lamination of swampy rivulets rivularia haematites stromatolites in a temperate climate abstract swampy rivulets rivularia haematites d c agardh stromatolites were collected from the same site on the plateau de langres chaugey france over a seven year period specimens were studied in thin sections in culture media under varying conditions of temperature and illumination and by scanning electron microscope sem in order to investigate the processes of calcification and lamination new sem observations confirm the polycrystalline structure of the lamellae reported by the authors and show that the crystals composing these lamellae are elongate and aligned in the same direction as the filaments linked by them the lamellae were also isolated in transverse sections under sem and found to correspond to what under the light microscope appear to be honeycomb structures the diameter of alveoli can be the same as that of a false branching their walls formed by the outer sheath have a dense network of microfibrils these observations confirm the formation of calcitic lamellae at the false branching points and in contact with the microfibrils of the outer sheath confirmation of earlier hypotheses was sought through observation in thin section of the stromatolites collected over seven years micritic dark laminae are thought to form in three stages i formation of a dark lamina by formation of a false branching zone in the wet season ii initial calcification in this zone as microsparitic and sparitic lamellae during even a short dry spell and iii subsequent bacterial micritization of the lamellae during an extended warm dry season the sparitic light laminae form around the hair bearing regions lamination was first considered with regard to the annual radial growth rate of r haematites the thin section may display either a single dark lamina for two years growth or a dark lamina thicker than the annual growth rate examination of local meteorological data accounts for the first phenomenon by a wet spell of more than 13 months and the second by a warm dry period of one month it may therefore take several years for a couplet to form as growth of the cyanobacterial colony is influenced by climatic variations the different calcification phenomena were also correlated with dry spells these interpretations are consistent with results obtained by experiments

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

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

influence of sand on hydraulics and gravel transport in a braided gravel bed river measurements in a braided reach of the proglacial white river washington are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel but maximum bed load diameter and total gravel transport rate significantly higher as were transport rates of given size fractions at similar shear stresses these differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment the influence on gravel transport of local variation in bed material size may explain the self sorting of bed load sheets described by other workers however in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed which partly offsets the relative size effects on competence and capacity

comparing different methods of bed shear stress estimates in simple and complex flow fields bed shear stress is a fundamental variable in river studies to link ow conditions to sediment transport it is however dif cult to estimate this variable accurately particularly in complex ow elds this study compares shear stress estimated from the log pro le drag reynolds and turbulent kinetic energy tke approaches in a laboratory ume in a simple boundary layer over plexiglas and over sand and in a complex ow eld around de ectors results show that in a simple boundary layer the log pro le estimate is always the highest over plexiglas the tke estimate was the second largest with a value 30 per cent less than the log estimate however over sand the tke estimate did not show the expected increase in shear stress in a simple boundary layer the reynolds shear stress seems the most appropriate method particularly the extrapolated value at the bed obtained from a turbulent pro le in a complex ow eld around de ectors the tke method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent uctuations close to the de ectors it is suggested that when single point measurements are used to estimate shear stress the instrument should be positioned close to 0 1 of the ow depth which corresponds to the peak value height in pro les of reynolds and tke shear stress copyright 2004 john wiley sons ltd

estimation of runoff critical shear stress for soil erosion from soil shear strength abstract critical shear stress is an important soil parameter governing detachment by runoff which appears in numerous erosion models soil shear strength has been often presented as the best soil property to predict critical shear stress however only a few studies have documented the relationship between soil shear strength and critical shear stress and the results obtained have sometimes seemed contradictory this is why we have put together all available data about the relationship between critical shear stress and soil shear strength in order to analyse and discuss its significance and its generality results showed that contradictions between different studies mainly arise because total shear stress is a poor predictor of detachment on rough soils when grain shear stress is used rather than total shear stress a significant relationship between saturated soil shear strength and critical shear stress does exist soil shear strength may thus be used to predict critical grain shear stress however it is recalled that in general only total shear stress is predicted by erosion models which makes detachment predictions difficult on rough natural soils

