does flow variance affect bedload flux when the bed is dominated by grain roughness abstract previous studies have shown that spatial variance in fluid and critical shear stress caused by form roughness can increase bedload flux others have revealed that variance in flow velocity increases with relative submergence and that bed mobility is reduced at lower submergences the paper explores the link between these observations and addresses the following questions is grain roughness sufficient to cause variance in fluid shear stress and an increase in bedload flux if this variance changes with submergence does this mean that the increase is dependent on submergence and does this explain the change in mobility with submergence a simple statistical bedload model based on spatial distributions of fluid and critical shear stress has been used to explore these effects over a water worked gravel deposit estimates of spatially distributed fluid shear stress were gained from laboratory flume measurements of near bed flow velocity and a distribution of critical shear stress was simulated using a discrete particle model of the sediment distribution used in the flume the velocity data were used to describe the change in the spatial distribution of near bed velocity with relative submergence which allowed the effects of submergence on flux to also be considered the main conclusions were i spatial variance in fluid shear stress from grain roughness was not sufficient to have an appreciable effect on bedload flux over water worked gravel beds with a spatial distribution of critical shear stress ii spatial variance in critical shear stress caused by grain roughness had a much larger influence and increased bedload flux this was a similar level of increase observed in studies for conditions where form roughness was high iii spatially averaged estimates of fluid and critical shear stress should not be used to estimate bedload flux even if form roughness is low and iv a rise in relative submergence increased bedload flux this was due to changes in the spatial distribution of near bed velocity and not due to a lowering in the local flow velocity as has been suggested by previous studies

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

topographic disturbance of subaqueous gravel substrates by signal crayfish pacifastacus leniusculus abstract the impact of signal crayfish pacifastacus leniusculus on the topography and fabric of six narrowly graded gravel substrates was investigated using repeat laser scanning of sediment surfaces in still water aquaria digital elevation models dems of the gravel surfaces were obtained before and after exposure to crayfish for five predetermined periods the impact on the substrate was quantified by establishing topographic and volumetric changes using dems of difference dod the presence of an individual medium sized crayfish for 24 h resulted in an average volume change in surface topography of 450 cm 3 over an area of 2400 cm 2 giving a sediment displacement of 1 7 kg m 2 d 1 the majority 78 of this volume change was associated with small scale 1 median grain diameter movements of surface grains this fabric adjustment altered grain orientations and friction angles crayfish also constructed pits and mounds that increased significantly the roughness of the gravel substrates and altered the protrusion of individual grains crayfish were able to move material up to 38 mm in diameter that had a submerged weight six times that of the individuals used in this study by modifying the arrangement of grains on the surface of fluvial substrates signal crayfish may counteract the low flow physical consolidation of gravel beds and reduce the entrainment stresses required to move river bed material the results of this study suggest that signal crayfish an internationally widespread invasive species may have substantial impacts on the physical environment of streams and rivers as well as on local benthic ecological communities

remote monitoring of volumetric discharge employing bathymetry determined from surface turbulence metrics current methods employed by the united states geological survey usgs to measure river discharge are manpower intensive expensive and during high flow events require field personnel to work in dangerous conditions indirect methods of estimating river discharge which involve the use of extrapolated rating curves can result in gross error during high flow conditions due to extrapolation error and or bathymetric change our goal is to develop a remote method of monitoring volumetric discharge that reduces costs at the same or improved accuracy compared with current methods while minimizing risk to field technicians we report the results of large scale particle image velocimetry lspiv and acoustic doppler velocimetry adv measurements conducted in a wide open channel under a range of flow conditions i e channel aspect ratio b h 6 6 31 9 reynolds number reh 4 950 73 800 and froude number fr 0 04 0 46 experiments were carried out for two different channel cross sections rectangular and asymmetric compound and two bathymetric roughness conditions smooth glass and rough gravel bed the results show that the mean surface velocity normalized by the depth averaged velocity the velocity index decreases with increasing h where is the boundary layer displacement thickness and that the integral length scales l11 1 and l22 1 calculated on the free surface vary predictably with the local flow depth remote determination of local depth averaged velocity and flow depth over a channel cross section yields an estimate of volumetric discharge

predicting incipient motion including the effect of turbulent pressure fluctuations in the bed 1 incipient motion of particles on a river bed can in principle be understood and predicted from a balance of the forces acting on the particles on a natural river bed the exposure of particles to the flow is variable the effect of exposure on the initiation of motion including the case of deep embedding is studied in this paper so far the turbulence induced lifting force has been derived exclusively from examinations of the surface flow the understanding of destabilization processes of the riverbed is improved when turbulence induced vertical pressure gradients in the upper sediment layer are included this is particularly important for particles that lie in the lee of larger ones an analytical solution for the critical shields parameter of spherical particles is found including the effects of pressure fluctuations in the bed macroscale flow structures average bed slope and shallow flow existing laboratory data designed to test the effect of exposure and roughness conditions are in excellent agreement with the new model

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

a consideration of the dune antidune transition in fine gravel hydraulic data defining the dune antidune transition in fine gravel are compared with potential flow theory and information is drawn from published experiments and field based studies attention is given to both transitional bedforms and the development of downstream migrating antidunes in the latter case most data pertain to sand beds and not to gravel empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower froude numbers at greater relative depths although a critical froude number of 0 84 may reasonably be applied for the beginning of the dune to antidune transformation lag effects and a possible depth limitation ensure that transitional bedforms may persist across a broad range of froude numbers from 0 5 to 1 8 this latter observation has great relevance for palaeohydraulic estimates derived from outcrop data whereas the application of theoretical bedform existence fields based upon potential flow theory to fine gravel was previously purely speculative the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems for the first time laboratory data pertaining to downstream migrating gravel antidunes are compared with theory these bedforms have been reported from certain experimental near critical flows above sand or gravel beds but have been observed infrequently in natural streams however there are no detailed studies from natural rivers and only a few contentious identifications from outcrops nevertheless the limited hydraulic data conform to theoretical expectations

