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

detection and reconstruction of large scale coherent flow structures in gravel bed rivers large scale flow structures lsfs in the streamwise direction are important features of gravel bed river flows because they may contribute to sediment transport and gas exchange in the present study these structures are detected using huang s empirical mode decomposition and reconstructed with phase averaging techniques based on a hilbert transform of the velocity signal the analysis is based on the fluctuating component of 15 quasi instantaneous velocity profiles measured with a three dimensional 3d acoustic doppler velocity profiler advp in an armoured gravel bed river with a low relative submergence of 2 9 ratio between flow depth and bed grain diameter lsfs were identified in most of the measured profiles and consistently showed similar features we were able to characterize the geometry of these large scale coherent structures the front has a vertical linear shift in the time domain and a vertical profile corresponding to a first quarter moon with the apex situated at z h approximate to 0 4 in the vertical the front scales with flow depth h and in the streamwise direction lsfs scale with three to seven times the mean flow depth on the bed the effect of lsfs is a periodic non linear variation of the friction velocity on average between 0 90 and 1 10 times the mean value a model for the friction velocity cycle resulting from lsfs oscillation is presented copyright c 2014 john wiley sons ltd

sampling surficial fluvial gravels the precision of size distribution percentile sediments abstract grid by number wolman sampling is commonly used to characterize the texture of surficial fluvial sediments assessments of sampling precision have concentrated on the median grain size but adequate representation of other grain size percentiles is paramount in a number of sedimentological contexts the field effort required to construct percentile sampling distributions by replication is prohibitive and the absence of any general size distribution for fluvial sediments precludes theoretical calculation of percentile standard errors bootstrapping is used in this paper to determine percentile standard errors for large samples from two gravel bar surfaces the improvement of percentile precision with sample size follows the theoretical expectation hence improvement in percentile recision is achieved only at the expense of much greater sampling effort once sample sizes reach about 400 stones however absolute percentile standard errors differ significantly from theoretically derived values of particular interest is the stability of coarser percentile estimates in coarse skewed distributions d95 estimates may be as precise as d50 estimates for a given sample size the sensitivity of the precision of estimated percentiles to distribution shape is examined using synthetic skewed and bimodal grain size distributions the absolute precision of percentile estimates is shown to vary with the grain size distribution of a particular sediment our field results provide a reference for further work

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

the linkage between velocity patterns and sediment entrainment in a forced pool and riffle unit a field based project was initiated in order to characterize velocities and sediment entrainment in a forced pool and riffle sequence three dimensional velocities and turbulence intensities were measured with an acoustic doppler velocimeter at 222 different points at three similar flows that averaged approximately 4 35 m 3 s 1 within a large pool riffle unit on north saint vrain creek colorado sediment sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow which reached a maximum level of 14 8 m 3 s 1 velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply jet flow flow separation vortex scour and turbulence generation enhance scour in the centre of pools ultimately multiple processes appear to play some role in maintenance of this forced pool and the associated riffle copyright 2008 john wiley and sons ltd

field measurements of three dimensional hydraulics in a step pool channel we investigated the effects of morphologic position and discharge on flow structure in a steep 0 10m m mountain channel by collecting three dimensional measurements of time averaged and turbulent velocity components with a sontek flowtracker handheld adv acoustic doppler velocimeter on a 30 m reach of a step pool channel in the colorado rockies velocity profiles were measured at morphologic positions characteristic of steep channels above steps step lips base of steps pools cascades runs and at five different discharges a marked three dimensionality of flow structure was documented in east st louis creek velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes cross stream and vertical components contributed averages of 20 and 15 respectively to overall vector magnitudes turbulence intensities were especially multi dimensional however with large contributions to turbulent kinetic energy from the vertical component of velocity analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities with substantially higher velocities upstream from steps than in pools discharge and morphology effects on cross stream and vertical velocity components however were not significant discharge and morphologic position also significantly affected turbulence intensities for all flow components with the greatest turbulence intensities occurring in pools and at high discharges these results illustrate the strong discharge dependence of hydraulics in step pool channels where relative submergence of bedforms changes rapidly with discharge and the substantial spatial variation in hydraulics created by step pool sequences

