turbulence structure over two dimensional bed forms implications for sediment transport turbulence measurements from the flow over two dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms the spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near bed flow field the measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components unlike measurements in uniform boundary layers these distributions and the higher moments of the velocity do not scale with the local shear velocity indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux this conclusion is supported by observations of sediment flux over a dune

recent and pleistocene beach dune sequences western australia abstract wave dominated sandy shores occur along much of the coast of western australia despite local variations there is a characteristic distribution of lithofacies corresponding to different geomorphic zones five lithofacies are recognised 1 trough bedded sand gravel 2 laminated sand 3 laminated bubble sand 4 laminated disrupted sand and 5 aeolian cross stratified sand the trough bedded sand gravel lithofacies is being deposited in the shallow shoreface below lwl the laminated sand and laminated bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone extensive bubble or vesicular sand is common towards hwl especially in berms the laminated disrupted sand lithofacies is being deposited on the backshore between hwl and storm water levels and consists of horizontally layered to homogeneous sands with storm debris especially wood weed and floatable skeletons e g sepla and spirula the aeolian cross stratified sand lithofacies is forming in beach ridge dune areas and consists of fine sands with large scale generally landward dipping forests soils and rootlets are common recognition of these lithofacies within a sedimentary sequence enables reconstruction of gross shoreline conditions in terms of wave and eolian environments tidal and storm heights and palaeogeography each of these lithofacies with their characteristic features is recognised in pleistocene sequences in perth basin the pleistocene sequences fit a model of coastal progradation with the trough bedded sand gravel lithofacies at the base and the aeolian sand lithofacies at the top the value of such a stratigraphic sequence however extends beyond the pleistocene

direct measurements of lift and drag on shallowly submerged cobbles in steep streams implications for flow resistance and sediment transport steep mountain streams have higher resistance to flow and lower sediment transport rates than expected by comparison with low gradient rivers and often these differences are attributed to reduced near bed flow velocities and stresses associated with form drag on channel forms and immobile boulders however few studies have directly measured drag and lift forces acting on bed sediment for shallow flows over coarse sediment which ultimately control sediment transport rates and grain scale flow resistance here we report on particle lift and drag force measurements in flume experiments using a planar fixed cobble bed over a wide range of channel slopes 0 004 s 0 3 and water discharges drag coefficients are similar to previous findings for submerged particles c d 0 7 but increase significantly for partially submerged particles in contrast lift coefficients decrease from near unity to zero as the flow shallows and are strongly negative for partially submerged particles indicating a downward force that pulls particles toward the bed fluctuating forces in lift and drag decrease with increasing relative roughness and they scale with the depth averaged velocity squared rather than the bed shear stress we find that even in the absence of complex bed topography shallow flows over coarse sediment are characterized by high flow resistance because of grain drag within a roughness layer that occupies a significant fraction of the total flow depth and by heightened critical shields numbers and reduced sediment fluxes because of reduced lift forces and reduced turbulent fluctuations

a new methodology for the quantitative visualization of coherent flow structures in alluvial channels using multibeam echo sounding mbes 1 in order to investigate the interactions between turbulence and suspended sediment transport in natural aqueous environments we ideally require a technique that allows simultaneous measurement of fluid velocity and sediment concentration for the whole flow field here we report on development of a methodology using the water column acoustic backscatter signal from a multibeam echo sounder to simultaneously quantify flow velocities and sediment concentrations the application of this new technique is illustrated with reference to flow over the leeside of an alluvial sand dune which allows for the first time in a field study quantitative visualization of large scale whole flow field turbulent coherent flow structures associated with the dune leeside that are responsible for suspending bed sediment this methodology holds great potential for use in a wide range of aqueous geophysical flows

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

physically based probabilistic analysis of sediment deposition in open channel flow abstractthis paper proposes a physically based probabilistic model for predicting the longitudinal distance of sediment deposition in an open channel flow numerical simulations were performed usin

real and apparent changes in sediment deposition rates through time 1 field measurements show that estimated sediment deposition rate decreases as a power law function of the measurement interval this apparent decrease in sediment deposition has been attributed to completeness of the sedimentary record the effect arises because of incorporation of longer hiatuses in deposition as averaging time is increased we demonstrate that a heavy tailed distribution of periods of nondeposition hiatuses produces this phenomenon and that observed accumulation rate decreases as t 1 over multiple orders of magnitude where 0 1 is the parameter describing the tail of the distribution of quiescent period length by using continuous time random walks and limit theory we can estimate the actual average deposition rate from observations of the surface location over time if geologic and geometric constraints place an upper limit on the length of hiatuses then average accumulation rates approach a constant value at very long times our model suggests an alternative explanation for the apparent increase in global sediment accumulation rates over the last 5 million years

a unified model for subaqueous bed form dynamics 1 bed form evolution remains dynamic even in the special case of steady uniform flow data from the sandy braided north loup river nebraska show that roughness features on the channel bottom display a statistical steady state and robust scaling that are maintained through the collective interactions of transient short lived bed forms motivated by such field data and laboratory observations of bed form growth we develop a nonlinear stochastic surface evolution model for the topography of bed load dominated sandy rivers in which instantaneous sediment flux explicitly depends on local elevation and slope this model quantitatively reproduces laboratory observations of initial growth and saturation of bed forms from a flat surface and also generates long term dynamical behavior characteristic of natural systems we argue that the variability in geometry and kinematics of bed forms in steady flow and the existence of roughness at all wavelengths up to the largest dunes are a consequence of the nonlinear relationship between sediment flux and topography subject to noise

