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

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

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

northern monterey bay upwelling shadow front observations of a coastally and surface trapped buoyant plume 1 during the upwelling season in central california northwesterly winds along the coast produce a strong upwelling jet that originates at point ano nuevo and flows southward across the mouth of monterey bay a convergent front with a mean temperature change of 3 77 0 29c develops between the warm interior waters and the cold offshore upwelling jet to examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern monterey bay oceanographic conditions were monitored using cross shelf mooring arrays drifters and hydrographic surveys along a 20 km stretch of coast extending northwestward from santa cruz california during the upwelling season of 2007 may september the alongshore location of the upwelling shadow front at the northern edge of the bay was driven by regional wind forcing through an alongshore pressure gradient buoyancy forces due to the temperature change across the front and local wind forcing the diurnal sea breeze the upwelling shadow front behaved as a surface trapped buoyant current which is superimposed on a poleward barotropic current moving up and down the coast up to several kilometers each day we surmise that the front is advected poleward by a preexisting northward barotropic current of 0 10 m s 1 that arises due to an alongshore pressure gradient caused by focused upwelling at point ano nuevo the frontal circulation onshore surface currents breaks the typical two dimensional wind driven cross shelf circulation offshore surface currents and introduces another way for water and the material it contains e g pollutants larvae to go across the shelf toward shore

formation and maintenance of a forced pool riffle couplet following loading of large wood abstract pool riffle maintenance has been documented in numerous studies but it has been almost impossible to characterize detailed natural pool riffle formation mechanisms because of the lack of baseline data prior to pool establishment in 2013 a study was conducted on the blackledge river in connecticut to document the formation of a new pool riffle couplet on a section of river that had previously been studied from 1999 to 2001 in 2001 the study reach contained a scour hole with a residual depth of 0 08 0 09 m downstream of a 1930s paired deflector with no identifiable riffle immediately downstream at this time a large severely undercut hemlock tree was noted along the left bank sometime between fall 2001 and 2004 the tree fell perpendicular to flow across the channel and formed a large wood lw jam and new pool riffle couplet several meters downstream of the old scour hole pool spacing along the reach decreased from 4 47 bankfull widths bfw in 1999 to 3 83 bfw after the new pool riffle couplet formed the new pool has a residual depth the water depth of the streambed depression below the elevation of the immediate downstream hydraulic control of 1 36 0 075 to 1 59 0 075 m which resulted from a combination of 1 32 0 09 m or less of incision below the old scour hole 95 6 or less of the depth increase and up to 0 18 0 09 m of downstream deposition and associated backwater formation 13 2 or less of the depth increase to assess dynamic stability of the pool riffle couplet over several flood cycles surficial fine sediment and organic material along the reach were quantified the 23 m long pool stores 25 7 of the surficial fine grained sediments and 15 4 of organic material along a 214 m long reach that includes one additional artificially created pool an adjacent 50 m long secondary channel impacted by the lw jam stores 65 3 of the surficial fine grained sediments and 54 8 of organic material along the full reach

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

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

the characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse grained forced pool a field based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit slope on the strength of turbulence generated by vortex shedding velocities were measured with an adv downstream of a boulder constriction within a shear zone of a large pool measurements were repeated for four flows that varied from 20 to 50 of bankfull discharge an additional longitudinal profile was conducted along the pool thalweg plots of velocity demonstrate high near bed velocities and turbulence levels in the pools turbulence levels were highest closer to the constriction in the shear zone turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges instantaneous velocities are large enough to temporarily lift pebbles at this depth it appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions the turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography characteristics of turbulence and sediment transport