field investigation of three dimensional flow structure at stream confluences 2 turbulence stream confluences are among the most highly turbulent locations in fluvial systems this paper examines the three dimensional structure of turbulence at three stream confluences in east central illinois the analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross sectional area but turbulence kinetic energy within this layer is 2 3 times greater than the turbulence kinetic energy of the ambient flow which has turbulence characteristics similar to those for flow in straight channels turbulence within the shear layer can be characterized as quasi two dimensional in the sense that large scale turbulence generated by transverse shear is predominantly two dimensional whereas small scale turbulence associated with bed friction is three dimensional spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms the shear layer dissipates rapidly as flow enters the downstream channel even though a well defined mixing interface persists at downstream locations

correlative velocity fluctuations over a gravel river bed velocity fluctuations in a steep coarse bedded river were measured in flow depths ranging from 0 8 to 2 2 m with mean velocities at middepth from 1 1 to 3 1 m s 1 analyses of synchronous velocity records for two and three points in the vertical showed a broad range of high coherence for wave periods from 10 to 100 s centering around 10 30 s streamwise correlations over distances of 9 and 14 m showed convection velocities near mean velocity for the same wave periods the range of coherent wave periods was a small multiple of predicted boil periods correlative fluctuations in synchronous velocity records in the vertical direction suggested the blending of short pulses into longer wave periods the highest spectral densities were measured beyond the range of coherent wave periods and were probably induced by migration of low relief bed forms

computational fluid dynamics modelling of boundary roughness in gravel bed rivers an investigation of the effects of random variability in bed elevation results from a series of numerical simulations of two dimensional open channel flow conducted using the computational fluid dynamics cfd code fluent are compared with data quantifying the mean and turbulent characteristics of open channel flow over two contrasting gravel beds boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra grid scale roughness elements e g particle clusters and small bedforms results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed typically at y h 0 1 0 3 this is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed use of the random elevation model to represent supra grid scale roughness also allows a reduction in the height of the near bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness despite these benefits the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution increasing the horizontal mesh resolution results in an increase in the near bed velocity gradient and turbulent kinetic energy effectively roughening the bed varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles but results in substantial changes to the shape of the turbulent kinetic energy profile these findings have significant implications for the application of cfd within natural gravel bed channels particularly with regard to issues of topographic data collection roughness parameterization and the derivation of mesh independent solutions copyright 2001 john wiley sons ltd

an experimental study on the ripple dune transition flume experiments were conducted on different bed stages across the ripple dune transition as flow velocity increases an initially flat bed surface made of fairly uniform sandy material is gradually transformed into a two dimensional rippled bed with further increase in velocity two dimensional ripples are replaced by irregular linguoid ripples as the average velocity necessary for the ripple dune transition to occur is imposed on the bed surface these non equilibrium linguoid ripples are further transformed into larger two dimensional dunes for each of these stages across the transition a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated an acoustic doppler velocimeter was used to study the flow characteristics above each bed surface detailed profiles were used along a transect located in the middle of the channel results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields section view they clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition turbulence intensity and reynolds stresses gradually increase throughout the transition two dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed linguoid ripples induce three dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region a very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes the turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability when compared with the rippled beds further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis together with an emphasis on quadrant analysis copyright 2001 john wiley sons ltd

the spatial characterization of turbulence around large roughness elements in a gravel bed river abstract this study characterizes the flow field above and around multiple instream submerged cobbles boulders and pebble clusters in order to obtain a better understanding of the hydrodynamics associated with large roughness elements lres in gravel bed rivers spatially distributed high frequency three dimensional velocity measurements were recorded in situ using acoustic doppler velocimeters at different flow stages the spatial distributions of turbulent kinetic energy ke longitudinal component integral timescales itsu and reynolds shear stresses were characterized and are presented for selected sites the longitudinal vertical reynolds shear stress increased with flow stage more strongly than the longitudinal lateral or lateral vertical reynolds shear stresses and dominate at the highest measured flows canonical redundancy analysis was used to relate lre morphometrics and mean flow conditions to the turbulence parameters estimated in the lre wakes i e ke itsu and reynolds shear stresses lre size and mean unobstructed velocity explained the highest proportion of the variance in the turbulent wake statistics multivariate regression models based on lre width mean unobstructed longitudinal velocity and flow depth are presented offering a tool to predict lre wake turbulence