holocene coastal dune formation in southern australia a case study abstract the 14 500 km long southern australian coast is exposed to high energy wave and wind regimes which have mobilized shelf derived sediments to produce extensive holocene shoreface deposits the 8000 km of sandy coast are backed by over 2000 km 2 of holocene dunes of varying degrees of stability in places dune deposition above present sea level accompanied the post glacial marine transgression commencing as early as 10 ka b p and has continued episodically to the present day the dunes are widespread in extent but vary in type size and evolution the oldest and often most extensive primary dune fields accompanied the post glacial marine transgression they occurred and in some places continue to form in areas of high wave and wind energy and abundant sand supply many of these systems exhausted their sediment supply by mid holocene and remain as stranded clifftop dunes where sand ramps were eroded or as eroding barriers secondary phases of dune activity occurred on moderate to high energy coasts following an initial period of post stillstand barrier progradation diminished sediment supply resulted in shoreline erosion and secondary dune formation the location size and evolution both of dunes types is largely controlled by the breaker wave regime which in turn influences the rate and volume of sediment supply beach morphodynamics and resulting aerodynamic boundary conditions and foredune erosion a study of dune formation along the 3700 km long south australian coast provides a basis for an overview of dune formation throughout southern australia

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

numerical computation of sand dune migration with free surface flow 1 this paper presents a morphodynamic model for free surface flows over bed forms the model reproduces bed form evolution and migration in a physically based manner the flow model component of the coupled morphodynamic model which is two dimensional in the streamwise and vertical directions explicitly treats unsteadiness and nonhydrostatic effects comparison with measurements shows that the flow model accurately simulates both mean flow and turbulence structure induced by bed forms under free surface flow the flow model performance is enhanced by the use of a high order godunov scheme known as the cubic interpolated pseudoparticle cip technique to compute the advection phase of the navier stokes equation in addition an enhanced k turbulence closure with nonlinear terms is incorporated in the present model in order to allow better predictions of reynolds stress anisotropy in regions with flow separation performance of the nonlinear k model is compared with standard k closure in the context of the flow as well as morphodynamic simulation nonequilibrium bed load sediment transport is treated using a eulerian stochastic formulation of the sediment exchange process in terms of pickup and deposition functions the proposed approach for sediment transport explicitly considers the local flow variability during morphodynamic computation model results show that the vertical grid distribution pattern affects the simulation result particularly for bed form morphodymics the model successfully simulates some important physical features of bed form evolution such as amalgamation and asymmetric geometry the simulation results are in agreement with laboratory experiments as well as some commonly used prediction methods for geometric characteristics of bed forms

in situ determination of particle friction angles of fluvial gravels particle friction angles represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows we determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force fd required to pivot or slide a particle out of its natural resting place within each site median values were very similar to previously reported relations yet different enough between sites that a location general predictive empirical relation would produce errors in of 10 degrees for di ks 1 furthermore within a di ks class at a given site the range in was as large as 80 much greater than the range of median values between classes for natural sediment mixtures using estimates of c from extensive bed load measurements made by andrews and erman 1986 at sagehen creek and the in situ measurements made in this study together with a theoretical model developed by wiberg and smith 1987 we show that measurements made with this new method can be used to accurately predict c for natural water worked sediments additionally these results confirm that a value 50 is more appropriate for predicting c of a given size class

a two dimensional discrete particle model of gravel bed river systems 1 the formation of bed forms in gravel bed rivers acts as a control on stream ecology and the response of rivers to floods available models do not reproduce the range of observed bed forms and do not consider interactions between the bed and flow hydraulics the model presented here considers a gravel bed river as a complex system in which sediment clasts are represented as discrete elements simple and local rules describe the sediment and flow dynamics using a trimodal sediment distribution irregular forms that scale with particle diameter develop without explicit feedback mechanisms because of the tendency of large particles to roll along the bed surface and collect into chains feedback mechanisms such as imbrication increase the effective entrainment threshold of groups of large particles and increase the stability of these imbricate forms a second type of bed form is associated with saltating grains and emerges where particles are transported at a preferred distance the development and maintenance of larger scale bed forms require feedback between the bed and flow properties by allowing mean velocity to adjust to bed morphology and considering the effect of acceleration on turbulence generation and mean velocity profiles we demonstrate the emergence of forms similar in morphology to gravel sheets dunes and riffle pools the model is best used to complement field based studies and is suitable for testing hypotheses of streambed behavior