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

flow and sediment dynamics in channel confluences confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large low gradient main channel commonly occur in nature but they have not yet been investigated measurements of the three dimensional velocity field turbulence sediment transport bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the upper rhone river switzerland the difference between the low flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone due to this bed discordance the tributary flow penetrates into the main channel mainly in the upper part of the water column whereas the main channel flow is hardly hindered by the tributary in the lower part of the water column giving rise to a two layer flow structure in the confluence zone in confluences with high supply of coarse sediment from the tributary the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity the sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near bed flow originating from the main channel the sediment transport capacity is further increased by the three dimensionality of the flow which is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary a conceptual model is proposed for the hydro morpho sedimentary processes and compared to existing conceptual models for confluences with different characteristics

water velocity and the nature of critical flow in large rapids on the colorado river utah 1 rapids are an integral part of bedrock controlled rivers influencing aquatic ecology geomorphology and recreational value flow measurements in rapids and high gradient rivers are uncommon because of technical difficulties associated with positioning and operating sufficiently robust instruments in the current study detailed velocity water surface and bathymetric data were collected within rapids on the colorado river in eastern utah with the water surface survey it was found that shoreline based water surface surveys may misrepresent the water surface slope along the centerline of a rapid flow velocities were measured with an adcp and an electronic pitot static tube integrating multiple measurements the adcp returned velocity data from the entire water column even in sections of high water velocity the maximum mean velocity measured with the adcp was 3 7 m s the pitot static tube while capable of only point measurements quantified velocity 0 39 m below the surface the maximum mean velocity measured with the pitot tube was 5 2 m s with instantaneous velocities up to 6 5 m s analysis of the data showed that flow was subcritical throughout all measured rapids with a maximum measured froude number of 0 7 in the largest measured rapids froude numbers were highest at the entrance of a given rapid then decreased below the first breaking waves in the absence of detailed bathymetric and velocity data the froude number in the fastest flowing section of a rapid was estimated from near surface velocity and depth soundings alone

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

dynamics of a river channel confluence with discordant beds flow turbulence bed load sediment transport and bed morphology 1 river channel confluences play a major role in the dynamics of all fluvial systems and yet our understanding of bed load routing at these sites is very sparse the dynamics of confluences are a function of the momentum ratio between the combining flows and the three dimensional geometry of the junction recent experiments have shown that discordance in bed height between the confluent rivers increases turbulence intensity and enhances upwelling of flow within the confluence however the significance of these flow characteristics on sediment transport is still unknown to examine the relations between flow and sediment transport we have measured near bed flow turbulence bed load transport rates and changes in bed morphology for eight different flow conditions at a sand bed discordant confluence detailed analysis of the near bed flow patterns reveals that within the shear layer low mean flow velocities are combined with the highest values of reynolds shear stresses and that turbulence generation is associated with intense upward movements of flow high sediment transport rates are found at the edges of the shear layer region where horizontal vertical cross stresses uw are high these patterns match changes in bed morphology where erosion occurs along the shear layer the relation between the shear layer and sediment transport confirms the role of bed discordance on the dynamics of the confluence migration of the shear layer into the confluence as a result of a change in momentum ratio modifies local near bed flow characteristics sediment transport rates and the spatial distribution of deposition and erosion zones

hydraulic conditions in experimental rill confluences and scour in erodible soils 1 few studies have examined processes active within rill networks or the effect of confluence geometry on rill sediment flux and it is not clear if observations from river confluences also apply at rill scale laboratory experiments were carried out in artificial rectangular rill channels with fixed and erodible beds to identify the effects of confluence geometry on hydraulic conditions and scour patterns symmetrical and asymmetrical confluences with angles from 19 to 90 were used hydraulic changes in the confluence zone in fixed bed experiments included decreased flow velocity increased shear velocity transition from supercritical to subcritical and from transitional to turbulent flow bed scour in the confluence zone resulted generally increasing with confluence angle but a more important influence was junction symmetry or asymmetry symmetrical confluences produced a symmetrical scour pattern mirroring the original channels but in asymmetrical confluences scour was more complex evolving to a symmetrical pattern with confluence angles higher than the original system results are consistent with channel evolution to an optimally branched system controlled by minimum power criteria but validation requires testing with more precise velocity measurement