frequency wavenumber velocity spectra taylor s hypothesis and length scales in a natural gravel bed river 1 macroscale turbulent coherent flow structures in a natural fast flowing river were examined with a combination of a novel 2 mhz acoustic doppler beam adb and a maximum likelihood estimator mle to characterize the streamwise horizontal length scales and persistence of coherent flow structures by measuring the frequency f streamwise wavenumber ks energy density velocity spectrum e f ks for the first time in natural rivers the adb was deployed under a range of froude numbers 0 1 0 6 at high reynolds numbers 106 based on depth and velocity conditions within a gravel bed reach of the kootenai river idaho the mle employed on the adb data increased our ability to describe river motions with relatively long 10 m length scales in 1 m water depths the e f ks spectra fell along a ridge described by v f ks where vis the mean velocity over depth consistent with taylor s hypothesis new consistent length scale measures are defined based on averaged wavelengths of the low frequencye f ks and coherence spectra energetic 50 of the total spectral energy low frequency f 1 m s lmwere found to be significantly longer than their corresponding coherence lengths suggesting that the turbulent structures evolve rapidly under these conditions this is attributed to the stretching and concomitant deformation of preexisting macroturbulent motions by the ubiquitous bathymetry induced spatial flow accelerations present in a natural gravel bed river

southern hemisphere circulation features associated with el nino southern oscillation events abstract composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the southern hemisphere sh circulation features associated with el nino southern oscillation enso events the period of analyses available 1972 83 has limited the composites to include only three enso events the reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period 1950 79 of sh rawinsonde station data in the sh winter a weak equivalent barotropic wavetrain pattern of anomalies extends over australia and the south pacific ocean to south america this wavetrain pattern is quite variable in amplitude and location between enso events although it is more stable over the subtropical pacific in the sh summer the circulation anomalies are more zonally symmetric with increased height at low and high latitudes and decreased height in middle latitudes the circulation anoma

hydraulic resistance of submerged flexible vegetation the main research objective consisted in analysing the influence of roughness caused by aquatic vegetation av in particular submerged macrophytes on the overall flow field these plants are highly flexible and behave differently depending on the flow situation they also react substantially to the flow field and thus the roughness becomes variable and dynamic conventional flow formulas such as the manning or the strickler formula are one dimensional and based on integral flow parameters they are not suitable for quantifying the roughness of av because the flow is complex and more dimensional due to the variable behaviour of the plants therefore the present investigation concentrates on the definition of a characteristic hydraulic roughness parameter to quantify the resistance of av within this investigation laboratory experiments were carried out with three different types of av chosen with respect to varying plant structures as well as stem lengths velocity measurements above these plants were conducted to determine the relationship between the hydraulic roughness and the deflected plant height the deflected plant height is used as the geometric roughness parameter whereas the equivalent sand roughness based on the universal logarithmic law modified by nikuradse was used as hydraulic roughness parameter the influence of relative submergence on the hydraulic roughness was also analysed the analysis of the velocity measurements illustrates that equivalent sand roughness and zero plane displacement of the logarithmic law are correlated to the deflected plant height and are equally to this height

a new geophone device for understanding environmental impacts caused by gravel bedload during artificial floods here to assess the contribution of gravel bedload on the removal of attached algae and aquatic plants from a cobble bed river during small floods we propose a geophone type method for measuring the local bedload of non uniform sized gravel due to limited peak discharge for focused events during our study a large fraction of bed material here cobbles was immobile and only a small fraction of bed material sand and gravel was expected to be transported during the flushing flows we analyzed the device we developed has a size equivalent to immobile bed material and a shape similar to bed material rounded cobbles at the site the instrument s design allows avoidance of disturbances in river bed micro topography during installation and local bedload transport during floods a flume experiment was conducted in order to establish an empirical algorithm for estimating the diameter of impacted gravel and here discuss uncertainty related to diameter estimations the proposed method was utilized to quantify gravel bedload in a cobble bed river during flushing flows in the text we also discuss the contribution of measured gravel bedload during flushing flows on the removal of attached algae up to a 37 reduction in chlorophyll a density and aquatic plants a reduction of 38 in dry mass per area based on time variation for the measured gravel bedload we also suggest the propagation of a bed form composed of the fine sediment fraction migrating on immobile larger sediment and implications for the propagation of the fine sediment wave for attached algae removal this article is protected by copyright all rights reserved

an investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel 1 conditions corresponding to the initiation of the erosion and deposition process 1 the present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start flat bed of the scouring process large eddy simulation les is performed at a relatively low channel reynolds number re 18 000 in the range where most flume studies with clear water scour conditions are conducted similar to these studies the incoming flow is fully turbulent and contains realistic turbulence fluctuations visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process it is found that the structure of the horseshoe vortex hv system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex however its intensity is the largest at vertical sections situated around the tip of the spur dike it is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes in one of them zero flow mode the necklace vortex is closer to the spur dike and more compact and the near bed jet flow beneath it is weak in the other one back flow mode a strong near bed jet flow convects the primary necklace vortex away from the spur dike and its core is more elongated and less compact this explains the large amplification by about 1 order of magnitude compared to the surrounding turbulent flow of the turbulent kinetic energy and pressure fluctuations inside the hv system in the region situated around the tip of the spur dike and the double peak distribution of the turbulent kinetic energy past the spur dike in the legs of the necklace vortex the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike it is found that the legs of the horseshoe vortices can interact at times with the vortex tubes shed in the detached shear layer dsl and with the tip of the spur dike these events typically result in a significant change in the coherence of the hv system the largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike but high bed shear stress values are also observed beneath the upstream part of the dsl the bed shear stress fluctuations around the local mean values can be very high especially in the region situated beneath the upstream part of the dsl at random times some of the vortices shed in the dsl merge or interact with eddies from the recirculation region this leads to an increase in their strength and to a large increase of the bed shear stress along their path

quadrant octant sequencing and the role of coherent structures in bed load sediment entrainment to permit the tracking of turbulent flow structures in an eulerian frame from single point measurements we make use of a generalization of conventional two dimensional quadrant analysis to three dimensional octants we characterize flow structures using the sequences of these octants and show how significance may be attached to particular sequences using statistical mull models we analyze an example experiment and show how a particular dominant flow structure can be identified from the conditional probability of octant sequences the frequency of this structure corresponds to the dominant peak in the velocity spectra and exerts a high proportion of the total shear stress we link this structure explicitly to the propensity for sediment entrainment and show that greater insight into sediment entrainment can be obtained by disaggregating those octants that occur within the identified macroturbulence structure from those that do not hence this work goes beyond critiques of reynolds stress approaches to bed load entrainment that highlight the importance of outward interactions to identifying and prioritizing the quadrants octants that define particular flow structures