characteristics of velocity profiles along riffle pool sequences and estimates of bed shear stress abstract numerous velocity profiles were measured along a riffle pool sequence for three different flow conditions for each flow condition ranging from baseflow to about 2 3 of bankfull depth ten to fifteen velocity profiles were measured along the same transect the objectives of the study were i to differentiate between components of bed roughness and shear stress attributed to surface friction and large scale bed undulations and ii to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles no previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle pool sequences and to determine how near bed velocity gradients change with increasing flow discharge at low flows and as expected riffles are characterized by higher near bed velocity gradients than pools similarly average bed shear stresses as determined from measurements of flow depth and energy gradient indicate a greater resistance and bed shear stress in riffle sections invariably however local shear stresses as determined from near bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope the shape of spatially averaged velocity profiles changes as discharges increases the difference in terms of near bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull further away from the bed surface velocity differences between pools and riffles do remain significant finally the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate perhaps a result of increasing density of small scale sedimentary structures

flow turbulence over fixed and weakly mobile gravel beds characteristics of turbulence structure in quasi 2d flows with static and weakly mobile gravel beds are presented three sets of measurements with acoustic doppler velocimeters in an irrigation canal were used two with subcritical bed shear stress static beds and one with the bed shear stress o close to critical oc weakly mobile bed the analyses included vertical distributions of local mean velocities turbulence intensities turbulent shear stresses velocity auto and cross spectra the quadrant method and high order velocity moments a number of properties of turbulence intensities high order moments streamwise bursting parameters and velocity spectra appeared to be similar for all three flows but some properties were different the most important one was an observed reduction in the von karman constant for the flow with weakly mobile bed comparison of these results with other studies and analogies with drag reducing flows suggest that at o oc 1 the drag on the bed for a given granula

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

thresholds of erosion and sediment movement in bedrock channels abstract this study examines the erosive resistance of the discontinuous loose joint blocks of various sizes that comprise the bed material of the etobicoke creek an incised bedrock channel found in an urbanized setting within the city of toronto ontario canada in order to accomplish this both field and laboratory experiments were conducted on the bed material found in this channel the field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events we found that rates of erosion and or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots with bed lowering 0 10 m in some locations the field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short the frequency distributions of distance of transport scaled with particle size is log normally distributed flume experiments demonstrated that the clasts offered different levels of resistance to movement depending on their orientation the platy nature of the rock fragments clast orientation and their imbrication significantly affect the threshold of erosion the results have significant implications for the understanding of bedrock rivers from a management perspective especially in the case of those rivers experiencing the effects of urban development in their drainage basins

does the permeability of gravel river beds affect near bed hydrodynamics the permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed however relatively little is known about the effect of bed permeability on overlying near bed flow dynamics and thus on fluid advection at the sediment water interface this study provides the first quantification of this effect for water worked gravel beds laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography the turbulence over permeable beds is less intense more organised and more efficient at momentum transfer because eddies are more coherent furthermore turbulent kinetic energy is lower meaning that less energy is extracted from the mean flow by this turbulence consequently the double averaged velocity is higher and the bulk flow resistance is lower over permeable beds and there is a difference in how momentum is conveyed from the overlying flow to the bed surface the main implications of these results are three fold first local pressure gradients and therefore rates of material transport across the sediment water interface are likely to differ between impermeable and permeable beds second near bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary third more sophisticated flow resistance models are required for coarse grained rivers that consider not only the bed surface but also the underlying permeable structure overall our results suggest that the effects of bed permeability have critical implications for hyporheic exchange fluvial sediment dynamics and benthic habitat availability

incomplete self similarity and flow velocity in gravel bed channels velocity measurements previously carried out using both a miniature current flowmeter and an acoustic doppler velocimeter are used to verify the applicability of the incomplete self similarity theory to deduce the velocity profile in a gravel bed channel then for the velocity profiles having the maximum value below the free surface and for the s shaped profiles the power velocity distribution is corrected using a new divergence function for each value of the depth sediment ratio the nondimensional friction factor parameter is calculated by integration of the measured velocity distributions in the different verticals of the cross section finally a semilogarithmic flow resistance equation is empirically deduced

macroturbulent structure of open channel flow over gravel beds the turbulent structure of open channel flow over a mobile gravel bed was investigated in an 8 m long 0 3 m wide and 0 3 m deep tilting flume a flow visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed the experiments reveal that turbulent flow consists of a sequence of large scale eddies with a vertical size close to the flow depth an average length equal to four to five depths and a width of about two depths the downstream motion of these eddies causes quasiperiodic fluctuations of the local flow velocity components the development of longitudinal troughs and ridges on the mobile bed and preferential transport of bed particles along troughs are related to the effect of the eddies the experimental results indicate that the depth scale eddies are an important turbulence mechanism contributing to sediment transport