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

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

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

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

canadian geomorphology 2000 geomorphology has a distinguished history in canada as old as the country itself initially the science of geomorphology of canada drew on rich british and french scientific traditions but later it developed its own character strongly influenced by the richness and diversity of the canadian landscape the strong imprint of glaciation and the importance of periglacial processes permafrost and sea ice canada is the second largest country in the world its coastline is longer and more varied than that of any other country nearly one third of canada is underlain by permafrost some up to 700 m thick the country s physiography ranges from nearly flat coastal and interior plains to the glacierised mountains of baffin and ellesmere islands british columbia and the yukon its climate and vegetation are equally diverse ranging from the cold semiarid tundra of the high arctic to the temperate rainforest of the pacific coast given this diversity it is not surprising that canada is active in many areas of geomorphic research an incredible spectrum of landforms and geomorphic processes has fostered world class research in canada both by canadians and by scientists from other countries much of this research is field based and empirical rooted in field observation in recent decades however there have been important advances in inductive laboratory and

reach scale variability of turbulent flow characteristics in a gravel bed river in gravel bed rivers bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts the turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale in this paper we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale the experiment was carried out in two reaches of 25 m long in the eaton north river quebec we produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements the flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod we collected 65 velocity profiles from which we derived turbulent flow characteristics results showed that in spite of the complex configuration of the bed velocity profiles are predominantly log linear mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements at the reach scale the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties

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

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

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

modelling aeolian sand transport using a dynamic mass balancing approach abstract knowledge of the changing rate of sediment flux in space and time is essential for quantifying surface erosion and deposition in desert landscapes whilst many aeolian studies have relied on time averaged parameters such as wind velocity u and wind shear velocity u to determine sediment flux there is increasing field evidence that high frequency turbulence is an important driving force behind the entrainment and transport of sand at this scale of analysis inertia in the saltation system causes changes in sediment transport to lag behind de accelerations in flow however saltation inertia has yet to be incorporated into a functional sand transport model that can be used for predictive purposes in this study we present a new transport model that dynamically balances the sand mass being transported in the wind flow the dynamic mass balance dmb model we present accounts for high frequency variations in the horizontal u component of wind flow as saltation is most strongly associated with the positive u component of the wind the performance of the dmb model is tested by fitting it to two field derived namibia s skeleton coast datasets of wind velocity and sediment transport i a 10 min 10 hz measurement resolution dataset ii a 2 h 1 hz measurement resolution dataset the dmb model is shown to outperform two existing models that rely on time averaged wind velocity data e g radok 1977 dong et al 2003 when predicting sand transport over the two experiments for all measurement averaging intervals presented in this study 10 hz 10 min the dmb model predicted total saltation count to within at least 0 48 whereas the radok and dong models over or underestimated total count by up to 5 50 and 20 53 respectively the dmb model also produced more realistic less peaky time series of sand flux than the other two models and a more accurate distribution of sand flux data the best predictions of total sand transport are achieved using our dmb model at a temporal resolution of 4 s in cases where the temporal scale of investigation is relatively short on the order of minutes and at a resolution of 1 min for longer wind and transport datasets on the order of hours the proposed new sand transport model could prove to be significant for integrating turbulence scale transport processes into longer term macro scale landscape modelling of drylands