teleconnections in the southern hemisphere abstract teleconnections are calculated from monthly mean anomalies of sea level pressure and 500 mb geopotential height for the southern hemisphere 10 90 s for five month winter and summer seasons the monthly means were calculated from australian analyses for the period from june 1972 to november 1980 zonally averaged anomalies at both sea level and 500 mb display an out of phase relation between low and high latitudes and in midlatitudes are negatively correlated with anomalies in the subtropics and polar regions in winter a striking zonal wavenumber 3 pattern is found over the southern ocean anomalies in 500 mb geopotential heights at 50 s 95 e 58 s 150 w and 38 s 15 w exhibit strong positive correlations while showing weaker negative correlations with heights over antarctica and in low latitudes a similar pattern appears in sea level pressure in summer anomalies in 500 mb geopotential height over the three subtropical continents appear to occur out of phase with anomalies over the su

mid late holocene climatic changes recorded by loess deposits in the eastern margin of the tibetan plateau implication for human migrations abstract the climatic fluctuations during the mid late holocene in northern sichuan southwest china were inferred from loess deposits using multi proxies of total organic carbon toc carbonates magnetic susceptibility stable isotopic composition of organic carbon 13 corg and grain size distribution in combination with the dates determined by the accelerator mass spectrometry ams 14 c and optically stimulated luminescence osl methods from 6300 to 5500 a bp the regional climate presented gradually increased warmth and humidity subsequently characterized by reduced precipitation but with relatively high air temperature between 5500 and 4800 a bp demonstrating that the east asian monsoon weakened a warmer and wetter climate prevailed since 4800 a bp and was interrupted by a sharp cold reversal at approximately 3300 a bp that was likely caused by solar irradiance forcing which resulted in a global cold climatic change and glacier advance large scale human migrations into the eastern margin of the tibetan plateau occurred in the cold and dry events that developed at 6000 2500 and 1200 a bp indicating a critical stimulus effect of climatic change on human migration

a zonal wave 3 index for the southern hemisphere 1 in this study 500 hpa geopotential heights from the ncar ncep reanalysis are used to calculate an index describing the spatial and temporal characteristics of zonal wave three zw3 in the high latitude southern hemisphere atmosphere the index spans the period 1958 2001 wavelet analysis of the index shows the strong interannual variation which is characteristic of zw3 and also reveals significant high amplitude decadal scale variation this interannual variation takes the form of marked fluctuation in sign and magnitude of the index it appears to occur when there are rapid changes from strongly meridional to strongly zonal flow or vice versa initial indications are that some of this variability may be associated with enso events

active interactions between turbulence and bed load conceptual picture and experimental evidence 1 this paper reports results from experiments with simultaneous measurement of areal bed load sediment concentration and water velocity horizontal and vertical components made at 1 5 particle diameters above the mean bed the same spatial and temporal scales were employed to measure the kinematic properties of flowing water and sediments the obtained data covering a range of low sediment discharges were analyzed in terms of bulk statistics autocorrelation and cross correlation functions and quadrant decomposition of fluctuations at the lowest transport rates the motion of sediment particles was found to be mostly associated with sweep events however with increasing flow rate and bed load discharge the process mechanics changed and the transport of sediments was also found to be significantly correlated with ejections the obtained results are integrated into a conceptual picture attempting to bridge previous depictions of the bed load transport mechanics

blocking in the southern hemisphere abstract the focus of this paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the southern hemisphere the analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more composite flow and anomaly fields for several cases are presented and examined in detail the geographical distribution of the frequency of lame amplitude 150 gpm anomalies at 500 mb that persist for only 1 3 days is very similar to that of the high frequency band 2 8 day period variances that identity the storm tracks in the southern hemisphere the primary location for blocking in the southern hemisphere is in the new zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerli

geostatistical analysis of the effects of stage and roughness on reach scale spatial patterns of velocity and turbulence intensity although previous research has documented well organized interactions between the turbulent flow field and an irregular boundary the spatial variability of turbulent flow characteristics at the reach scale remains poorly understood in this paper we present detailed field measurements of three dimensional flow velocities and turbulence intensities in a high gradient cobble bed riffle from three discharges additional data on sediment grain size and bed topography were used to characterize boundary roughness an acoustic doppler velocimeter was used to measure velocities along five cross sections within a 6 in long reach of the north fork cache la poudre river vertical profiles were also measured along the channel thalweg we adopted a spatially explicit stochastic hydraulic approach and focused not on coherent flow structures per se but rather time averaged reach scale variability and spatial pattern scaling velocities and turbulence intensities by the reach averaged friction velocity u accounted for changes in flow depth and enabled comparisons among the three discharges we quantified the effects of stage and roughness by assessing differences among probability distributions of hydraulic quantities and by examining geostatistical metrics of spatial variability we computed semivariograms for streamwise and transverse directions and fit parametric models to summarize the spatial structure of each variable at each discharge cross correlograms were also used to describe the local and lagged effects of boundary roughness on flow characteristics although the probability distributions yielded some insight incorporating spatial information revealed important elements of stage dependent flow structure the development of secondary currents and flow convergence at higher stages were clearly documented in maps and semivariograms in general the spatial structure of the flow field became smoother and more continuous as stage increased and the effects of boundary roughness diminished although roughness elements do influence velocities and turbulence intensities our data suggest that the flow primarily responds to the gross morphology of the channel and that flow depth is the primary control on flow structure the geostatistical framework proved useful and our results indicate that a complete stochastic description must also be explicitly spatial c 2006 elsevier b v all rights reserved