turbulent flow across a natural compound channel 1 the measurements and primary analysis of turbulent flow across a compound channel river severn england are presented the velocity was measured using a three dimensional acoustic doppler velocimeter in combination with a directional current meter the statistical flow structure is examined against existing analytical formulations derived for single channel flows based on laboratory studies the existence of a vertically two layer structure around the interface between the main channel and the floodplain is demonstrated indicating 1 a vertical shear dominated flow zone near the bed and 2 away from the bed a transverse shear dominated flow zone with enhanced turbulent mixing the temporal spectra clearly reveal the occurrence of anisotropic turbulence both in the main channel and over the floodplain the present findings necessitate the resolution of both transverse and vertical structures for advanced modeling of compound channel flows the measured data can be used to assess the performance of mathematical river models

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

fine scale characterization of the turbulent shear layer of an instream pebble cluster this study characterizes the shear layer and associated vortex shedding around an isolated submerged pebble cluster in a gravel bed river the approach combines flow visualization and high frequency three dimensional velocity acoustic doppler velocimeter measurements two vortex shedding modes in the wake of the cluster were identified a small scale high frequency initial instability mode and a lower frequency mode that scales with cluster height the lower frequency mode arose from the intermittent interaction and amalgamation of the small scale instability vortices reynolds shear stresses velocity spectra and coherence functions indicated a dominance of longitudinal vertical shedding vortices in the wake of the cluster simultaneous flow visualization was required to determine the nature and behavior of the shedding modes quadrant analysis revealed that q2 and q4 events contributed 80 of the local longitudinal vertical component reynolds shear stress and demonstrated a dominance of ejection events in the wake of the cluster through flow visualization the behavior of the shear layer was seen to vertically expand and contract with the passage of q2 and q4 events respectively

resolving two dimensional flow structure in rivers using large scale particle image velocimetry an example from a stream confluence large scale particle image velocimetry lspiv has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems lspiv has typically been used in the field to obtain velocity or discharge measurements in relatively simple one dimensional flow detailed two dimensional or three dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling piv limiting factors such as poor particle seeding the need for camera rectification and challenging field conditions in this study we implement a low cost lspiv setup using a high resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch time averaged 2d velocities derived from lspiv are compared with those measured with an acoustic doppler velocimeter adv in the camera s field of view we also assess the capabilities of this setup to resolve both turbulent and time averaged flow structures at a stream confluence our results reveal that even in challenging field conditions a basic lspiv setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers the resulting dataset contains velocity information at high spatial and temporal resolution a significant advance in understanding flow processes at stream confluences our lspiv analysis provides support for previous numerical modeling studies that have distinguished between kelvin helmholtz and wake modes of turbulent behavior within the mixing interface at confluences this study shows that lspiv should be considered as both an alternative for traditional methods and a tool that can provide unprecedented levels of resolution of surface velocity patterns on rivers that can be used to evaluate numerical predictions of flow structure in complex fluvial environments this article is protected by copyright all rights reserved

scales of boundary resistance in coarse grained channels turbulent velocity profiles and implications gravel bed surfaces are characterized by morphological features occurring at different roughness scales the total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag grain stress and form drag components created by bed forms to facilitate a better understanding of total resistance and bed load transport processes there is a need to mathematically separate shear stress into its component parts one way to do so is to examine the properties of vertical velocity profiles above such surfaces these profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance a flume based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an acoustic doppler velocimeter average vertical velocity profiles above each roughness scale were clearly segmented the vertical extent of the inner flow region was directly related to the scale of roughness present on the bed and independent of flow depth increasing with increased roughness on a rough but rather uniform plane bed made of heterogeneous coarse sediments with no bed forms the shape of the velocity profile was clearly dominated by the local variations in grain characteristics when pebble clusters were superimposed the average shear stress in the outer flow region increased by 100 from the plane bed conditions the ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0 18 under shallow flow conditions and 0 3 under deep flow conditions the grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15 and 30 of the total channel stress increasing with decreased resistance roughness height ks d50 values at the grain scale for the plane bed and pebble cluster experiments were 0 73 and 0 63 respectively these are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling

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

turbulence structure in an ice covered sand bed river although many high latitude rivers are characterized by seasonal ice cover few empirical studies have examined the effect of surface ice on the turbulent properties of the underlying flowing water this study presents three dimensional turbulence measurements obtained with an acoustic doppler velocimeter adv in a straight reach of an ice covered sand bed river the statistical properties of three dimensional turbulence were analyzed it is shown that the spatial structure of the flow was nonuniform due to nonuniformity of the ice cover in the cross stream direction and due to the riverbed geometry these factors are responsible for the complex spatial structure of the flow field which limits the predictability of available physical models observed distributions over depth of reynolds stresses and mean streamwise velocities generally conform with distributions derived from piecewise solutions of a two dimensional flow model at least in the central part of the flow but observed and expected values of some parameters differ substantially we have hypothetically addressed these differences to the effect of flow stratification and the values of a stratification parameter have been estimated the velocity measurements allowed to analyze the coherent structures near the bed and beneath the ice cover their spatial scales were evaluated to be as large as the river depth for ejections and 0 7 0 8 times the river depth for sweeps whereas low frequency variations of the streamwise velocity were observed to have scales of two to four river depths

coarse bedload transport in a mountain river coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in squaw creek montana usa during 1991 and 1992 a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters for larger discharges coarser fractions of the bed material mobilized and bedload transport rates and average hydraulic parameters stabilized as discharge reduced mobile coarse particles became less frequent and deposited fine particles were removed resulting in an increase in bed roughness these observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph bedload pulses were aperiodic but spatially variable flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches 1998 john wiley sons ltd

clinopyroxene of spinel peridotite mantle xenoliths from lake nji cameroon volcanic line w africa crystal chemistry and petrological implications the crystal chemistry of six clinopyroxenes enclosed in protogranular spinel peridotite mantle xenoliths from lake nji cameroon w africa was studied by means of single crystal x ray diffraction and electron microprobe these spinel peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by lee et al 1996 for spinel peridotite xenoliths of the same region 19 11 and 15 8 respectively and by high clinopyroxene orthopyroxene ratios 0 7 2 7 present study and about 0 4 lee et al 1996 the clinopyroxene crystal chemistry indicates that the xenoliths show the compositional features of a mantle source contaminated by small volume melts responsible for the high clinopyroxene content the protogranular texture of the cameroon xenoliths upper portion of lithospheric mantle testifies that the spinel peridotite and the associated small volume melts completely re equilibrated at the spinel peridotite facies this is also supported by the petrological and geochemical data of lee et al 1996 on the etinde biu plateau spinel peridotite xenoliths which underwent late proterozoic early paleozoic enrichment in incompatible trace elements

detection of significant climatic precession variability in early pleistocene glacial cycles abstract despite having a large influence on summer insolation climatic precession is thought to account for little variance in early pleistocene proxies of ice volume and deep water temperature various mechanisms have been suggested to account for the dearth of precession variability including meridional insolation gradients interhemispheric cancellation of ice volume changes and antiphasing between the duration and intensity of summer insolation we employ a method termed empirical nonlinear orbital fitting enof to estimate the amplitudes of obliquity and precession forcing in early pleistocene proxies and their respective leads or lags relative to the timing of orbital variations analysis of a high resolution north atlantic benthic 18 o record comprising data from iodp sites u1308 and u1313 indicates a larger precession contribution than previously recognized with an average precession to obliquity amplitude ratio of 0 51 0 30 0 76 95 confidence interval in the rate of change of 18 o between 3 and 1 ma averaged when eccentricity exceeds 0 05 this ratio rises to 1 18 0 84 1 53 additional support for precession s importance in the early pleistocene comes from its estimated amplitude covarying with eccentricity analyses of other benthic 18 o records yielding similar orbital amplitude ratios and use of an orbitally independent timescale also showing significant precession precession in phase with northern hemisphere summer intensity steadily intensifies throughout the pleistocene in agreement with its more common identification during the late pleistocene a northern hemisphere ice sheet and energy balance model run over the early pleistocene predicts orbital amplitudes consistent with observations when a cooling commensurate with north atlantic sea surface temperatures is imposed these results provide strong evidence that glaciation is influenced by climatic precession during the late pliocene and early pleistocene and are consistent with hypotheses that glaciation is controlled by northern hemisphere summer insolation