response of bed surface patchiness to reductions in sediment supply 1 river beds are often arranged into patches of similar grain size and sorting patches can be distinguished into free patches which are zones of sorted material that move freely such as bed load sheets forced patches which are areas of sorting forced by topographic controls and fixed patches of bed material rendered immobile through localized coarsening that remain fairly persistent through time two sets of flume experiments one using bimodal sand rich sediment and the other using unimodal sand free sediment are used to explore how fixed and free patches respond to stepwise reductions in sediment supply at high sediment supply migrating bed load sheets formed even in unimodal sand free sediment yet grain interactions visibly played a central role in their formation in both sets of experiments reductions in supply led to the development of fixed coarse patches which expanded at the expense of finer more mobile patches narrowing the zone of active bed load transport and leading to the eventual disappearance of migrating bed load sheets reductions in sediment supply decreased the migration rate of bed load sheets and increased the spacing between successive sheets one dimensional morphodynamic models of river channel beds generally are not designed to capture the observed variability but should be capable of capturing the time averaged character of the channel when applied to our experiments a 1 d morphodynamic model rte bookagdegnormgravmixpw xls predicted the bed load flux well but overpredicted slope changes and was unable to predict the substantial variability in bed load flux and load grain size because of the migration of mobile patches our results suggest that 1 the distribution of free and fixed patches is primarily a function of sediment supply 2 the dynamics of bed load sheets are primarily scaled by sediment supply 3 channels with reduced sediment supply may inherently be unable to transport sediment uniformly across their width and 4 cross stream variability in shear stress and grain size can produce potentially large errors in width averaged sediment flux calculations

taking the river inside fundamental advances from laboratory experiments in measuring and understanding bedload transport processes bedload transport in gravel bed rivers impacts channel stability the lifespan of hydraulic structures physical components of aquatic habitat and long term channel evolution field measurements of bedload transport are notoriously difficult which precludes understanding many of the processes and mechanics associated with grain motion such uncertainties are exacerbated when using bedload transport equations most of which were derived using data from a single river or set of laboratory flume experiments recently laboratory experiments have focused on better quantifying the processes that impact bedload fluxes which can then be used to improve sediment transport predictions we highlight recent advances in laboratory instrumentation that can be used in bedload transport studies in particular more accurate ways to measure bedload fluxes near bed turbulence bed grain sizes and topography hold great promise laboratory experiments have also fundamentally improved our understanding of the influence of sediment supply and armoring processes on bedload fluxes and channel conditions the importance of flow hydrographs in controlling total bedload transport rates and bedload hysteresis has also been demonstrated using flume experiments finally many details about the mechanics of grain motion including flow turbulence bed arrangement and particle transport statistics are only possible through laboratory investigations and we feature key knowledge gaps that can be improved with further study

numerical simulation of turbulence and sediment transport of medium sand a model of sand transport in water is produced by combining a turbulence resolving large eddy simulation les with a discrete element model dem prescribing the motion of individual grains of medium sand the momentum effect of each particle on the fluid is calculated at the les cell containing the particle and the fluid velocity and pressure interpolated to each particle center is used to derive fluid force on each particle in the dem eleven numerical experiments are conducted of an initially flat bed of particles the experiments span a range of motion from essentially no motion to vigorous suspension hydraulic roughness is found to increase abruptly at the transition from bed load to suspended load transport suspended sediment extracts momentum from the flow and decreases the rate of shear whereas slightly higher in the flow vertical drag by suspended grains damps turbulence and increases the rate of shear vertical sediment diffusivity and effective particle settling velocity are much smaller than is commonly assumed in suspended sediment models the bed load experiments suggest that saltation by itself is a poor model of bed load sand transport in contrast to expectations from saltation models the peak bed load flux occurs at essentially the same level as the bed and grains move slowly in frequent contact with other grains higher and faster moving bed load grains that can be considered to be in saltation represent a smaller portion of the total flux entrainment of bed load grains occurs in response to fluid penetration of the bed by high vorticity turbulence structures embedded within broader high speed fluid regions referred to as a sweeps or high speed wedges

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

spatial variations in surface sediment structure in riffle pool sequences a preliminary test of the differential sediment entrainment hypothesis dseh riffle pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles this preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows however the differential sediment entrainment hypothesis dseh postulates that differential entrainment can instead result from spatial sedimentological contrasts here we use a novel suite of in situ grain scale field measurements from a riffle pool sequence to parameterise a physically based model of grain entrainment field measurements include pivoting angles lift forces and high resolution dems acquired using terrestrial laser scanning from which particle exposure protrusion and surface roughness were derived the entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles this is because pool grains have looser packing hence greater exposure and lower pivoting angles conversely riffle and pool exit grains have denser packing lower exposure and higher pivoting angles a cohesive matrix further stabilises pool exit grains the resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2d computational fluid dynamics cfd model of the reach the cfd model predicts that under bankfull conditions pools experience lower shear stresses than riffles and pool exits however the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles resulting in differential entrainment under bankfull flows significantly this differential entrainment does not require a reversal in flow velocities or shear stress suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle pool sequences this finding has implications for the prediction of sediment transport and the morphological evolution of gravel bed rivers

understanding heavy mineral enrichment using a three dimensional numerical model layered deposits of relatively light and heavy minerals can be found in many aquatic environments quantification of the physical processes which lead to the fine scale layering of these deposits is often limited with flumes or in situ field experiments therefore the following research questions were addressed i how can selective grain entrainment be numerically simulated and quantified ii how does a mixed bed turn into a fully layered bed and iii is there any relation between heavy mineral content and bed stability herein a three dimensional numerical model was used as an alternative measurement to study the fine scale process of density segregation during transport the three dimensional model simulates particle transport in water by combining a turbulence resolving large eddy simulation with a discrete element model prescribing the motion of individual grains the granular bed of 0 004 m in height consisted of 200 000 spherical particles d50 500 m five suites of experiments were designed in which the concentration ratio of heavy 5000 kg m to light particles i e 2560 kg m was increased from 6 15 35 60 to 80 all beds were tested for 10 s at a predefined flow speed of 0 3 m s analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time the latter accumulated at the bottom of the domain forming a layer whereas the lighter particles were transported over the layer forming sweeps particles below the heavy particle layers indicated that the layer was able to armour the particles below consequentially enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart which enhances current understanding of heavy mineral placer formation this article is protected by copyright all rights reserved