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

coupled dynamics of the co evolution of gravel bed topography flow turbulence and sediment transport in an experimental channel a series of flume experiments were conducted in a large experimental channel at the st anthony falls laboratory to understand the coupled dynamics of flow and bed forms above the sediment water interface simultaneous high resolution measurements of velocity fluctuations bed elevations and sediment flux at the downstream end of the channel were made for a range of discharges the probability density functions pdfs of bed elevation increments and instantaneous reynolds stress reveal a power law tail behavior and a wavelet cross correlation analysis depicts a strong dependence of these series across a range of scales indicating a feedback between bed form dynamics and near bed turbulence these results complement our previous findings in which the signature of bed form evolution on the near bed velocity fluctuations was confirmed via the presence of a spectral gap and two distinct power law scaling regimes in the spectral density of velocity fluctuations we report herein a strong asymmetry in the probability distribution of bed elevation increments and instantaneous reynolds stresses the latter being further analyzed and interpreted via a quadrant analysis of velocity fluctuations in the longitudinal and vertical directions we also report the presence of intermittency multifractality in bed elevation increments and interpret it in view of the asymmetric nature of the pdfs as the result of scale coupling in other words the geometric asymmetry at the bed form scale gets transferred down to a probabilistic asymmetry at all smaller scales indicating a local anisotropy in the energy transfer finally we propose a predictive relationship between bed form averaged sediment transport rates and bed form averaged instantaneous reynolds stress and validate it using our experimental data

coherent flow structures in a depth limited flow over a gravel surface the influence of surface roughness 1 turbulent flows moving over a gravel bed develop large scale macroturbulent flow structures that are initiated at anchor clasts in the bed and grow and dissipate as they move upward through the flow depth this paper extends previous research in which we investigated the influence of the reynolds number on coherent flow structures generated over a gravel bed by assessing the importance of effective bed roughness here we report on flume experiments in which flows over beds of decreasing surface roughness have been quantified through the application of digital particle imaging velocimetry which allows study of the downstream and vertical components of velocity over the entire flow field these results indicate that as the effective roughness increases 1 the visual distinctiveness of the coherent flow structures becomes more defined throughout the flow depth 2 the upstream angle of slope of the coherent flow structure increases and 3 the reduction in streamwise flow velocity and turbulence intensity toward the upstream side of the structure becomes greater applying standard scaling laws these structures appear shear generated and form through a combination of both wake flapping and the reattachment of localized shear layers associated with flow separation around individual topographic protrusions as the effective protrusion decreases the scale of these coherent flow structures also decreases

bridging the gap between turbulence and larger scales of flow motions in rivers although flow turbulence in rivers is of critical importance to earth scientists ecologists and engineers its relations with larger flow scales are not well understood thus leaving a fundamental gap in our knowledge from an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel bed river we show that the signature of the fundamental turbulent flow structures e g ejections and sweeps is embedded within increasingly larger flow scales in a self similar manner the imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two orders of magnitude from a few seconds to nearly 10 minutes this property is explained by the clustering of turbulent events creating an emergent pattern at larger scales the size of the larger flow pulsations scales with the spacing of the pools and riffles in the river this implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology these results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport habitat selection and fish behaviour copyright 2011 john wiley sons ltd

characterizing spatial variability in velocity and turbulence intensity using 3 d acoustic doppler velocimeter data in a plane bed reach of east st louis creek colorado usa abstract we investigated the influence on flow resistance of flow structure and turbulence at the reach scale in a mountain channel using 3 d velocity measurements and geostatistical analysis to understand the complexity of the flow structure in a reach with limited bed irregularities the increase in flow resistance at low flows in a plane bed reach was not fully explained by grain resistance therefore detailed 3 d velocity measurements were made to investigate spatial variability in velocity and turbulence components and potential controls on flow resistance one plane bed reach was surveyed over two stages in fraser experimental forest colorado using a combination of a total station lidar light detection and ranging and a sontek flowtracker handheld adv acoustic doppler velocimeter lidar was used to capture bank and channel geometry at low flows whereas the water surface and bed data were collected with the total station at all flows we used the standard deviation of bed elevation b within a moving window as an index of roughness height ks and calculated the relative submergence of the bed at different stages as h ks where h is the local flow depth adv measurements were collected on a grid with a 0 3 m to 0 5 m spacing geostatistical analysis of the velocity data indicated that the flow was highly three dimensional and varied based on stage demonstrating that even small irregularities in the bed have a significant influence on the flow characteristics the streamwise component was the largest at both low and high flow but varied more throughout the reach at low flow at high flow the greatest streamwise velocities were located within the thalweg areas of upwelling and downwelling also varied based on stage with this component being strongly influenced by small changes in the morphology at high flow and by protuberant grains at low flows the cross stream velocity and turbulence components were controlled by the flow structure and less by the roughness of the bed the turbulence intensity is significant when considering hydraulics for predicting sediment transport and for habitat assessment the plane bed flow patterns were compared to results from a pool riffle reach in the north fork cache la poudre river a step pool reach in east st louis creek and a step pool reach in italy on the rio cordon the comparison of the 3 d velocities among channel types showed that the plane bed reach has higher streamwise velocities but similar values of cross stream and transverse velocities streamwise turbulence intensities were similar in both the plane bed and step pool reach in east st louis creek the analysis revealed that the connection between the flow characteristics and the channel was not only related to the gross morphology and location of larger clasts but also to the development of shear layers from the convergence of flow therefore it is essential to understand how water moving in one direction can alter the characteristics of another component of flow and how this interaction is connected to the bed morphology

influence of relative submergence on spatial variance and form induced stress of gravel bed flows 1 spatial flow variance has a strong control on sediment transport sediment water interface exchange mechanisms and the distribution and behavior of aquatic organisms in rivers thus being able to quantify spatial flow variance and how it varies with different water levels is important for understanding how fluvial processes change during periods of time varying flow in this paper laboratory flume measurements of near bed flow velocity were used to quantify spatial flow variance and form induced stress and their variation with flow submergence within and above the surface of porous gravel beds with differing grain roughness the analysis revealed spatial flow variance was usually four or five times higher within the roughness layer than above a rise in relative submergence resulted typically in a decrease in spatial variance relative to bed shear velocity in streamwise form induced intensity streamwise turbulence intensity and form induced momentum flux both within and above the roughness layer flow submergence had no consistent influence on spatial variance in the vertical flow direction and in reynolds stress form induced stress was significant within the roughness layer more so at shallow depths the greater significance was driven primarily by higher spatial flow variance in time averaged streamwise velocity at these depths the implication is the relative role of momentum transfer mechanisms within the roughness layer and thus sediment water interface exchange processes in rivers will change during periods of time varying flows