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

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

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

a traversing system to measure bottom boundary layer hydraulic properties this study describes a new convenient and robust system developed to measure benthic boundary layer properties with emphasis placed on the determination of bed shear stress and roughness height distribution within estuarine systems by using velocity measurements this system consisted of a remotely operated motorised traverser that allowed a single adv to collect data between 0 and 1 m above the bed as a case study we applied the proposed traversing system to investigate bottom boundary layer bbl hydraulic properties within coombabah creek queensland australia four commonly employed techniques 1 log profile lp 2 reynolds stress rs 3 turbulent kinetic energy tke and 4 inertial dissipation id used to estimate bed shear stresses from velocity measurements were compared bed shear stresses estimated with these four methods agreed reasonably well of these the lp method was found to be most useful and reliable additionally the lp method permits the calculation of roughness height which the other three methods do not an average value of bed shear stress of 0 46 n m2 roughness height of 4 3 mm and drag coefficient of 0 0054 were observed within coombabah creek results are consistent with that reported for several other silty bed estuaries

changes in velocity profiles at roughness transitions in coarse grained channels heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness distinct spatial zones in terms of grain size have been reported for example sand ribbons and bedload sheets the transition from areas of finer to coarser grained surface sediment is often abrupt however the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels this paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistance on the structure of the turbulent boundary layer measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface boundary shear stress and roughness length increase considerably downstream from this transition velocity profiles become concave upwards downstream and upstream sections show significant differences in terms of near bed velocities deceleration downstream of the transition velocity gradient and turbulence intensity of the streamwise velocity component comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity or shear stress associated with the pressure drag produced by large and isolated obstacles

measurements of turbulence in the benthic boundary layer over a gravel bed it is now generally recognized that the mathematical description of sediment dynamics will be improved through an understanding of geophysical turbulence in this study therefore the turbulence characteristics of the benthic boundary layer over coarse sediments are examined as a step towards the realization of such a description examination of the turbulence has been conducted using spectral correlation quadrant and statistical analyses and a number of features have been identified mean eddy length scales obtained using spectral and autocorrelation techniques were found to be similar the interpretation of the duration and interval between the intermittent bursting events scaled using outer flow variables was found to be dependent upon the sampling interval autocorrelation analysis revealed a high correlation between bedload transport and the instantaneous horizontal flow component rather than instantaneous kinematic stress from the standpoint of developing prognostic transport models based on physically realistic benthic flow processes it is considered that turbulence measurements will lead to such formulation

can the discharge of a hyperconcentrated flow be estimated from paleoflood evidence 1 many ood events involving water and sediments have been characterized using classic hydraulics principles assuming the existence of critical ow and many other simplications in this paper hyperconcentrated ow discharge was evaluated by using paleo ood reconstructions based on paleostage indicators psi combined with a detailed hydraulic analysis of the critical ow assumption the exact location where this condition occurred was established by iteratively determining the corresponding cross section so that specic energy is at a minimum in addition all of the factors and parameters involved in the process were assessed especially those related to the momentum equation existing shear stresses in the wetted perimeter and nonhydrostatic and hydrostatic pressure distributions the superelevation of the hyperconcentrated ow due to the ow elevation curvature was also estimated and calibrated with the psi the estimated peak discharge was established once the iterative process was unable to improve thet between the simulated depth and the depth observed from the psi the methodological approach proposed here can be applied to other higher gradient mountainous torrents with a similar geomorphic conguration to the one studied in this paper likewise results have been derived with fewer uncertainties than those obtained from standard hydraulic approaches whose simplifying assumptions have not been considered

estimating shear stress from moving boat acoustic doppler velocity measurements in a large gravel bed river 1 moving boat acoustic doppler current profiling adcp is increasingly used to measure discharge in large rivers we investigate whether useful information about bed shear stress can be recovered from such data alternative ways to estimate local bed shear stress using the logarithmic law of the wall and spatial averaging are tested using adcp transects across lower fraser river canada repeatability is assessed by comparing estimates from outward and return boat tracks the most precise method uses the vertically averaged mean velocity and a zero velocity height based on bed grain size information the accuracy of the assumed zero velocity height can be judged by consistency between estimates using mean velocity and near bed velocity shear stress estimates from unconstrained log law fits are less repeatable and tend to overpredict and mean shear stress estimates using the depth slope product are unreliable in this river because of nonuniform flow