experiments on dispersion of tracer stones under lower regime plane bed equilibrium bed load transport 1 a common approach for estimating the bed load transport rate in gravel bed streams is to relate it to deterministic channel averaged driving parameters and corresponding resistance properties of the bed material notwithstanding the proven success of this approach in modeling various morphodynamic scenarios it does not contain the mechanics necessary to relate the bulk sediment transport rate to the displacement patterns of individual particles experiments on entrainment transport and deposition of tracer stones in a flume described here were designed to address this issue predictors of statistics of bed elevation fluctuations at short timescales total and elevation specific particle entrainment rates particle step lengths mean and associated probability density function for particle virtual velocity and thickness of the active layer were developed the working hypothesis tested in this paper is that the statistics of tracer displacements can be related to channel averaged hydraulic parameters and thus linked to macroscopic aspects of bed load transport

lateral bed load experiments in a flume with strong initial transversal slope in sub and supercritical conditions 1 the role of lateral slope in the generation of bed load transport and bed evolution is investigated because of the substantial lack of detailed laboratory or field data we designed new tests specifically devoted to acquiring accurate measurements in transient flows where the bed surface gradient has a dominant effect on the direction and magnitude of the bed load the bed was prepared with well sorted sand and constant nonzero slopes both longitudinally and transversally over the flume the flow in each experiment evolved to steady uniform conditions characterized by a horizontal bed profile in each section and unchanged longitudinal slope both sub and supercritical conditions were investigated the bounding edges of the bed profile were surveyed on a selected transversal section some detailed kinematic description of the grain motion in the bed load layer has been pursued a shallow water model for mobile bed able to include the most recent theories for the treatment of lateral bed load was developed and numerically applied to the experimental tests comparison between measured and calculated data and between sub and supercritical conditions offers several points of interest

a physical model for seismic noise generation by turbulent flow in rivers previous studies suggest that the seismic noise induced by rivers may be used to infer river transport properties and previous theoretical work showed that bedload sediment flux can be inverted from seismic data however the lack of a theoretical framework relating water flow to seismic noise prevents these studies from providing accurate bedload fluxes and quantitative information on flow processes here we propose a forward model of seismic noise caused by turbulent flow in agreement with previous observations modeled turbulent flow induced noise operates at lower frequencies than bedload induced noise moreover the differences in the spectral signatures of turbulent flow induced and bedload induced forces at the riverbed are significant enough that these two processes can be characterized independently using seismic records acquired at various distances from the river in cases with isolated turbulent flow noise we suggest that riverbed stress can be inverted finally we validate our model by comparing predictions to previously reported observations we show that our model captures the spectral peak located around 6 7 hz and previously attributed to water flow at hance rapids in the colorado river united states we also show that turbulent flow causes a significant part of the seismic noise recorded at the trisuli river in nepal which reveals that the hysteresis curve previously reported there does not solely include bedload but is also largely influenced by turbulent flow induced noise we expect the framework presented here to be useful to invert realistic bedload fluxes by enabling the removal of the turbulent flow contribution from seismic data

role of pressure gradients in sheet flow of coarse sediments under sawtooth waves 1 discrete particle model dpm simulations have been performed to directly examine the role of horizontal pressure gradients acting on the wave bottom boundary layer wbbl during the process of sheet flow transport in the surf zone the dpm is a two phase wbbl model that directly couples a one dimensional eddy viscosity fluid flow to a three dimensional particle model newton s third law is enforced at every simulation time step through fluid particle interaction forces of buoyancy drag and added mass simulations are able to reproduce bedload transport rates from a laboratory data set for coarse sediment distributions consequently simulations of a monochromatic sawtooth wave were performed with three coarse grain size distributions to investigate the relative importance of bed shear stresses horizontal pressure forces and fluid particle drag forces on bedload transport processes the simulation results demonstrate that the magnitude of the horizontal pressure force acting directly on sediment embedded in the wbbl is small compared to the magnitude of the particle drag force or particle stress however the phasing of the onshore peak in the horizontal pressure force with respect to the particle drag force is important to sediment mobilization and enhances the observed onshore bias in bed load flux removing the horizontal pressure force on the sediment particles resulted on average in a 30 reduction in net bed load flux in the surf zone free stream fluid accelerations are typically equated to the horizontal pressure gradient thus the simulations may explain why the fluid acceleration can be a successful parameter for aiding in sediment transport predictions in the surf zone

interactions between bed forms topography turbulence and transport 1 results are presented examining the interaction between two sandy bed forms under low sediment transport conditions in a small laboratory flume the initial artificially made bed forms were out of equilibrium with the flow field temporal evolution of bed forms was monitored using time lapse photography in order to characterize bed form adjustment to the imposed flow velocity measurements were collected using an acoustic doppler velocimeter to characterize both mean flow and turbulence associated with different bed form geometries sandy bed forms all had identical initial geometries however the initial distance between bed form crests was varied between experiments overall deformation of the bed varied as a function of initial bed form spacing however bed forms evolved unpredictably as periods of relatively slow change were punctuated by periods of rapidly changing geometry subtle changes in bed form trough geometry were found to have a strong influence on turbulence and therefore sediment transport comparison with field studies suggests that the mechanisms described herein are active in natural systems