frequency spectra and intermittency of the turbulent suspension process in a sand bed river previous research suggests that the turbulence driven suspension process in sand bed channels is dominated by intermittent energetic eddies with length scales of the order of channel depth because of the scarcity of data on the turbulent suspension process in alluvial channels the possible variability in suspension intermittency and turbulent frequency content due to contrasts in flow depth velocity or bedforms remains unclear however the present study analyses eddy correlation suspension signals from seven deployments in varied flow conditions around a sandy meander bend deployment depths at near bankfull flow stages varied from 2 to 5 5 m velocities at 0 75 1 m height from 0 6 to 0 9 m s 1 and local mean suspended sand concentrations ranged from 30 to 150 mg l 1 in the intermittence and spectral content of sand suspension between the various deployments are analysed and results are compared with previously published findings study data suggest that the dominant eddy sizes involved in sediment mixing across the sensor level are consistently of the order of 1 5 times flow depth and lie within the energy bearing turbulent range when sand suspension is analysed in the time domain in the various deployments energetic burst like suspension events occupying only 1 5 of the record duration account for 20 90 of the suspension work the degree of intermittence in the suspension process was observed to increase in deeper flows where mixing events contributing extreme vertical sediment fluxes appear to be relatively more frequent

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

remote measurement of river discharge using thermal particle image velocimetry piv and various sources of bathymetric information abstract although river discharge is a fundamental hydrologic quantity conventional methods of streamgaging are impractical expensive and potentially dangerous in remote locations this study evaluated the potential for measuring discharge via various forms of remote sensing primarily thermal imaging of flow velocities but also spectrally based depth retrieval from passive optical image data we acquired thermal image time series from bridges spanning five streams in alaska and observed strong agreement between velocities measured in situ and those inferred by particle image velocimetry piv which quantified advection of thermal features by the flow the resulting surface velocities were converted to depth averaged velocities by applying site specific calibrated velocity indices field spectra from three clear flowing streams provided strong relationships between depth and reflectance suggesting that under favorable conditions spectrally based bathymetric mapping could complement thermal piv in a hybrid approach to remote sensing of river discharge this strategy would not be applicable to larger more turbid rivers however a more flexible and efficient alternative might involve inferring depth from thermal data based on relationships between depth and integral length scales of turbulent fluctuations in temperature captured as variations in image brightness we observed moderately strong correlations for a site aggregated data set that reduced station to station variability but encompassed a broad range of depths discharges calculated using thermal piv derived velocities were within 15 of in situ measurements when combined with depths measured directly in the field or estimated from field spectra and within 40 when the depth information also was derived from thermal images the results of this initial proof of concept investigation suggest that remote sensing techniques could facilitate measurement of river discharge

comparison of turbulent flows over clusters of varying density abstractgravel bed rivers commonly form an armored surface layer within which cluster bedforms develop clusters can form in isolation or develop cluster groups of increasing density flume experiments are presented in which a clustered armored surface was formed on a gravel bed flows were measured over an area without a cluster and areas with an isolated cluster a coupling of two clusters and a group of three clusters turbulent flow parameters reynolds stresses and turbulent kinetic energy were calculated from acoustic doppler velocimeter measured flows and visualizations of the flow patterns around clusters flow separated over the cluster crest creating an increase in the turbulent flow properties and a high turbulence intensity region when flow reattached downstream turbulent kinetic energy increased around the coupled clusters but around the grouped clusters the flow field was distinguished by a high degree of uniformity the net effect of increasing density of cluster grouping was an incr

velocity profiles and the structure of turbulence at the outer bank of a compound meander bend abstract few studies have quantified near bank turbulence at the field scale in meander bends as a result details of the structure of turbulence at the outer bank of bends are poorly understood despite recognized linkages among turbulence bank erosion and channel migration this study uses high frequency measurements of flow velocities to analyze the characteristics of turbulence in close proximity to the outer bank of an actively migrating compound meander bend results show that the structure of turbulence in the bend is linked to curvature induced effects through the progressive advection of high momentum fluid toward the outer bank as flow moves through the bend vertical profiles of streamwise vertical reynolds stresses near the outer bank differ considerably from those in wide straight channels because of the effects both of curvature induced helical motion and of local frictional effects associated with the complex bank morphology the results of the study provide the basis for a conceptual model of the structure of outer bank turbulence in this meander bend

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

three dimensional measurement of river channel flow processes using acoustic doppler velocimetry this paper describes and assesses i the use of a new instrument for the determination of three dimensional flow velocities in natural rivers the acoustic doppler velocimeter adv and ii a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface the adv uses the doppler shift principle to measure the velocity of small particles assuming to be moving at velocities similar to the fluid velocity is resolved into three orthogonal components and measured in a volume 5 cm below the sensor head minimizing interference of the flow field and allowing measurements to be made close to the bed a simple method for positioning and orienting the instrument using digital tacheometry is described and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography the paper includes a preliminary field assessment of the adv by comparing velocity profiles with those generated from marsh mcbirney electromagnetic current meters and a full field assessment of the position and orientation methodology these results suggest that the recommended methods in combination with an adv are able to provide reliable mean three dimensional velocity field information and accurate bed and surface topography copyright 1998 john wiley sons ltd

rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions confluences are important locations for river mixing within drainage networks yet few studies have examined in detail the dynamics of mixing within confluences this study examines the influence of momentum flux ratio the scale of the flow cross sectional area and the density differences between incoming flows on thermal mixing at a small stream confluence results reveal that rates and patterns of thermal mixing depend on event specific combinations of the three factors the mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel as the momentum flux from the lateral tributary increases mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel the cross sectional area of the flow is negatively correlated with mixing rates suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow density differences are not strongly related to rates of mixing results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions but that in general length scales of mixing are short and rates of mixing are high at this small confluence compared with those typically documented at large river confluences copyright 2015 john wiley sons ltd

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

sensitivity of bed shear stress estimated from vertical velocity profiles the problem of sampling resolution bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile however most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed the lack of near bed measurements is a critical loss of information which may affect bed shear stress estimates detailed velocity profiles obtained from a field acoustic doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near bed points is critical for the estimation of bed shear stress in a shallow river environment moreover the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform 1998 john wiley sons ltd

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

1 min in the life of a river selecting the optimal record length for the measurement of turbulence in fluvial boundary layers in the analysis of instantaneous velocity measurements it is important to consider carefully the properties of the instruments as well as the sampling frequency and the record length although several studies have examined the influence of instrument settings on velocity measurements in rivers little information is available on the effects of the record length on these measurements and on the turbulence parameters that can be extracted from the time series decisions on the record length are often made ad hoc here we quantify the effects of the sampling period on the precision of turbulence statistics e g mean standard deviation skewness reynolds stress quadrant analysis obtained from velocity measurements in fluvial turbulent boundary layers a bootstrapping method is used to determine the standard errors s e of turbulence statistics from 19 time series the time series were sampled in three gravel bed rivers using both electromagnetic current meters ecm and acoustic doppler velocimeters adv the record lengths ranged between 20 and 60 min the sampling frequencies were 20 and 25 hz for the ecms and advs respectively we show that for most turbulence statistics the optimal record length minimum sampling effort to achieve low standard errors ranges between 60 and 90 s we discuss the implication of these record lengths for standard errors estimates and variance losses

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

predicting macroturbulence energy and timescales for flow over a gravel bed experimental results and scaling laws abstract the fluvial science community lacks predictive equations for macroturbulence energy and timescales which may serve useful for predicting sediment transport in rivers therefore we investigated the systematic variation of macroturbulence scales by varying hydraulic parameters in an experimental facility collecting velocimetry data and measuring macroturbulence scales with spectral analyses and testing scaling laws for macroturbulence prediction that consider inner variables outer variables and turbulence kinetic energy experiments spanned across 11 energy gradients and aspect ratio combinations including 116 testing conditions each with 30 minute instantaneous turbulence time series collected results showed velocimetry data and time average turbulence parameters supported the presence of an equilibrium outer region and coherent double layer assumed in the experimental design macroturbulence and bursting wavenumbers were constant in the outer region when scaled with the flow depth and were independent of distance above the boundary aspect ratio energy gradient froude number or shields parameter data results agreed with the derivation herein and showed macroturbulence energy scales with the streamwise turbulence kinetic energy semi theoretical equations for turbulence kinetic energy scaled macroturbulence energy reasonably well a novel equation is presented for the macroturbulence period and the equation s transferability is supported by consistency with past macroturbulence studies bursting formula and flow visualization studies novel equations are presented for macroturbulence energy and transferability is supported on theoretical and empirical grounds however open questions remain regarding a universal function for streamwise turbulence kinetic energy and the net contribution of macroturbulence to total turbulence energy results showed the macroturbulence wavenumber required a relatively long duration of data i e 17 min to stabilize results were used to formulate a time scale for minimum measurement length of turbulence data when information about macroturbulence is sought after we discuss macroturbulence linkages with sediment transport including prediction of the sediment carrying capacity via macroturbulence formula as one application to help close the gap between turbulence and fluvial sediment transport

flow resistance in open channel flows with sparsely distributed bushes the paper faces the problem of the resistance due to vegetation in a river characterized by fully submerged vegetation formed by concentrated colonies of bushes the flow presents strong spatial variations between plants that make unreliable the traditional approach based on time averaging of turbulent fluctuations a more useful model based on time and spatial averaging is proposed in the paper the necessary closure hypotheses are also discussed the vertical distribution of mean velocity and turbulence stress have been measured with laser doppler anemometry techniques by means of spatial and time averaging rules based on the double averaged velocity and reynolds stress profiles an analytical two layer model is proposed in order to describe uniform flow conditions in the whole flow depth theoretical results are compared with the results of a series of experimental tests carried out in a laboratory flume

hydrodynamics of a forced riffle pool in a gravel bed river 2 scale and structure of coherent turbulent events 1 in the companion article we present mean velocity and turbulence intensity statistics at a range of discharges in a forced riffle pool unit and show that the distributions of mean velocity and turbulence are characteristic of nonuniform flows the objectives of this paper are to describe the spatial and temporal variability of the structure of turbulence in the forced riffle pool unit and to examine turbulence generation mechanisms and the links between the scales of macrobed forms and coherent turbulent flow structures our results show that while larger scale structures develop outside of the thalweg the scales of coherent turbulent flow structures decrease with velocity and discharge in the forced riffle pool flow separation is shown to be an important factor with full separation occurring in lateral shear zones and intermittent separation near the bed in the pool head these zones generate new turbulence that disrupts a number of coherent turbulent flow structures that are typical of uniform flows coherent turbulent flow structures do not appear to be the source of morphological scaling in the forced riffle pool

the effect of bedload transport on mean and turbulent flow properties abstract this paper reports the results of a flume experiment that was designed to investigate the effect of bedload transport on mean and turbulent properties of the flow the experiment consisted of varying the bedload transport rate for a given hydraulic condition and of measuring the flow velocity profiles using an acoustic doppler velocimeter adv for each transport rate in order to allow for comparison bedload transport was produced by injecting gravel size particles d50 7 4 mm with a conveyer belt mounted at the upstream end of the flume the results indicate that the effect of bedload on flow characteristics is complex it is shown that bedload transport causes opposite effects on flow velocity depending on the roughness of the bed and the relative magnitude of flow and sediment transport variables a better understanding of these conflicting results is obtained from the application of an energy budget approach to the analysis of velocity data this approach demonstrates that bedload affects flow velocity by modifying the rate of dissipation of turbulent kinetic energy however the mechanisms responsible for the modification of turbulent dissipation are still unknown

measurements of the relationship between turbulence and sediment in suspension over mobile sand dunes in a laboratory flume 1 the relationship between fluid and particle motions over a mobile dune bed was investigated using a laboratory flume with mobile sand dunes fluid turbulence data from a laser doppler velocimeter ldv and suspended sediment data from an acoustic backscatter system bss were collected from the same sampling volume at a constant position relative to passing sand dunes in a laboratory channel by using a motion control system to follow bed forms through the test section a second bss continually traversed the test section to map the bed profile the data from the ldv and the two bss units were correlated in order to link turbulence data with variations in suspended sediment concentration and position on bed forms local values of suspended sediment concentration were poorly correlated with quadrant classification indicating a weak relationship between local turbulence and sediment in suspension reynolds stress values were not strongly affected by dune size over the scales covered in the experiments sweeps quadrant 4 events were the most numerous but bursts quadrant 2 events generally had higher relative magnitudes bursts were best correlated with changes in suspended sediment concentration and with cumulative sediment suspension

turbulent and advective momentum fluxes in streams 1 the scope of the present paper is the evaluation of the exchange of momentum in natural streams to explore the contribution that turbulence and secondary currents have on the shear stress distributions within the flow and hence evaluate the overall hydraulic resistance existing methods for the evaluation of shear stresses in rivers are examined with respect to their applicability to complex flow fields and a new mathematical formulation for analyzing the shear stresses affecting the longitudinal velocity distribution is presented this new method enables one to estimate the time averaged principal shear stress and its particular components due to turbulence and secondary currents directly from three dimensional velocity data an acoustic doppler velocimeter adv was used to measure the currents and turbulence at five cross sections of two field sites over a wide range of discharges the results of the cross sectional measurements reflect the nonuniformity of the river bed roughness cross sectional form and planform limited applicability was found for methods based on the theory of two dimensional boundary layer flows shear stresses estimated using the new method emphasize the dominance of secondary currents on the total momentum flux momentum fluxes caused by secondary currents were an order of magnitude larger than momentum fluxes due to turbulence