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

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

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

great differences in the critical erosion threshold between surface and subsurface sediments a field investigation of an intertidal mudflat jiangsu china abstract understanding of bottom sediment erodibility is necessary for the sustainable management and protection of coastlines and is of great importance for numerical models of sediment dynamics and transport to investigate the dependence of sediment erodibility on degree of consolidation we measured turbidity waves tidal currents intratidal bed level changes and sediment properties on an exposed macrotidal mudflat during a series of tidal cycles we estimated the water content of surface sediments in the uppermost 2 cm of sediment and sub surface sediments at 2 cm below the sediment surface bed shear stress values due to currents c waves w and combined current wave action cw were calculated using a hydrodynamic model in this study we estimate the critical shear stress for erosion using two approaches and both of them give similar results we found that the critical shear stress for erosion ce was 0 17 0 18 n m2 in the uppermost 0 2 cm of sediment and 0 29 n m2 in sub surface sediment layers depth 2 cm as determined by time series of cw values and intratidal bed level changes and values of ce obtained using the water content of bottom sediments were 0 16 n m2 in the uppermost 2 cm and 0 28 n m2 in the sub surface depth 2 cm sediment these results indicate that the value of ce for sub surface sediments depth 2 cm is much greater than that for the uppermost sediments depth 0 2 cm and that the ce value is mainly related to the water content which is determined by the extent of consolidation our results have implications for improving the predictive accuracy of models of sediment transport and morphological evolution by introducing variable ce values for corresponding sediment layers and can also provide a mechanistic understanding of bottom sediment erodibility at different sediment depths on intertidal mudflats as related to differences in the consolidation time

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

continuous measurements of discharge from a horizontal acoustic doppler current profiler in a tidal river 1 acoustic doppler current profilers adcps can be mounted horizontally at a river bank yielding single depth horizontal array observations of velocity across the river this paper presents a semideterministic semistochastic method to obtain continuous measurements of discharge from horizontal adcp hadcp data in a tidal river in the deterministic part single depth velocity data are converted to specific discharge by applying the law of the wall which requires knowledge of local values of the bed roughness length z0 a new filtration technique was developed to infer cross river profiles of z0 from moving boat adcp measurements width averaged values of z0 were shown to be predominantly constant in time but differed between ebb and flood in the stochastic part of the method specific discharge was converted to total discharge on the basis of a model that accounts for the time lag between flow variation in the central part of the river and flow variation near the banks model coefficients were derived using moving boat adcp data the consistency of mutually independent discharge estimates from hadcp measurements was investigated to validate the method analyzing river discharge and tidal discharge separately inaccuracy of the method is attributed primarily to mechanisms controlling transverse exchange of momentum which produce temporal variation in the discharge distribution over the cross section specifically development of river dunes may influence the portion of the discharge concentrated within the range of the hadcp

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

evaluation of grain shear stresses required to initiate movement of particles in natural rivers shear stresses were evaluated at different sites on two rivers the first the rulles is characterized by a pebbly bedload and a meandering bed with riffles and pools the second the rouge eau has mainly a sandy rippled bed where meandering is well developed but also flat gravelly sectors without meandering system shear stresses calculated from friction velocities using a redefined y1 roughness height parameter were compared with total shear stresses calculated from the energy grade line and the hydraulic radius divergence between these shear stresses seems to increase in the presence of bedforms and large scale irregularities of the channel the ratio is close to 0 5 in the gravelly sector of the rouge eau and reaches 0 65 in the riffles of the rulles generally located at the inflexion point of the meanders while it is less than 0 3 in the pools of the same river located in the loops and only 0 2 in the sandy rippled sector of the rouge eau grain and bedform shear stresses were evaluated at these same sites by different methods the grain shear stress represents on average 30 per cent of the total shear stress in the riffles of the rulles and the gravelly sector of the rouge eau but less than 15 per cent in the pools in the rulles and the sandy sectors of the rouge eau however it emerges from experiments conducted with marked pebbles and in situ observations of erosion and transport of sandy and gravelly particles that the grain shear stresses are underestimated and cannot explain the movements and modifications actually observed conversely shear stresses calculated from friction velocities at the sites where erosion actually occurred or failed to occur despite very high velocities provide a better explanation of the observed movements

bed disturbance via foraging fish increases bedload transport during subsequent high flows and is controlled by fish size and species abstract benthic foraging by fish can modify the nature and rates of fine sediment accrual and the structure and topography of coarse grained fluvial substrates with the potential to alter bed material characteristics particle entrainment thresholds and bedload transport fluxes however knowledge of what controls the nature extent and intensity of benthic foraging and the consequent influence of these controls on geomorphic impact remain rudimentary an ex situ experiment utilising barbel barbus barbus and chub leuciscus cephalus extended previous work by considering the role of fish size and species as controls of sediment disturbance by foraging and the implications for bed material characteristics and bedload transport in a laboratory flume changes in bed microtopography and structure were measured when a water worked bed of 5 6 22 6 mm gravels was exposed to four size classes of barbel 4 5 5 6 6 8 8 10 in length and a single size class of chub 8 10 in line with other studies that have investigated animal size as a control of zoogeomorphic agency increasing the size of barbel had a significant effect on measured disturbance and transport metrics specifically the area of disturbed substrate foraging depth and the fish s impact on microtopographic roughness and imbrication all increased as a function of fish size in a comparison of the foraging effects of like sized barbel and chub 8 10 in length barbel foraged a larger area of the test bed and had a greater impact on microtopographic roughness and sediment structure relative to water worked beds that were not foraged bed conditioning by both species was associated with increased bedload transport during the subsequent application of high flows however the bedload flux after foraging by barbel which is a specialist benthivore was 150 higher than that following foraging by chub which feed opportunistically from the bed and the total transported mass of sediment was 98 greater an interesting implication of these results given the abundance and widespread distribution of foraging fish is that numerous fish species belonging to a variety of functional groups may be acting as zoogeomorphic agents in rivers directly affecting bed material conditions and sediment transport fluxes in proportion to their body size and feeding traits

the influence of the water saturation on the strength of volcanic tuffs used as building stones moisture content is one of the most important factors influencing the strength of building stones ayazini and seydiler afyonkarahisar turkey tuffs were investigated with the goal to determine the influence of the water on their strength mercury porosimetry was used to determine the pore size distribution ayazini tuffs have pore sizes ranging from about 200 000 to 10 nm and seydiler tuffs ranging from about 7000 to 10 nm the tuff samples are immersed in water with different time periods of 1 6 24 and 48 h the following physico mechanical properties were measured for all the samples both in dry and saturated condition compressive strength and flexural strength test results were indicated by simple regression analysis good correlations were found between physico mechanical properties with dry and saturated tuff samples the results had shown their mechanical strength decreases in wet conditions for dry samples tested had 22 21 mpa uniaxial compression strength while 1 h immersed samples had 15 11 mpa which indicates a decrease of 31 96 after the 48 h water immersion time period the value of compression strength decreases by 43 99 and becomes 12 44 mpa

increase in coarse sediment transport associated with disturbance of gravel river beds by signal crayfish pacifastacus leniusculus there is growing acknowledgement of the interaction between animals and the river bed on which they live and the implications of biological activity for geomorphic processes it has been observed that signal crayfish pacifastacus leniusculus disturb gravel substrates potentially promoting sediment transport and impacting ecological communities however the mechanisms involved and the extent of their impact remain poorly understood especially in relation to other processes that affect grain mobility in gravel bed rivers a series of flume experiments using loose and water worked gravel beds of narrowly graded grain sizes that were exposed to 6 h of crayfish activity under low velocity flows showed a substantial increase in the number of grains entrained by subsequent higher velocity flows when compared with control runs in which crayfish were never introduced crayfish alter the topography of their substrate by constructing pits and mounds which affect grain protrusion when walking and foraging they also alter gravel fabric by reorienting and changing the friction angle of surface grains in water worked surfaces this fabric rearrangement is shown to lead to a statistically significant partial reversal of the structuring that had been achieved by antecedent flow for these previously water worked surfaces the increase in entrainment arising from disturbance by crayfish was statistically significant with grain transport nearly twice as great this suggests that signal crayfish an increasingly widespread invasive species in temperate latitudes beyond their native nw north america have the potential to enhance coarse grained bedload flux by altering the surface structure of gravel river beds and reducing the stability of surface grains this study illustrates further the importance of acknowledging the impact of mobile organisms in conditioning the river bed when assessing sediment entrainment mechanics in the context of predicting bedload flux

discharge estimation from h adcp measurements in a tidal river subject to sidewall effects and a mobile bed horizontal acoustic doppler current profilers h adcps can be employed to estimate river discharge based on water level measurements and flow velocity array data across a river transect a new method is presented that accounts for the dip in velocity near the water surface which is caused by sidewall effects that decrease with the width to depth ratio of a channel a boundary layer model is introduced to convert single depth velocity data from the h adcp to specific discharge the parameters of the model include the local roughness length and a dip correction factor which accounts for the sidewall effects a regression model is employed to translate specific discharge to total discharge the method was tested in the river mahakam representing a large river of complex bathymetry where part of the flow is intrinsically three dimensional and discharge rates exceed 8000 m3 s 1 results from five moving boat adcp campaigns covering separate semidiurnal tidal cycles are presented three of which are used for calibration purposes whereas the remaining two served for validation of the method the dip correction factor showed a significant correlation with distance to the wall and bears a strong relation to secondary currents the sidewall effects appeared to remain relatively constant throughout the tidal cycles under study bed roughness length is estimated at periods of maximum velocity showing more variation at subtidal than at intratidal time scales intratidal variations were particularly obvious during bidirectional flow conditions which occurred only during conditions of low river discharge the new method was shown to outperform the widely used index velocity method by systematically reducing the relative error in the discharge estimates

measurements in a braided river chute and lobe 1 flow pattern sediment transport and channel change this paper and its companion ashworth et al this issue discuss measurements of channel change and associated flow and sediment transport processes in a representative chute and bar reach within a proglacial gravel bed river the sunwapta river in the canadian rockies during a week in which water discharge through the reach increased then decreased a sediment wave passed into and partly along the reach at first the chute aggraded then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries measurements of velocity shear stress and gravel transport rate revealed day to day changes in the divergent pattern of flow and sediment transport the off center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport but as the lobe grew the flow and transport gradually switched away to the other distributary width averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations by the end of the study the new lobe had almost accreted onto the original bar head supporting the idea that most braid bars are of compound origin temporal and spatial patterns in grain size distributions of the bed bed load and deposited sediment are discussed in the companion paper

variable and conflicting shear stress estimates inside a boundary layer with sediment transport this paper presents a comparison between two methods for estimating shear stress in an atmospheric internal boundary layer over a beach surface under optimum conditions using wind velocities measured synchronously at 13 heights over a 1 7 m vertical array using ultrasonic anemometry the reynolds decomposition technique determines at a point shear stresses at each measurement height while the law of the wall yields a single boundary layer estimate based on fitting a logarithmic velocity profile through the array data analysis reveals significant inconsistencies between estimates derived from the two methods on both a whole event basis and as time series despite a near perfect fit of the law of the wall the point estimates of reynolds shear stress vary greatly between heights calling into question the assumed presence of a constant stress layer a comparison with simultaneously measured sediment transport finds no relationship between transport activity and the discrepancies in shear stress estimates results do show however that reynolds shear stress measured nearer the bed exhibits slightly better correlation with sand transport rate the findings serve as a major cautionary message to the interpretation and application of single height measurements of reynolds shear stress and their equivalence to law of the wall derived estimates and these concerns apply widely to boundary layer flows in general 2015 the authors earth surface processes and landforms published by john wiley sons ltd

error propagation for velocity and shear stress prediction using 2d models for environmental management pasternack g b gilbert a t wheaton j m buckland e m 2006 error propagation for velocity and shear stress prediction using 2d models for environmental management journal of hydrology 328 1 2 227 241 sponsorship us fish and wildlife service contracting entity for calfed bay delta ecosystem restoration program cooperative agreement dcn 113322g003