pool riffle sedimentation and surface texture trends in a gravel bed stream a 3 year field campaign was completed to investigate spatial and temporal variability of sedimentation trends for a single pool riffle pair located in the santa cruz mountains california our measurements represent a range of hydrologic conditions over 11 sediment mobilizing events two different statistical methods were used to explore riffle sedimentation cochran s q and mcnemar s nonparametric tests one method indicate that riffle sediment surface texture was spatially and temporally varied at the transect level for mcnemar s test variation was significant at p 0 05 with several trends evident including strong riffle fining triggered by a 20 year flood event a nonlinear empirical orthogonal function method known as self organizing maps soms the second method shows that riffle sediment surface texture is well described by two characteristic temporal signals and one transitional signal at the sampling node level som mapping to each sampling node clearly shows riffle sediment surface texture change was spatially organized over the 11 sediment mobilizing events observations of pool sediment storage indicate that the pool riffle pair exhibited a coupled sedimentation response i e similar texture trends between pool and riffle following the 20 year flood the coupled response was characterized by a trend toward overall sedimentation conditions that were similar to those measured at the beginning of the study the reported texture trends may be of interest to salmonid habitat studies that examine factors contributing to successful versus unsuccessful fry emergence

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

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

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

particle friction angles in steep mountain channels sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers one hypothesis for this observation is that particles are more stable in mountain channels due to particle particle interlocking or bridging across the channel width this hypothesis has yet to be tested however because we lack direct measurements of particle friction angles in steep mountain channels here we address this data gap by directly measuring the minimum force required to dislodge sediment pebbles to boulders and the sediment weight in mountain channels using a handheld force gauge at eight sites in california with reach averaged bed angles ranging from 0 5 to 23 and channel widths ranging from 2 m to 16 m we show that friction angles in natural streams average 68 and are 16 larger than those typically measured in laboratory experiments which is likely due to particle interlocking and burial results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps however the mean particle friction angle does not vary systematically with bed slope these results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels instead the spatial pattern and grain size dependence of particle friction angles may indirectly lower transport rates in steep narrow channels by stabilizing large clasts and channel spanning steps which act as momentum sinks due to form drag

bed surface adjustments to spatially variable flow in low relative submergence regimes in mountainous rivers large relatively immobile grains partly control the local and reach averaged flow hydraulics and sediment fluxes when the flow depth is similar to the size of these grains low relative submergence heterogeneous flow structures and plunging flow cause spatial distributions of bed surface elevations textures and sedimentation rates to explore how the bed surface responds to these flow variations we conducted a set of experiments in which we varied the relative submergence of staggered hemispheres simulated large boulders between runs all experiments had the same average sediment transport capacity upstream sediment supply and initial bed thickness and grain size distribution we combined our laboratory measurements with a 3d flow model to obtain the detailed flow structure around the hemispheres the local bed shear stress field displayed substantial variability and controlled the bedload transport rates and direction in which sediment moved the divergence in bed shear stress caused by the hemispheres promoted size selective bedload deposition which formed patches of coarse sediment upstream of the hemisphere sediment deposition caused a decrease in local bed shear stress which combined with the coarser grain size enhanced the stability of this patch the region downstream of the hemispheres was largely controlled by a recirculation zone and had little to no change in grain size bed elevation and bed shear stress the formation development and stability of sediment patches in mountain streams is controlled by the bed shear stress divergence and magnitude and direction of the local bed shear stress field

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

entrainment and motion of coarse particles in a shallow water stream down a steep slope we investigate the entrainment deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope this is an idealization of bed load transport in mountain streams earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics the intermittent character of particle transport at low water discharges led us to consider it as a random process sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles we develop a birth death immigration emigration markov process to describe the particle exchanges between the bed and the water stream a key feature of the model is its long autocorrelation times and wide frequent fluctuations in the solid discharge a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments we present experimental data obtained using a nearly two dimensional channel and glass beads as a substitute for sediment entrainment trajectories and deposition were monitored using a high speed digital camera the empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model experiments confirmed the existence of wide and frequent fluctuations of the solid discharge and revealed the existence of long autocorrelation time but theory overestimates the autocorrelation times by a factor of around three particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model which performs well when a single particle is moving for our experiments the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity we also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased the more vigorous the stream was the more continuous the solid discharge became moreover although 90 of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 particles dissipate more and more energy when the bed slope is increased but surprisingly the dissipation rate is nearly independent of fluid velocity a movie is available with the online version of the paper