the influence of pool length on local turbulence production and energy slope a flume experiment the in uence of pool length on the strength of turbulence generated by vortex shedding was investigated in a 6 m long recirculating ume the experiment utilized a 38 constriction of ow and an average channel bed slope of 0 007 the base geometry for the intermediate length pool experiment originated from a highly simpli ed 0 10 scale model of a forced pool from north saint vrain creek colorado discharge in the ume was 31 6 l s which corresponds to a discharge in the prototype channel of 10 m3 s three shorter and four longer pool lengths also were created with a xed bed to determine changes in turbulence intensities and energy slope with pool elongation three dimensional velocities were measured with an acoustic doppler velocimeter at 31 40 different 0 6 depth and near bed locations downstream of the rectangular constriction the average velocity and root mean square rms of the absolute magnitude of velocity at both depths are signi cantly related to the distance from the constriction in most pool locations downstream of the constriction in many locations pool elongation results in a non linear change in turbulence intensities and average velocity based on the overall ow pattern the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy the lateral location of this shear zone is sensitive to changes in pool length energy slope also was sensitive to pool length due to a combination of greater length of the pool and greater head loss with shorter pools the results indicate some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools and lower their rate of energy dissipation copyright 2004 john wiley sons ltd

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

3 d flow and scour near a submerged wing dike adcp measurements on the missouri river 1 detailed mapping of bathymetry and three dimensional water velocities using a boat mounted single beam sonar and acoustic doppler current profiler adcp was carried out in the vicinity of two submerged wing dikes located in the lower missouri river near columbia missouri during high spring flows the wing dikes become submerged creating a unique combination of vertical flow separation and overtopping plunging flow conditions causing large scale three dimensional turbulent flow structures to form on three different days and for a range of discharges sampling transects at 5 and 20 m spacing were completed covering the area adjacent to and upstream and downstream from two different wing dikes the objectives of this research are to evaluate whether an adcp can identify and measure large scale flow features such as recirculating flow and vortex shedding that develop in the vicinity of a submerged wing dike and whether or not moving boat single transect data are sufficient for resolving complex three dimensional flow fields results indicate that spatial averaging from multiple nearby single transects may be more representative of an inherently complex temporally and spatially variable three dimensional flow field than repeated single transects results also indicate a correspondence between the location of calculated vortex cores resolved from the interpolated three dimensional flow field and the nearby scour holes providing new insight into the connections between vertically oriented coherent structures and local scour with the unique perspective of flow and morphology in a large river

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

three dimensional numerical modeling of mixing at river confluences lateral mixing of a pollutant is considered as a slow process that is usually complete within 100 300 river widths recent studies on flow dynamics at river confluences revealed that lateral mixing can be markedly enhanced when the tributary channel is shallower than the main channel this study uses a three dimensional model to examine mixing processes immediately downstream of confluences as well as further downstream in the mainstream simulations are presented for a concordant and discordant laboratory junction and a field confluence for a low and a high flow condition the decrease in standard deviation at a cross section of a tracer over a distance of 5 channel widths is 30 for discordant beds but only 10 for concordant beds in the laboratory simulation at the natural site bed discordance is more important at the low flow than at the high flow with corresponding decreases in the standard deviation of 31 and 18 over 3 5 channel widths mixing is completed after a distance of 25 and 37 channel widths for the low and high flow conditions respectively further downstream mixing is mainly affected by planform curvature of the channel

riparian reforestation and channel change how long does it take repeated measurements of two small streams in northeastern vermont document change in channel width and suggest variable rates of widening because of passive reforestation over four decades historic data on channel width are available for several tributaries to sleepers river in danville vt usa from the 1960s in 2004 and 2008 we re measured channel dimensions in two of these tributaries in two reaches of upper pope brook and along seven reaches of an unnamed tributary w12 four reaches had reforested since 1966 two reaches remained nonforested the other three reaches have been forested since at least the 1940s comparisons between 1966 and 2004 showed that reforested reaches widened significantly and comparisons between 2004 and 2008 showed continued widening but at a greater rate between 1966 and 2004 reforested reaches widened at an average rate of 4 1 cm year while the rate more than doubled for the last four years 8 7 cm year additionally turbulence data collected during five peak flows in the spring of 2005 showed significantly greater turbulent kinetic energy tke in the reforested reach than in either the forested or nonforested reach our data add supporting information to the conceptual model of stream w12 that describes a process of incision widening and recovery of a stream reach transitioning from nonforested to forested riparian vegetation

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