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

robust classification for the joint velocity intermittency structure of turbulent flow over fixed and mobile bedforms two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity intermittency characteristics and suggest a universality to the velocity intermittency structure for flow over bedforms one dataset was obtained by sampling flow over static bedforms in different locations and the other was based on a static position but mobile bedforms a flow classification based on the velocity intermittency behaviour is shown to reveal some differences from that based on an analysis of reynolds stresses boundary layer correlation and turbulent kinetic energy this may be attributed to the intermittency variable which captures the local effect of individual turbulent flow structures locations in the wake region or the outer layer of the flow are both shown to have a velocity intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms the combined effect of this yields a velocity intermittency structure unique to bedform flow the use of a time series of a single velocity component highlights the potential power of our approach for field numerical and laboratory studies the further validation of the velocity intermittency method for non idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology hydraulics meteorology and environmental fluid mechanics 2014 the authors earth surface processes and landforms published by john wiley sons ltd

on the evolution and form of coherent flow structures over a gravel bed insights from whole flow field visualization and measurement the microtopography of a gravel bed river has been shown to generate turbulent flow structures that originate from shear flow generated in the near bed region although field and laboratory measurements have shown that such flows contain a range of coherent flow structures cfs the origin evolution and characteristics of the turbulent structures are poorly understood here we apply a combined experimental methodology using planar laser induced fluorescence and particle imaging velocimetry lif piv to measure simultaneously the geometric kinematic and dynamic characteristics of these cfs the flow structures were analyzed by applying standard reynolds decomposition and lagrangian vortex detection methods to understand their evolution propagation and growth in the boundary layer and characterize their internal dynamical complexity the lif results identify large individual fluid packets that are initiated at the bed through shear that generate a bursting mechanism when these large individual fluid packets are analyzed through direct flow measurement they are found to contain several smaller scales of fluid motion within the one larger individual fluid parcel flow measurements demonstrate that near bed shear controls the initiation and evolution of these cfs through merging with vortex chains that originate at the bed the vortex chains show both the coalescence in the formation of the larger structures and also the shedding of vortices from the edges of these packets which may influence the life span and mixing of cfs in open channels the life span and geometric characteristics of such cfs are critical in influencing the duration and intensity of near bed stresses that are responsible for the entrainment of sediment

the pattern of surface waves in a shallow free surface flow 1 this work presents new water surface elevation data including evidence of the spatial correlation of water surface waves generated in shallow water flows over a gravel bed without appreciable bed forms careful laboratory experiments have shown that these water surface waves are not well known gravity or capillary waves but are caused by a different physical phenomenon in the flow conditions studied the shear present in shallow flows generates flow structures which rise and impact on the water air interface it is shown that the spatial correlation function observed for these water surface waves can be approximated by the following analytical expression w e 2 2 w2cos 2 l0 1 the proposed approximation depends on the spatial correlation radius w characteristic spatial period l0 and spatial lag this approximation holds for all the hydraulic conditions examined in this study it is shown that l0 relates to the depth averaged flow velocity and carries information on the shape of the vertical velocity profile and bed roughness it is also shown that w is related to the hydraulic roughness and the flow reynolds number

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

flow resistance in natural turbulent channel flows the need for a fluvial fluid mechanics in fluvial environments feedbacks among flow bed forms sediment and macrophytes result in a complex fluid dynamics the assumptions underpinning standard tools in hydraulics are commonly violated and alternative approaches must be formulated i argue that we should question the assumption that classical notions in fluid mechanics provide the foundations for the techniques of the future recent work on turbulent dissipation interscale modulation of the dynamics intermittency and the role of complex forcings is discussed an agenda for future work is proposed that involves improving our characterization of complex forcings and developing better understanding of the behavior of the velocity gradient tensor in complex fluvial environments this leads to the formulation of modeling tools relevant to fluvial fluid mechanics rather than a reliance on methods developed elsewhere one avenue by which such methods might be developed is suggested based on the stretched spiral vortex as a baseline topology this would result in a nonequilibrium model for turbulence that has greater potential to capture the dynamics in which we are interested although these ideas are raised in the context of a future fluvial fluid mechanics they are applicable to any situation where turbulent flows are forced in complicated ways

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

emergence of coherent flow structures over a gravel surface a numerical experiment 1 gravel bed rivers have complex porous and irregular surfaces characterized by a range of morphological forms these topographical structures determine the flow structures that develop over the river bed primarily by the shedding of vortices in the downstream wake of protruding clasts previous research into these flow structures has come from experimental studies which have used either flow visualization or single point measurements techniques here we present a numerical experiment where large eddy simulation les is used to study the generation evolution and destruction of these flow structures over a naturally water worked gravel surface the numerical simulations reported in this paper show that there are two distinct scales of boundary influence upon the shallow flow and emphasize that the measured flow variability at any one point in a natural river will contain both locally derived and upstream inherited flow structures according to the range of scales of bed topography present

the application of computational fluid dynamics to natural river channels eddy resolving versus mean flow approaches in the last decade as computing power has increased there has been an explosion in the use of eddy resolving numerical methods in the engineering earth and environmental sciences for complex geomorphic flows where accurate field investigations are difficult to perform and where experiments may be difficult to scale these numerical approaches are beginning to give key insights into the nature of these flows eddy resolving methods such as large and detached eddy simulation les des may be contrasted with the time averaged three dimensional simulations that only really began to be applied seriously in geomorphology fifteen years ago while the potential of les for geomorphology has been examined previously des is a relatively recent method that deserves further consideration in this paper we explain the method and then utilise examples from meander and confluence flows as well as flow near the bed of a gravel bed river to highlight the improvements to both the representation of the mean flow and to the representation of time varying processes that result from the use of les des some suggestions are provided for the future use of such techniques in geomorphology

the antarctic circumpolar wave a combination of two signals abstract a simplified view of the possible mechanisms behind the antarctic circumpolar wave acw interannual variability is provided by a frequency domain decomposition of several observed atmospheric and oceanic variables two significant interannual signals with different temporal and spatial characteristics are identified in the southern ocean and most of the variance of the acw in the interannual band can be accounted for by a linear combination of them the first signal has a period of oscillation of around 3 3 yr and a zonal wavenumber 3 structure across the southern ocean it involves self sustained fluctuations inherent in the southern ocean and driven by coupled air sea interactions in which the atmosphere and the ocean mutually force one another this signal is represented by an atmospheric standing oscillation with centers at fixed locations around antarctica and a propagating oceanic pattern in which the surface antarctic circumpolar current plays an essential role the second signal has a

turbulence characteristics of new zealand gravel bed rivers gravel bed river turbulence is investigated on the basis of field measurements in new zealand rivers the data include instant longitudinal velocities obtained by two methods point measurements an

effect of bed sand content on the turbulent flows associated with clusters on an armored gravel bed surface abstractas a gravel bed river armors a bed surface often develops clusters as part of its structure the influence of sand on armoring and the impact of clusters on the surrounding flow processes were investigated through flume experiments armored beds were created from four different sediments which progressively increased from 1 38 sand in the bed sediment extending from gravel framework to sand matrix beds turbulent kinetic energy tke and xz direction reynolds stress calculated from velocity profiles showed the clusters increased turbulence and induced formation of recirculation cells similar recirculation cells formed downstream of every cluster but the strength and orientation of these cells varied with bed condition the increase in sediment sand content and change in bed condition from gravel framework to sand matrix correlated to an increase in the energy and momentum in the recirculation cell but a reduction in cluster influence across the flow profile results show that the sand conten

turbulence and secondary flow over sediment stripes in weakly bimodal bed material longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction this paper quantifies mean and turbulent flow structures over self formed sediment stripes in a weakly bimodal sand and gravel mixture turbulence anisotropy generates two secondary circulation cells across the channel half width which produce a cross stream perturbation in boundary shear stress the interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline finer sediments are entrained from regions of high shear stress transported laterally by the secondary flow and deposited in regions of lower shear stress lateral changes in bed texture further enhance the near bed secondary flow which provides a positive feedback mechanism for stripe growth in bimodal sediments at shear stresses just above the entrainment threshold stripes may replace lower stage plane beds at higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms read more http ascelibrary org doi abs 10 1061 asce 0733 9429 1999 125 5 463

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

a comparative study of the turbulent flow field with and without a pebble cluster in a gravel bed river 1 this paper investigates the processes and spatial patterns of the turbulent flow around a naturally formed in situ pebble cluster and the interactions with large scale coherent flow structures instantaneous high frequency velocities were measured using the simultaneous deployment of four acoustic doppler velocimeters with the pebble cluster and after the cluster was removed results indicate a twofold increase in turbulent kinetic energy over the sampling area when the pebble cluster is present individual vertical component velocity spectra in the wake of the pebble cluster contain a marked peak which decreases in frequency from 1 8 hz near the cluster to 1 0 hz farther downstream this peak is associated with eddies shedding from the shear layer in the lee of the cluster and it agrees well with the values estimated using a strouhal number of 0 18 space time correlations indicate the presence of coherent depth scale macroturbulent flow structures these structures in the intermediate flow region are relatively unaffected by eddies shedding from the pebble cluster the results suggest the effects of the pebble cluster on the surrounding flow dynamics are quite localized and limited to the near bed region

decadal variation of the annual cycle in the australian dataset abstract during the 1970s and 1980s elements of the atmospheric circulation over the southern hemisphere changed markedly from their state in the two previous decades the change was evident in the zonal asymmetry especially in the zonal harmonic wave 3 at higher latitudes and in the semiannual wave in pressure and wind the semiannual wave changed in the same sense in all three southern oceans but by the largest amount in the south pacific ocean the second harmonic in a 12 month series by which we describe the semiannual wave dominates the shape of the long term mean annual curve of sea level pressure in mid and high latitudes this harmonic weakened during the period and its midlatitude peak disappeared in the south pacific ocean as part of the low frequency changes during the 18 years from 1972 to 1988 the central pressure in the subantarctic trough fell the trough moved northward and the subtropical ridge moved south the meridional movement of the trough and the ridge and the concurrent

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

estimating bed shear stress from remotely measured surface turbulent dissipation fields in open channel flows synoptic information on bed shear stress is necessary in predicting the transport of sediments and environmental contaminants in rivers and open channels existing methods of estimating bed shear stress typically involve measuring vertical profiles of streamwise velocity or reynolds stress and taking advantage of the logarithmic or the constant stress region respectively to determine friction velocity and subsequently bed shear stress while effective these methods yield local measurements of bed shear stress only direct measurements of bed shear stress can also be obtained through measurements with a drag plate however this method yields average shear stress information over the area of the plate and is impractical for large scale implementation in the field here we present a method capable of providing continuous synoptic measurements of bed shear stress over a large field of view a series of large scale particle image velocimetry lspiv and acoustic doppler velocimetry adv measurements were made in a variety of flows generated in a wide open channel facility turbulent dissipation is calculated on the free surface from the 2 d lspiv results and is correlated with near surface adv measurements of turbulent dissipation in the water column the adv results are consistent with the nezu 1977 established relationship for the vertical variation of turbulent dissipation in the water column knowledge of the correlation between free surface and near surface dissipation values coupled with nezu s 1977 relationship allow a robust and accurate estimate of friction velocity to be made and subsequently shear stress at the bed can be estimated this article is protected by copyright all rights reserved

water worked gravel beds in laboratory flumes a natural analogue an investigation has been conducted to identify the key parameters that are likely to scale laboratory sediment deposits to the field scale two types of bed formation were examined one where sediment is manually placed and screeded and the second where sediment is fed into a running flume this later technique created deposits through sequential cycles of sediment transport and deposition detailed bed surface topography measurements have been made over a screeded bed and three fed beds in addition bulk subsurface porosity and hydraulic conductivity have been measured by comparing the four beds results revealed that certain physical properties of the screeded bed were clearly different from those of the fed beds the screeded bed had a random organization of grains on both the surface and within the subsurface the fed beds exhibited greater surface and subsurface organization and complexity and had a number of properties that closely resembled those found for water worked gravel beds the surfaces were water worked and armoured and there was preferential particle orientation and direction of imbrication in the subsurface this suggested that fed beds are able to simulate in a simplified manner both the surface and subsurface properties of established gravel bed river deposits the near bed flow properties were also compared it revealed that the use of a screeded bed will typically cause an underestimation in the degree of temporal variability in the flow furthermore time averaged streamwise velocities were found to be randomly organized over the screeded bed but were organized into long streamwise flow structures over the fed beds it clearly showed that caution should be taken when comparing velocity measurements over screeded beds with water worked beds and that the formation of fed beds offers an improved way of investigating intragravel flow and sediment water interface exchange processes in gravel bed rivers at a laboratory scale copyright 2008 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

measurement of fluctuating pressures on coarse bed material bed protections are usually characterized by low mobility transport conditions and nonequilibrium turbulence profiles as the present knowledge of the influence of turbulence on stability of cover layer units is minimal an in depth investigation was undertaken regarding the influence of turbulence on the stability of rough granular beds detailed measurements of fluctuating pressures on a bed element are used to evaluate certain concepts that are often used in modeling the entrainment of bed material from hydraulically rough beds three pressure transducers are placed in a cube that is part of a rough granular bed under open channel flow and velocities are measured using laser doppler velocimetry the measurements show that the magnitude of the fluctuating pressure at a certain point of the cube is a function of the exposure relative to the stones upstream of the cube a quadrant analysis reveals that the drag force is not only directly dependent on the horizontal near bed velocity but on the vertical velocity as well further the effect of small scale eddies shedding from the stone during large scale increases of longitudinal velocity is shown the fact that large scale velocity fluctuations create a large part of the pressure or force variance indicates that downstream of a roughness transition these fluctuations have to be taken into account in order to evaluate the stability of the bed

comment on pore water flow due to near bed turbulence and associated solute transfer in a stream or lake sediment bed by m higashino et al

the pacific south american modes and tropical convection during the southern hemisphere winter abstract atmospheric circulation features and convection patterns associated with two leading low frequency modes in the southern hemisphere sh are examined in multiyear global reanalyses produced by ncep ncar and nasa dao the two leading modes referred to as the pacific south american psa modes are represented by the first two eof patterns the two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the pacific south american sector in the pacific anomalies in the subtropics and in the midlatitudes are opposite in phase taken together the two psa modes represent the intraseasonal oscillation in the sh with periods of roughly 40 days the evolution of the psa modes shows a coherent eastward propagation a composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the psa modes outgoing longwave radiation olr anomaly composites during the mature phase

the influence of migrating bed forms on the velocity intermittency structure of turbulent flow over a gravel bed 1 modeling turbulent flows at high reynolds number requires solving simplified variants of the navier stokes equations the methods used to close the resulting reynolds averaged or eddy simulation equations usually follow classical theory and at small enough scales postulate universal scaling for turbulence that is independent of the velocity itself this may not be the best way to conceptualize geophysical turbulence turbulent intermittency may be defined in terms of the local roughness of the velocity signal as measured by pointwise holder exponents this study investigates the joint velocity intermittency structure of flow over a gravel bed surface with migrating bed forms we report clear velocity intermittency dependence and quantify its nature above the moving bed form profile our results imply differences in energy transfer close to bed forms at shorter wavelengths than those forced directly hence progress in modeling flows of geophysical relevance may require a reconsideration of the principles on which turbulence closures are based

estimation of flow resistance in gravel bedded rivers a physical explanation of the multiplier of roughness length the need to estimate velocity and discharge indirectly in gravel bedded rivers is a commonly encountered problem semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information although such equations have a semi theoretical basis in natural gravel bed channels an empirical constant 6 8 or 3 5 has to be introduced to scale up the characteristic grain size d50 or d84 to represent the effective roughness length in this paper two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small scale form resistance reflecting the occurrence of microtopographic bedforms in gravel bedded environments first spatial elevation dependence in short detailed bed profiles from a single gravel bedded river is investigated using semivariogram and zero crossing analyses this leads to objective identification of two discrete scales of bed roughness associated with grain and microtopographic roughness elements second the autocorrelation structure of the three dimensional near bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms apart from improving the capacity to determine friction factors for velocity and discharge estimation the findings have implications in general for the initial motion of gravelly bed material

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

spatial scale partitioning of in situ turbulent flow data over a pebble cluster in a gravel bed river 1 spatial structures within turbulent flow data were investigated through the use of a new multivariate variation partitioning analysis technique involving principal coordinates of neighbor matrices pcnm which is a form of distance based eigenvector maps dbem the analysis revealed a significant 0 01 spatial dependence 58 for the mean and turbulent flow variables the flow variables were obtained from instantaneous two dimensional velocities collected in situ along a streamwise section that crosses over a pebble cluster in a gravel bed river using the orthogonal property of the pcnm variables the explained variation was partitioned over four significant 0 01 spatial scales very large vl 17 large l 24 medium m 6 and fine f 2 nearly 75 of the variance of the main turbulent flow indicators such as the root mean square of the streamwise and vertical velocity components and the mean uv component reynolds shear stress was explained by the vl and l scale pcnm submodels which have streamwise and vertical length scales of the order of x 5 3h 2 6h and y 1 0h 0 5h where h is the flow depth respectively through a multivariate mapping procedure clear spatial patterns within the explained flow variables emerge around the cluster where the flow separation zone seems to play a significant role at a range of scales as well intervariable correlations at each spatial scale obtained through eigenvector scatterplots show intricate relationships between the flow variables the interdependence of the reynolds shear stress and the u component turbulent energy is much stronger at the vl scale than at the l and m scales the application of pcnm analysis on the turbulent flow field shows the power of the technique to resolve the relevant spatial scales and patterns and demonstrates its potential use in a variety of water resources studies

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

reply to comment by f boano et al on pore water flow due to near bed turbulence and associated solute transfer in a stream or lake sediment bed

bedload transport of fine gravel observed by motion picture photography motion pictures taken at duck creek a clear stream 6 5 m wide and 35 cm deep near pinedale wyoming provide detailed quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them bed shear stress was approximately 6 pa 60 dynes cm 2 which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material and transport was almost entirely as bedload the displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones and rarely by brief sliding of large angular ones entrainment was principally by rollover of the larger particles and liftoff of the smaller ones and infrequently by ejection caused by impacts whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones the displacement times averaged about 0 2 0 4 s and generally were much shorter than the intervening repose times the collective motions of the particles were characterized by frequent brief localized random sweep transport events of very high rates of entrainment and transport which in the aggregate transported approximately 70 of the total load moved these events occurred 9 of the time at any particular point of the bed lasted 1 2 s affected areas typically 20 50 cm long by 10 20 cm wide and involved bedload concentrations approximately 10 times greater than background the distances travelled during displacements averaged about 15 times the particle diameter despite the differences in their dominant modes of movement the 8 16 mm particles typically travelled only about 30 slower during displacement than the 2 4 mm ones whose speeds averaged 21 cm s 1 particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely with diffusivities of 4 6 and 0 26 cm 2 s 1 respectively the bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler the 2 4 mm particles were entrained 6 times faster on finer areas of the bed where 8 16 mm particles covered 6 of the surface area than on coarser ones where they covered 12 even though 2 4 and 4 8 mm particles covered practically the same percentage areas in both cases the 4 8 and 8 16 mm particles in contrast were entrained at the same rates in both cases to within the statistical uncertainty the rates of distrainment balanced the rates of entrainment for all three sizes and were approximately proportional to the corresponding concentrations of bedload

comments on southern mid to high latitude variability a zonal wavenumber 3 pattern and the antarctic circumpolar wave in the csiro coupled model abstract variability in the southern atmospheric circulation at mid to high latitudes with a dominant quasi stationary wavenumber 3 pattern has been reported in many observational studies the variability is barotropic in nature with signals in the middle troposphere as well as at the atmosphere ocean interface moreover there are preferred fixed centers for the strongest anomalies these features are well reproduced by the commonwealth scientific and industrial research organisation coupled model on various timescales on the interannual timescale an index of the modeled wavenumber 3 pattern shows little correlation with the modeled southern oscillation index suggesting that the variability associated with wavenumber 3 anomalies is separate to modeled enso like events however the variation of the pattern index is strikingly similar to and highly correlated with the modeled oceanic variability the associated oceanic anomalies move eastward and are similar to those of the observed antarctic circum

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

spatially representative velocity measurement over water worked gravel beds 1 recent studies have examined the spatial heterogeneity of velocity fields over natural river boundaries a key challenge is understanding how many velocity measurements are required to provide spatially representative estimates of the flow this paper describes a series of laboratory experiments in which flow velocities have been measured in a detailed spatial pattern over two water worked gravel beds these data have been utilized to deduce the minimum density of measurements required to provide representative estimates of several spatially averaged flow parameters this was coupled with an investigation into the influence of measurement density on the level of error in the estimation of these parameters empirical relationships are developed that can be used to estimate a priori the required minimum measurement density for a known precision and accuracy over macroscopically flat water worked gravel beds within a range of submergences these estimates can be based on measurements of the bed roughness length scale bed shear velocity and flow depth alone for all of the flow parameters the level of error in adopting a lower measurement density than the required minimum and its change with measurement density was especially large at the lower densities given this dependence on measurement density caution should be taken when comparing flow estimates from studies that have used similar flow and bed conditions but different measurement densities

recent large scale changes in the extratropical southern hemisphere atmospheric circulation abstract the zonal anomalies in the 500 hpa geopotential height field of the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalyses 1958 96 the zonal anomalies were constructed by removing the zonal mean from each grid point and principal components analysis was performed on the resulting spatial anomalies the principal components of the leading eof of the seasons march april may mam to september october november son indicate that a change has occurred in the extratropical large scale atmospheric circulation of the southern hemisphere it is estimated to have begun after 1975 and is limited to the southern late fall winter and early spring the elements of this change are a deeper pacific trough and higher indian ocean ridge at subtropical latitudes at higher latitudes the ridge over the pacific strengthened and shifted east while troughing occurred in the southwest atlantic weddell sea region consistent wi

coherent flow structures in a depth limited flow over a gravel surface the role of near bed turbulence and influence of reynolds number 1 in gravel bed rivers the microtopography of the bed exerts a significant effect on the generation of turbulent flow structures although field and laboratory measurements have indicated that flows over gravel beds contain coherent macroturbulent flow structures the origin of these phenomena and their relationship to the ensemble of individual roughness elements forming the bed is not quantitatively well understood here we report upon a flume experiment in which flow over a gravel surface is quantified through the application of digital particle imaging velocimetry which allows study of the downstream and vertical components of velocity over the entire flow field the results indicate that as the reynolds number increases 1 the visual distinctiveness of the coherent flow structures becomes more defined 2 the upstream slope of the structures increases and 3 the turbulence intensity of the structures increases analysis of the mean velocity components the turbulence intensity and the flow structure using quadrant analysis demonstrates that these large scale turbulent structures originate from flow interactions with the bed topography detection of the dominant temporal length scales through wavelet analysis enables calculation of mean separation zone lengths associated with the gravel roughness through standard scaling laws the calculated separation zone lengths demonstrate that wake flapping is a dominant mechanism in the production of large scale coherent flow structures in gravel bed rivers thus we show that coherent flow structures over gravels owe their origin to bed generated turbulence and that large scale outer layer structures are the result of flow topography interactions in the near bed region associated with wake flapping

effect of flow depth and velocity on the scales of macroturbulent structures in gravel bed rivers 1 laboratory and field data show that the length of macroturbulent structures consistently scales with flow depth y scalings with average streamwise flow velocity u only exist for laboratory data our aim is to establish the relations between macroturbulent structures low speed wedges and ejections and u and y in gravel bed rivers fifteen velocity profiles covering a wide range of u while controlling for y were measured the duration time interval between the beginning and the end of an event and length distance between two successive similar events of the macroturbulent events are analyzed strong scalings of the duration and length of the low speed wedges and ejections exist for y ejection length is strongly related to u because the sampling design ensured the statistical independence between u and y these results should provide robust estimates of the scalings of macroturbulent structures in gravel bed rivers

examining the physical components of boundary shear stress for water worked gravel deposits it is argued in this commentary that in order to understand better the physical mechanisms that generate boundary shear stress over water worked gravel beds flow velocity data should be re evaluated by spatial averaging the reynolds equations to produce time and space averaged double averaged momentum equations a series of laboratory experiments were conducted in which the flow velocities were measured using a piv system over two water worked gravel deposits combined with detailed data on the bed surface topography and vertical porosity the physical components of shear stress were obtained this enabled the various momentum transfer mechanisms present above within and at the interface of a porous fluvial deposit to be quantified this included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer it is demonstrated that double averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment water interface exchange mechanisms within the roughness layer in water worked gravel deposits by considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel sand mixtures and the transfer of solutes into porous gravel deposits this analysis reveals the importance of obtaining co located high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress copyright 2010 john wiley sons ltd

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

analyzing turbulence intensity in gravel bed channels in this paper the results of an experimental investigation of the turbulence intensity in gravel bed channels are described the runs were carried out by measuring with an acoustic doppler velocimeter the turbulence intensity profile along six verticals of a given cross section in a laboratory flume the analysis of the measured intensity distributions has shown the existence of two different regions above and below the tops of the roughness elements in which different intensity profiles occur furthermore the measured profiles have shown a maximum of the turbulence intensity that decreases for increasing values of the roughness height confirming that the turbulence damping efficiency increases when the roughness elements protrude inside the flow the applicability of nezu s relationship derived for a hydraulically smooth bed for the experimental intensity profiles above the roughness elements is positively tested finally a new intensity distribution for a rough bed applicable to the whole water depth is proposed in this profile two coefficients having a known physical meaning the maximum turbulence intensity and the depth at which this maximum is located appear