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

probabilistic description of grain resistance from simultaneous flow field and grain motion measurements experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry piv system individual grain movements were surveyed using data from time series of images near bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross correlation techniques to the collected plan view images statistics of grain motions were collected through a semiautomatic procedure significant changes in the flow field were observed in the proximity of the entrained or deposited particles a strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains the theory of grass is revisited and developed based on the experimental results the probability distribution of individual grain resistance has been derived from the statistics of the near bed velocity field and of the entrainment risk

assessment of chevron dikes for the enhancement of physical aquatic habitat within the middle mississippi river usa summary blunt nosed chevron dikes a new invention now being widely constructed on the middle mississippi river mmr have been justified as a tool for enhancing physical aquatic habitat chevron dikes were initially designed to concentrate flow induce channel scour and thus facilitate river navigation more recently these structures have been justified in part for promoting habitat heterogeneity the ability of chevrons to create and diversify physical aquatic habitat however has not been empirically evaluated to assess the ability of chevrons to create and diversify physical aquatic habitat we compiled hydrologic and geospatial data for three channel reference conditions along a 2 0 km 140 ha reach of the mmr where three chevrons were constructed in late 2007 we used the hydrologic and hydraulic data to construct detailed 2 d hydrodynamic models for three reference condition historic circa 1890 pre chevron and post chevron channel conditions these models documented changes in depths and flow dynamics for a wide range of in channel discharges depth velocity habitat classes were used to assess change in physical aquatic habitat patches and spatial statistical tools in order to evaluate the reach scale habitat patch diversity comparisons of pre and post chevron conditions revealed increases in deep to very deep 3 0 m areas of slow moving 3 0 m low velocity constructing chevrons dikes or other dike like structures in the river channel can change the physical aquatic habitat patch mosaic and likely contribute to small increases in physical aquatic habitat heterogeneity however differences in the types quantity and diversity of physical aquatic habitat patches created by chevron dikes in comparison to the physical aquatic habitat patch mosaic of historic channel underscore the need for additional research to determine which physical aquatic habitat patches are critical for the recovery of endangered or threatened aquatic organisms

detailed simulation of morphodynamics 2 sediment pickup transport and deposition the paper describes a numerical model for simulating sediment transport with eddy resolving 3 d models this sediment model consists of four submodels pickup transport over the bed transport in the water column and deposition all based on a turbulent flow model using large eddy simulation the sediment is considered as uniform rigid spherical particles this is usually a valid assumption for sand bed rivers where underwater dune formation is most prominent under certain shear stress conditions these particles are picked up from the bed due to an imbalance of gravity and flow forces they either roll and slide on the bed in a sheet of sediment or separate from the bed and get suspended in the flow sooner or later the suspended particles settle on the bed again each of these steps is modeled separately yielding a physics based process model for sediment transport suitable for the simulation of bed morphodynamics the sediment model is validated with theoretical findings such as the rouse profile as well as with empirical relations of sediment bed load and suspended load transport the current model shows good agreement with these theoretical and empirical relations moreover the saltation mechanism is simulated and the average saltation length height and velocity are found to be in good agreement with experimental results

effects of bed forms and large protruding grains on near bed flow hydraulics in low relative submergence conditions in mountain rivers bedforms large relatively immobile grains and bed texture and topographic variability can significantly alter local and reach averaged flow characteristics the low relative submergence of large immobile grains causes highly three dimensional flow fields that may not be represented by traditional shear stress flow velocity and turbulence intensity equations to explore the influence of large protruding grains and bedforms on flow properties we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bedforms respectively detailed information on the flow was obtained by combining our measurements with a 3d numerical model commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered however at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near bed shear stresses are likely to be inaccurate zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow the reach averaged shear stress did not correlate to depth or slope as commonly assumed but instead was controlled by the relative boulder submergence and degree of plunging flow for accurate flow predictions in mountain rivers the effects of bedforms and large boulders must be considered

untangling boulder dislodgement in storms and tsunamis is it possible with simple theories boulders can move during storms and tsunamis it is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field in this contribution we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of newton s second law of motion in polar coordinates and defining a critical position for boulder dislodgement we find that the boulder dislodgement is not only a function of the causative wave but also of the roughness in the vicinity of the boulder and the slope angle we employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis as the main result of our numerical experiments we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values this allows us to conclude that simple theories are applicable to answer the questions asked in the title but we argue only if they contain a critical dislodgement condition like the one presented here

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

computational modeling of dissipation and regeneration of fluvial sand dunes under variable discharges it is observed during flood events that bed forms initially grow in height and make the riverbed rougher but later under high discharge the bed forms grow longer with the opposite effect of making the riverbed smoother after the discharge drops to a lower value new bed forms regenerate on top of the elongated bed forms this mechanism leads to a significant variation in the bed roughness and the water stage and hence determines the behavior of floods and the risk of flood disasters this work presents detailed modeling of bed forms under discharge hydrographs and simulates the conditions under which the bed is flattened out in the upper plane bed regime the flow was simulated by large eddy simulation and the sediments were considered as rigid spheres and modeled in a lagrangian framework the bed morphodynamics were the result of entrainment and deposition of sediment particles we examined several discharge hydrographs in the first case we increased the discharge linearly and then kept it constant after reaching the upper plane bed condition the dunes were generated and grew during the rising stage of discharge when the flow conditions reached the upper plane bed regime high frequency ripples were generated and helped to flatten the bed the results also showed that in contrast with mechanisms in the dune regime the flattening of the bed was associated with a distinct pattern of sediment transport which deposited sediment mainly in the lee side of the dunes and led to flattening of the bed after flattening the sediments were mainly transported in suspension mode as long as flow conditions stayed in the upper plane bed regime the bed remained flat with small high frequency ripples we also examined two other scenarios one with an immediate falling stage of discharge after the rising stage and the other with a period of constant discharge between the rising and falling stages dunes were regenerated during the falling stage of discharge for both cases in the first case the dunes were regenerated before the bed was flattened because of a time lag between the flow and the bed deformation bed flattening was followed by dune regeneration in the second case moreover a significant reduction in the form drag coefficient was observed after the flattening in the second case the reduction in form drag led to a reduction in the flood intensity the model showed its ability to reproduce the key phenomena of the development of subaqueous dunes under conditions of a passing flood wave

friction angle measurements on a naturally formed gravel streambed implications for critical boundary shear stress we report the first measurements of friction angles for a naturally formed gravel streambed for a given test grain size placed on a bed surface friction angles varied from 10o to over 100o friction angle distributions can be expressed as a function of test grain size median bed grain size and bed sorting parameter friction angles decrease with increasing grain size relative to the median bed grain size and are a systematic function of sorting with lower friction angles associated with poorer sorting the probability distributions of critical shear stress for different grain sizes on a given bed surface as calculated from our friction angle data show a common origin but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses the potential mobility of a grain as defined by its probability distribution of critical shear stress may be overestimated for larger grains in this analysis because our calculations do not take into account the effects of grain burial and altered near bed flow fields

statistical analysis of triggered seismicity in the kresna region of sw bulgaria 1904 and the umbria marche region of central italy 1997 a version of the restricted trigger model is used to analyse the temporal behaviour of some aftershock sequences the conditional intensity function of the model is similar to that of the epidemic type aftershock sequence etas model with the restriction that only the aftershocks of magnitude bigger than or equal to some threshold mtr can trigger secondary events for this reason we have named the model restricted epidemic type aftershock sequence retas model varying the triggering threshold we examine the variants of the retas model which range from the modified omori formula mof to the etas model including such models as limit cases in this way we have a quite large set of models in which to seek the model that fits best an aftershock sequence bringing out the specific features of the seismotectonic region struck by the crisis we have applied the retas model to the analysis of two aftershock sequences the first is formed by the events which followed the strong earthquake of m 7 8 which occurred in kresna sw bulgaria in 1904 the second includes three main shocks and a large swarm of minor shocks following the quake of 26 september 1997 in the umbria marche region central italy the mof provides the best fit to the sequence in kresna that leads to the thought that just the stress field changes due to the very strong main shock generate the whole sequence on the contrary the complex behaviour of the seismic sequence in umbria marche appears when we make the threshold magnitude vary setting the cut off magnitude m0 2 9 the best fit is provided by the etas model while if we raise the threshold magnitude m0 3 6 and set mtr 5 0 the retas model turns out to be the best model in fact observing the time distribution of this reduced data set it appears more evident that especially the strong secondary events are followed by a cluster of aftershocks

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

bed load transport in bedrock rivers the role of sediment cover in grain entrainment translation and deposition 1 bedrock rivers exert a critical control over landscape evolution yet little is known about the sediment transport processes that affect their incision we present theoretical analyses and field data that demonstrate how grain entrainment translation and deposition are affected by the degree of sediment cover in a bedrock channel theoretical considerations of grain entrainment mechanics and sediment continuity each demonstrate that areas of exposed bedrock and thin sediment depths cause sediment transport to be size independent albeit excluding extreme grain sizes we report gravel and cobble magnetic tracer data from three rivers with contrasting sediment cover the bedrock river calder 20 cover the bedrock south fork eel river 80 and the alluvial allt dubhaig 100 these data sets show that 1 transport distances in the river calder are controlled by sediment patch location whereas in the other rivers transport distances are described by gamma distributions representing local dispersion 2 river calder transport distances are size independent across all recorded shear stresses whereas the other rivers display size selectivity 3 river calder tracers are entrained at a dimensionless shear stress of 0 038 which is relatively low compared to alluvial rivers and 4 virtual grain velocities in the river calder are higher than in a comparable reach of the allt dubhaig these contrasts result from differences in the thicknesses and spatial distribution of sediment in the three rivers and support the theoretical analysis sediment processes in bedrock rivers systematically vary along a continuum between bedrock and alluvial end members citation hodge r a t b hoey and l s sklar 2011 bed load transport in bedrock rivers the role of sediment cover in grain entrainment translation and deposition j geophys res 116 f04028 doi 10 1029 2011jf002032

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

using hydraulic equivalences to discriminate transport processes of volcanic flows we characterized stratified deposits from the upper toluca pumice at toluca volcano mexico to distinguish the various modes of transport at play in their genesis using the concept of hydraulic equivalence we determined that deposits resulted from a combination of suspended load fallout saltation and rolling in particular some well sorted coarse stratified beds have a single pumice mode most likely indicative of clasts having traveled through both the transport system and the traction bed such beds are likely remnants of the sorting operated within the large scale transport system other coarse beds have pumice and lithic modes suggesting rolling in the traction bed we propose that boundary layer processes control the sorting of those beds and all finer beds by helping to discriminate between transport mechanisms hydraulic equivalences have a general applicability in geophysical flows involving clasts of contrasted densities

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

trouble with diffusion reassessing hillslope erosion laws with a particle based model 1 many geomorphic systems involve a broad distribution of grain motion length scales ranging from a few particle diameters to the length of an entire hillslope or stream studies of analogous physical systems have revealed that such broad motion distributions can have a significant impact on macroscale dynamics and can violate the assumptions behind standard local gradient flux laws here a simple particle based model of sediment transport on a hillslope is used to study the relationship between grain motion statistics and macroscopic landform evolution surface grains are dislodged by random disturbance events with probabilities and distances that depend on local microtopography despite its simplicity the particle model reproduces a surprisingly broad range of slope forms including asymmetric degrading scarps and cinder cone profiles at low slope angles the dynamics are diffusion like with a short range thin tailed hop length distribution a parabolic convex upward equilibrium slope form and a linear relationship between transport rate and gradient as slope angle steepens the characteristic grain motion length scale begins to approach the length of the slope leading to planar equilibrium forms that show a strongly nonlinear correlation between transport rate and gradient these high probability long distance motions violate the locality assumption embedded in many common gradient based geomorphic transport laws the example of a degrading scarp illustrates the potential for grain motion dynamics to vary in space and time as topography evolves this characteristic renders models based on independent stationary statistics inapplicable an accompanying analytical framework based on treating grain motion as a survival process is briefly outlined

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

sensitivity of incipient particle motion to fluid flow penetration depth within a packed bed a discrete element method is applied to a three dimensional analysis related to sediment entrainment on a micro scale sediment entrainment is the process by which a fluid medium accelerates particles from rest and advects them upward until they are either transported as bedload or suspended by the flow modelling of the entrainment process is a critically important aspect for studies of erosion pollutant resuspension and transport and formation of bedforms in environmental flows previous discrete element method studies of sediment entrainment have assumed the flow within the particle bed to be negligible and have only allowed for the motion of the topmost particles at the same time micro scale experimental studies indicate that there is a small slip of the fluid flow at the top of the bed indicating the presence of non vanishing fluid velocity within the topmost bed layers the current study demonstrates that the onset of particle incipient motion which immediately precedes particle entrainment is highly sensitive to this small fluid flow within the topmost bed layers using an exponential decay profile for the inner bed fluid flow the discrete element method calculations are repeated with different fluid penetration depths within the bed for several small particle reynolds numbers for cases with slip velocity corresponding to that observed in previous experiments with natural sediment the predicted particle velocity is found to be a few percent of the fluid velocity at the top of the viscous wall layer which is a reasonable range of velocities for observation of incipient particle motion this method for prescribing the fluid flow within the particle bed allows for the current discrete element method to be extended in future studies to the analysis of sediment entrainment under the influence of events such as turbulent bursting additionally predictions for the slip velocities and fluid flow profile within the bed suggest the need for further experimental studies to provide the data necessary for additional improvement of the discrete element method models

a physically based bedload transport model developed for 3 d reach scale cellular modelling abstract a bedload transport model is outlined which aims to provide a more physically based description of bed surface and bedload grain size distributions gsds than existing empirical bedload models but is simple enough to be incorporated in a reduced complexity modelling framework one motivation for this is the increasing need for 3 d models of decadal reach scale channel and floodplain evolution for use in river management the computational demands of simulation at these scales mean that the appropriate models are necessarily of the reduced complexity type the bedload model is derived through analysis of sediment beds created using a discrete element model these beds are eroded using a probabilistic entrainment algorithm which predicts the bedload gsd there are two parts to the bedload model the first identifies a functional relationship between the applied shear stress and the fraction of the bed undergoing active transport while the second uses this fraction in conjunction with the surface gsd to produce a bedload gsd the model produces bedload coarsening as shear stress increases from a value of one to a value of five times a reference shear stress this model is tested in a series of numerical experiments applied using a simple cellular model of a straight flume which is run in both recirculation and feed modes in both modes the equilibrium gsds of the bedload and surface are consistent with expected flume behaviour suggesting that the model provides a reasonable reproduction of sediment entrainment transport and sorting

quantifying the role of bed surface topography in controlling sediment stability in water worked gravel deposits 1 this study uses measured bed topography scans and fractional bed load transport rate data obtained from previous laboratory studies of gravel transport to examine the influence of the local grain scale topography on grain entrainment and fractional transport rates a simple steady state fluid model is developed to impose fluid forces onto the exposed areas of the bed surface image analysis is used to identify the surface grains and simple physically based algorithms are used to include the effects of sheltering comparisons of modeled and experimental fractional transport rates show that remote sheltering has a very strong influence on grain entrainment with the effects of remote sheltering included the model was found to be able to predict fractional transport rates well for a range of bed topographies and feed conditions further studies using a wider range of bed shear stresses would allow the effects of turbulence and bed re arrangement to be included in the model

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

bedload a granular phenomenon bedload the transport of sediment remaining in contact with the stream bed has mainly been studied from the perspective of the correlation between fluid driving forces and the responding sediment flux yet grain grain interactions are important and bedload should also be considered as a granular phenomenon we review progress made recently in the study of granular flows especially on segregation and rheology that better illuminates the nature of bedload granular flows may exhibit gas like or fluid like flow or quasi solid deformation all three conditions might be duplicated in bedload understanding of intense bedload transport occurring continuously in a layer several grains deep typical of sand beds might greatly benefit from results in granular physics as illustrated by grain inspired bedload results however processes restricted to the surface of the bed when particles move intermittently and the bed becomes structured while characteristic in gravel bed channels are not well addressed in granular physics mutual study of these phenomena may benefit both physics and fluvial geomorphology we intend therefore to contribute to an enhanced dialogue between granular physics and bedload science communities copyright 2010 john wiley sons ltd