generalized to cover applications in the wider context of field conditions including a range of grain sizes and flow conditions

the application of computational fluid dynamics to natural river channels three dimensional versus two dimensional approaches this paper addresses the recent trend in fluvial geomorphology towards using computational fluid dynamics to explore the adjustment between flow processes sediment transport and river channel morphology it aims to evaluate the extent to which three dimensional 3d models improve predictive ability and prediction utility compared to two dimensional 2d applications this is achieved through comparing the predictions of both 3d and 2d models with high quality field data identical boundary conditions obtained from a confluence within a gravel bed river system with high relative roughness are defined for each model evaluation of the 3d model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity however comparison with the 2d model shows that the 3d model has a higher predictive ability particularly if the 2d model is not corrected for the effects on flow structure of secondary circulation further the 3d model provides more reliable estimates of bed shear stress and other more useful information such as the three dimensional flow field important for mixing processes this suggests that there is significant merit in the move towards 3d models but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties

on the entrainment of sediment and initiation of bed defects insights from recent developments within turbulent boundary layer research sediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds results of visualization experiments suggest that these sweep impacts are grouped and may define patches of entrainment upon a mobile bed that may be wider than an individual sweep impact additionally the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks these observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed these features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripples