flow and bedform dynamics in an alluvial channel with downward seepage abstract in this paper we report the findings from an experimental study on a parabolic cross sectional sand bed channel with uniform fine sand under no seepage and downward seepage conditions through experiments we observed that an alluvial channel which remained at threshold condition of sediment movement during the no seepage experiment started transporting sediments with the application of seepage in the downward direction shields stress of the threshold channel increased significantly from its critical value after the application of downward seepage which led to the deformation of the cross sectional shape and consequent development of the bedforms the role of turbulence in the development of bedforms has also been analyzed measures of turbulent statistics show that the time mean velocities and reynolds stresses were increased significantly with the application of downward seepage under the action of seepage conditions increase in the flux of streamwise turbulent kinetic energy in the streamwise direction was observed in the region close to the channel boundary also quadrant analysis exhibited an increase in the contributions from all the bursting events and the thickness of the sweep dominated zone in near bed region after the application of downward seepage a bedform tracking tool has been used to evaluate the variability in the geometry of bedforms we have classified these developing bedforms as current ripples and linguoid ripples according to their evolution with time under the downward seepage condition it has been further observed that the variation in shields stress and corresponding bedform geometry reached an equilibrium state in the presence of downward seepage when the experiments were run over a longer period of time 24 31 h

on the genesis and evolution of barchan dunes morphodynamics barchan dunes are crescent shaped formations of sand that can dominate both desert and subaqueous landscapes when the supply of sand is scarce because of the complexity and scale of the underlying phenomena the mechanisms governing the entire process from the genesis to the long term evolution of barchan fields still remain to be better understood herein we attempt to present a description of this process in a subaqueous environment by employing a large eddy simulation approach that couples turbulent flow and sand bed morphodynamics we show that the seeds of the emergent structure in barchan fields are random turbulent flow motions near the initially flat bed we also provide high resolution insights into phenomena such as barchan migration and merging and show how transverse sand waves are formed and migrate over the barchan horns furthermore the transverse sand waves over the barchan horns are shown to be the seeds of the newly born barchans at the end points of the two horns of a barchan through the process known as calving to show this we examine the celerity wavelength and amplitude of the transverse sand waves over the barchan as they approach the end of its horn the celerity and wavelength of these transverse sand waves are shown to be the defining factors in determining the frequency of the calving process the amplitude of the newly born barchans through calving is also shown to be associated with the amplitude of the transverse waves near the end of the horn the simulation data also show that the wavelength of the newly born barchans the distance between individual dunes is closely related to that of the transverse sand waves over their maternal barchan finally we use the simulation results to discuss past conclusions derived from theory conventional models and field observations

numerical simulation of sand waves in a turbulent open channel flow we develop a coupled hydro morphodynamic numerical model for carrying out large eddy simulation of stratified turbulent flow over a mobile sand bed the method is based on the curvilinear immersed boundary approach of khosronejad et al adv water resour vol 34 2011 pp 829 843 we apply this method to simulate sand wave initiation growth and evolution in a mobile bed laboratory open channel which was studied experimentally by venditti church j geophys res vol 110 2005 f01009 we show that all the major characteristics of the computed sand waves from the early cross hatch and chevron patterns to fully grown three dimensional bedforms are in good agreement with the experimental data both qualitatively and quantitatively our simulations capture the measured temporal evolution of sand wave amplitude wavelength and celerity with good accuracy and also yield three dimensional topologies that are strikingly similar to what was observed in the laboratory we show that near bed sweeps are responsible for initiating the instability of the initially flat sand bed stratification effects which arise due to increased concentration of suspended sediment in the flow also become important at later stages of the bed evolution and need to be taken into account for accurate simulations as bedforms grow in amplitude and wavelength they give rise to energetic coherent structures in the form of horseshoe vortices which transport low momentum near bed fluid and suspended sediment away from the bed giving rise to characteristic boil events at the water surface flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines which coupled with sediment erosion from the accelerating flow over the stoss side provides the mechanism for continuous bedform migration and crestline rearrangement the statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations furthermore and in agreement with recent experimental findings singh et al water resour res vol 46 2010 pp 1 10 the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed the spectral gap delineates the spectrum of turbulence from the low frequency range associated with very slowly evolving albeit energetic coherent structures induced by the migrating sand waves overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed

aeolian and subaqueous bedforms in shear flows a sediment bed sheared by an unbounded flow is unconditionally unstable towards the growth of bedforms called ripples under water and dunes in the aeolian case we review here the dynamical mechanisms controlling this linear instability putting the emphasis on testing models against field and laboratory measurements we then discuss the role of nonlinearities and the influence of finite size effects namely the depth of the atmospheric boundary layer in the aeolian case and the water depth in the case of rivers

shear velocity estimates in rough bed open channel flow shear velocity u is an important parameter in geophysical flows in particular with respect to sediment transport dynamics in this study we investigate the feasibility of applying five standard methods the logarithmic mean velocity profile the reynolds stress profile the turbulent kinetic energy tke profile the wall similarity and spectral methods that were initially developed to estimate shear velocity in smooth bed flow to turbulent flow over a loose bed of coarse gravel d 50 15cm under sub threshold conditions the analysis is based on quasi instantaneous three dimensional 3d full depth velocity profiles with high spatial and temporal resolution that were measured with an acoustic doppler velocity profiler advp in an open channel the results of the analysis confirm the importance of detailed velocity profile measurements for the determination of shear velocity in rough bed flows results from all methods fall into a range of 20 variability and no systematic trend between methods was observed local and temporal variation in the loose bed roughness may contribute to the variability of the logarithmic profile method results estimates obtained from the tke and reynolds stress methods reasonably agree most results from the wall similarity method are within 10 of those obtained by the tke and reynolds stress methods the spectral method was difficult to use since the spectral energy of the vertical velocity component strongly increased with distance from the bed in the inner layer this made the choice of the reference level problematic mean shear stress for all experiments follows a quadratic relationship with the mean velocity in the flow the wall similarity method appears to be a promising tool for estimating shear velocity under rough bed flow conditions and in field studies where other methods may be difficult to apply this method allows for the determination of u from a single point measurement at one level in the intermediate range 036 copyright c 2013 john wiley sons ltd

bed form genesis from bed defects under unidirectional oscillatory and combined flows new experimental data on bed form initiation under unidirectional oscillatory and combined flows are presented to gain quantitative insight into bed form genesis from artificially generated defects on a flat sediment laden bed planform changes revealed in time lapse photography allowed study of the evolution of the downstream and upstream edges of defects from their initial geometric center based on this temporal data set and flow velocity profiles it was observed that combined flow bed forms share the same bed form initiation processes as unidirectional and oscillatory flows which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions the development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle under current dominated combined flow conditions no defect propagation occurred in the upstream direction despite the presence of flow reversal in addition spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that i periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows ii wave dominated combined flows possess a period driven peak with respect to defect growth in experiments with relatively short wave periods but this peak is absent in experiments possessing relatively long wave periods t 15 s and iii current dominated combined flows and pure unidirectional flows do not display a period driven peak in defect growth these results suggest that the occurrence of long period combined flow bed forms may be underrepresented in the sedimentary record

initiation of bed forms on a flat sand bed the results of a series of 47 bed development experiments are combined with appropriate experimental results of other authors the generation of initial bed waves termed sand wavelets on a plane sand bed is found to be instigated by the occurrence of a random pileup of sediment this pileup subsequently growing as further sediment is trapped by the pileup when the height of the pileup approaches the bed roughness height further pileups are generated at preferred spacings downstream the wavelength l of the sand wavelets so generated is relatively insensitive to the applied bed shear stress and primarily a function of the size d g of the sediment this wavelength can be described by the empirical formula l dg r c0 2 10 2 5 from a theoretical viewpoint this wavelength is approximately described by a reassessment of the flow system instability analyses of richards both ripples and dunes are found to develop from the sand wavelets

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

mean and turbulent flow structure during the amalgamation process in fluvial bed forms 1 fluvial channels present bed forms such as dunes and ripples that alter instantaneous hydrodynamics parameters such as flow velocities water surface profiles bed shear stresses and reynolds stresses and create turbulent coherent structures that are significantly different from those presented in flat bed conditions it is known that les based models are more suitable than rans models to reproduce the complex hydrodynamics around bed forms herein a les model is applied to describe the mean and turbulent flow structure under superimposed bed forms three cases were simulated run i train of ripples run ii superimposed bed forms and run iii amalgamated bed forms the les modeling was performed using a free surface condition to allow the model to develop undulations and boils on the water surface caused by effect of the bed forms some important conclusions from this study are the division of high and low shear stresses on the stoss side of the dune the progression of the flow field topology from run i and run iii and the type of turbulent coherent structures found in each stage the region of high shear stresses was related to turbulence production in which the streamwise velocity fluctuations where strips structures are related to streaks were associated to the modification of the bed morphology the turbulence horseshoes vortices thv were more frequent in run i than in the other two cases where streamwise rolls were more frequent finally the frequency of the bursting events increased from run i to run ii and decreased from run ii to run iii implications of detailed hydrodynamics into bed forms processes are also presented and discussed

bedform spurs a result of a trailing helical vortex wake the formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein bedform spurs are formed by helical vortices that trail from the lee surface of oblique segments of bedform crestlines trailing helical vortices quickly route sediment away from the lee surface of their parent bedform scouring troughs and placing this bed material into the body of the spur the geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross stream strouhal number when present spur bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms field measurements collected at the north loup river nebraska and flume experiments described by venditti et al 2005a 2005b 2005c demonstrate that this trailing helical vortex mediated sediment transport is a mechanism for bedform deformation interactions and transitions between two dimensional and three dimensional bedforms this article is protected by copyright all rights reserved

morphodynamics of small scale superimposed sand waves over migrating dune bed forms 1 the kinematics and morphodynamics of low amplitude small scale sand waves developed over migrating dunes are examined using data drawn from laboratory experiments we refer to the superimposed features as sand sheets a general descriptive term for low amplitude bed waves that are not easily classified as ripples dunes or bars within the experiments the sheets formed downstream of the reattachment point at a distance that was invariant with dune size some sheets lacked slip faces composed of sand grains avalanching down a slope near the angle of repose over equilibrium dunes three to four sand sheets were observed per 100 s sheet thickness was 10 of the height of the dune upon which they were superimposed they migrated at 8 to 10 times the dune rate they had nearly constant lengths over the full range of dune lengths and flow conditions and they had aspect ratios of 0 025 dunes and sand sheets represent distinct scales of sediment transport with different migration rates however sediment transport rates calculated from the sand sheet and dune morphologies are nearly identical for transport equivalence to occur sand sheets migrating at 10 times the dune rate must be 0 1 times the size which is consistent with the morphological observations superimposed bed waves on dunes are often considered simply as additional roughness elements but these results indicate that such bed waves are the agency by which the dune bed form itself moves downstream

bedforms in a turbulent stream formation of ripples by primary linear instability and of dunes by nonlinear pattern coarsening it is widely accepted that both ripples and dunes form in rivers by primary linear instability the wavelength of the former scaling on the grain size and that of the latter being controlled by the water depth we revisit here this problem in a theoretical framework that allows to give a clear picture of the instability in terms of dynamical mechanisms a multi scale description of the problem is proposed in which the details of the different mechanisms controlling sediment transport are encoded into three quantities the saturated flux the saturation length and the threshold shear stress hydrodynamics is linearized with respect to the bedform aspect ratio we show that the phase shift of the basal shear stress with respect to the topography responsible for the formation of bedforms appears in an inner boundary layer where shear stress and pressure gradients balance this phase shift is sensitive to the presence of the free surface and the related effects can be interpreted in terms of standing gravity waves excited by topography the basal shear stress is dominated by this finite depth effect in two ranges of wavelength when the wavelength is large compared to the flow depth so that the inner layer extends throughout the flow and in the resonant conditions when the downstream material velocity balances the upstream wave propagation performing the linear stability analysis of a flat sand bed the relation between the wavelength at which ripples form and the flux saturation length is quantitatively derived it explains the discrepancy between measured initial wavelengths and predictions that do not take this lag between flow velocity and sediment transport into account experimental data are used to determine the saturation length as a function of grain size and shear velocity taking the free surface into account we show that the excitation of standing waves has a stabilizing effect independent of the details of the flow and sediment transport models consequently the shape of the dispersion relation obtained from the linear stability analysis of a flat sand bed is such that dunes cannot result from a primary linear instability we present the results of field experiments performed in the natural sandy leyre river which show the formation of ripples by a linear instability and the formation of dunes by a nonlinear pattern coarsening limited by the free surface finally we show that mega dunes form when the sand bed presents heterogeneities such as a wide distribution of grain sizes

bed and flow dynamics leading to sediment wave initiation 1 new piv based experiments show that the nascent seed waves from which both ripples and dunes develop are generated on planar mobile sediment beds in a two stage process the first stage comprises the motion of random sediment patches that reflect the passage of sediment transport events caused by attached eddies these eddy transport events propagate at speeds that are proportional to their size and less than overhead eddy convection velocities but potentially larger than local average fluid and sediment velocities in the second stage interactions of the moving patches result in a bed disturbance that exceeds a critical height and interrupts the bed load layer quasi regular seed waves are then generated successively downstream of this stabilised growing disturbance via a scour deposition wave that arises from the requirement of sediment mass conservation and the sediment transport and bed stress distributions downstream of a bed perturbation seed waves are thereby of preferred lengths that scale with the grain size i e length o 130 grain diameters agreeing with compiled measurements this two stage generation mechanism is valid for fully turbulent hydraulically smooth and rough bed flows of small to large sediment transport rates it is furthermore valid for laminar flows although the critical disturbances leading to seed wave generation arise through bed discontinuities and not eddy based sediment transport events the identified generation mechanism which accounts for turbulence effects explains the observed similar scaling of alluvial closed conduit and lightweight sediment seed waves the present measurements highlight further aspects of the flow dynamics preceding seed wave generation including decreases in von karman s constant due to bed mobility near bed eddy convection speeds in excess of local double averaged in time and space streamwise velocities and the validity of the four range spectral scaling model for open channel flows proposed earlier by nikora

turbulent characteristics and evolution of sheet flow in an alluvial channel with downward seepage abstract experimental investigation of the flow hydrodynamics and temporal changes in the cross sectional profile of an alluvial channel has been studied in the present work experiments were carried out in a curvilinear cross sectional shaped channel with no seepage and with a downward seepage condition to ascertain seepage effects on channel geometry and turbulent characteristics of flow measures of turbulent characteristics such as time averaged near bed velocities and reynolds stresses were found to increase with the application of downward seepage stream power and the value of shields parameter are increased under the action of downward seepage causing bed particles to move in the form of a sheet layer integral scales of flow suggest that the size of eddies increases with the application of downward seepage which is linked to the evolution of the sheet layer sheet development causes reduction in flow depth and rapid channel widening cross sectional parabolic shape of the threshold channel is transformed into a trapezoidal shape with the presence of the sheet layer with the passage of time 11 h the channel attains another equilibrium geometrical state with the value of shields stress around 0 074 from the no seepage value of 0 0399 an empirical equation for sheet flow rate is derived with the consideration of seepage in the downward direction

turbulence measurements in open channel flows over artificial bed forms measurements of mean and turbulence characteristics in uniform open channel flows of constant mean depth over artificial fixed one dimensional periodic bed features were obtained using two component laser doppler velocimetry two types of bed forms based on triangular elements of the same height n 20 of the flow depth and wavelength n 12 5 times the height but of different shape were studied 1 45 triangle ridges separated by a flat region and 2 triangular elements approximating dunes with a mildly sloping upstream face instead of a flat region the role of the total shear velocity based on the mean energy slope and the mean flow depth as a possible velocity scale is studied velocity defect and the reynolds shear stress profiles were generally well approximated by the flat bed profiles for regions of flow far from the bed sufficiently far from the recirculation and reattachment zones turbulence characteristics collapsed fairly well when scaled with the total shear velocity over most of the depth but do not follow the flat bed behavior in the lower part of the flow possible implications of the clear water measurements for the description of suspended sediment concentration profiles and total suspended load computations are explored

bed form initiation from a flat sand bed 1 bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0 5 mm sand velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent uniform open channel flows under these conditions two separate modes of bed form initiation are observed defect and instantaneous initiation defect initiation occurs at lower flow stages where sediment transport is sporadic and patchy and is characterized by defect propagation associated with flow separation instantaneous initiation occurs at larger flow strengths where sediment transport is general and widespread this form of bed form initiation begins with the imprinting of a cross hatch pattern on the flat sediment bed which leads to chevron shaped forms that migrate independently of the initial pattern the chevrons eventually align to form incipient crest lines this mode of bed form initiation does not appear to be linked to turbulent structures but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales

numerical modeling of subaqueous sand dune morphodynamics the morphodynamic evolution of subaqueous sand dunes is investigated using a 2 d reynolds averaged navier stokes numerical model a laboratory experiment where dunes are generated under stationary unidirectional flow conditions is used as a reference case the model reproduces the evolution of the erodible bed until a state of equilibrium is reached in particular the simulation exhibits the different stages of the bed evolution e g the incipient ripple generation the nonlinear bed form growing phase and the dune field equilibrium phase the results show good agreement in terms of dune geometrical dimensions and time to equilibrium after the emergence of the first ripple field the bed growth is driven by cascading merging sequences between bed forms of different heights a sequence extracted from the simulation shows how the downstream bed form is first eroded before merging with the upstream bed form superimposed bed forms emerge on the dune stoss sides during the simulation an analysis of the results shows that they emerge downstream of a slight deflection on the dune profile the deflection arises due to a modification of the sediment flux gradient consecutive to a reduction in the turbulence relaxation length while the upstream bed form height decreases as they migrate superimposed bed forms grow on the dune stoss side and eventually provoke the degeneration of the dune crest cascading merging sequences and superimposed bed forms dynamics both influence the dune field evolution and size and therefore play a fundamental role in the dune field self organization process

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

probability distributions of particle hop distance and travel time over equilibrium mobile bedforms the joint probability distribution of streamwise particle hop distance lateral particle hop distance and travel time constrains the relationships between topographic change and sediment transport

the trouble with shear stress abstract predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability sediment motion is driven by a temporal distribution of applied fluid forces which are often simplified into time averaged flow velocities or shear stresses in sediment transport equations spatial variations in these flow parameters are also often neglected given the nonlinearities in the relation between flow and sediment flux the reach averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux such spatial and temporal flow variations are ignored with the commonly used reach averaged shear stress ds that is a function of the flow depth and channel slope to demonstrate the uncertainties with using reach averaged shear stresses and even local shear stresses to predict bedload transport rates we have three goals first we outline the derivations and assumptions involved with commonly used estimates of shear stress second we summarize the literature on the uncertainties involved with using shear stress and other space and time averaged flow parameters to predict sediment flux third we use a new data set from experiments to add further support to a growing body of literature that suggests ds may not be linked to the mechanics of bedload transport we show considerable uncertainties exist in estimating a reach averaged shear stress by comparing ds and near bed shear stresses calculated from velocity profiles reynolds stresses and local turbulence for example ds was systematically greater than these near bed stresses even for experiments without large bed roughness we also find that correlations between local time averaged sediment fluxes and local reynolds stresses or turbulence intensities are relatively poor the near bed time averaged local flow velocity has the strongest relation with the time averaged local bedload flux because it is closely related to the forces that actually drive sediment transport however sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity suggesting that additional work is needed to identity a parameter or distributions of parameters that will best predict sediment fluxes

on interfacial instability as a cause of transverse subcritical bed forms 1 recent experimental results suggest there are at least two distinct bed form initiation processes bed forms may be generated from local bed defects that are propagated down and across stream by flow separation processes when sediment transport is patchy and sporadic alternatively bed form development may occur over the whole bed at once when sediment transport is general and widespread herein we critically test a simple model for this latter bed form initiation mode that was originally presented by h k liu in 1957 but not tested because of the complex nature of the measurements required the theory is based on the idea that a moving sand bed might be likened to a dense fluid and that a hydrodynamic instability develops at the interface between the sediment transport layer and the near bed fluid leading to the organization of the transport system into laterally extended discrete bed forms our predicted initial bed form lengths are within 10 of the lengths observed in a series of flume experiments suggesting that transverse waveforms on sand beds sheared by unidirectional currents can be initiated as an interfacial hydrodynamic instability of kelvin helmholtz type when the current is sufficiently vigorous to produce general sediment movement that is to create a sediment transport layer that acts as a pseudofluid with density composed of solid and fluid components

fluvial dunes initiation characterization flow structure research into fluvial dunes spans disciplines studies at grain to reach scales and methodological approaches that include theoretical experimental numerical and field investigations despite significant research efforts to date it remains difficult to provide definitive answers to some fundamental questions regarding dunes this paper reviews three notable challenges that remain regarding fluvial dunes namely scale consistent linking of bed morphologies with turbulent flow fields the intriguing question of what causes trains of highly ordered sediment waves to form in beds of river sediments and how to define the important characteristics of a dune covered bed including lengths shapes and their statistical nature in each case the particular challenge is discussed and then recent research and ways forward are presented copyright 2010 john wiley sons ltd

spectral description of migrating bed forms and sediment transport this paper considers the problem of spatiotemporal bed topography evolution and sediment transport estimation in rivers with migrating bed forms of different types and sizes in statistical equilibrium conditions instead of resorting to bed form classification we propose to evaluate the evolution of multiscale bed topography as the integral of unit contributions defined through a space time fourier decomposition of bed elevations using joint 2 d spectra in the frequency and wave number domain a functional relationship between the length scales and the timescales in which migrating bed forms are decomposed is proposed and developed into a dimensionless expression for scale dependent convection velocities this formulation highlights the violation of taylor s hypothesis for migrating bed forms confirming statistically that larger bed forms travel slower as compared to smaller bed forms this phenomenological description leads to a spectral extension of the simons et al 1965 formula for sediment transport to incorporate a range of multiscale migrating features both the scaling of convection velocities and the spectral estimate of sediment transport rate were validated through extensive bed elevation data from laboratory experiments conducted at the st anthony falls laboratory for a range of froude numbers 0 2 fr 0 5 under varying discharge and bed material composition

on the role of fluid infiltration into gravel dunes using a 3d numerical model abstract both the hydrodynamic conditions as well as the related sediment transport processes of sandy bedforms are understood fairly well however little is known about the genetic processes of bedforms involved in environments dominated by gravel in the coastal environment storm waves spring tides bore waves and to a lower frequency tsunamis are common phenomena that can provide the required energy to form gravel bedforms given the high energy of such wave events and the formidable practical difficulties of fixed instrumentation and in situ measurements knowledge on the hydrodynamic processes in the direct vicinity and in the interior of gravel bedforms is still limited a 3d numerical flume tank model using smoothed particle hydrodynamics sph was generated to study the nature of flows for an extreme wave above and in the interior of gravel bedforms as a complementary method to in situ measurements with typical flume tank dimensions x 2 68 m y 0 25 m z 0 8 m three experiments were undertaken to test a range of simplified bedform geometries stoss side slopes 42 23 13 using a unimodal grain fraction the water column was represented by 11 million fluid particles and the bedforms by 480 immobile solid particles a single numerical wave was introduced using a vertical paddle that moved to x 0 3 m perpendicular to the bottom of the tank fluid velocities extracted at each timestep at each point of the tank were used to determine free stream pore water flow velocities and residence times from all experiments results showed that when the wave crest was positioned before the bedform crests pore water flows appeared predominantly in the downstream direction however when the position of wave crest was located after the bedform crests a vortex developed at the leeside of all bedforms causing a change of the pore water flow towards the upstream direction regions of inflow outflow and pore water flow through the pore spaces were evident in all experiments but changed as the stoss side slope angles were decreased it is concluded that variations in the stoss side steepness appear to control the locations and the position of the wave crest the directions of pore water flow which was illustrated in a conceptual model

initial stages of erosion and bed form development in a turbulent flow around a cylindrical pier 1 bed load transport and erosion in fine sediment beds are mainly driven by the dynamics of the near bed turbulent flow in situations when the shear stress is not sufficiently high to produce significant transport the presence of an obstacle can initiate erosion and trigger the development of bed forms which are produced by the emergence of the turbulent horseshoe vortex thv system we develop a numerical model to investigate the initial stages of erosion and the development of ripples produced by the thv system in the vicinity of a surface mounted cylindrical pier the flow is simulated using the detached eddy simulation approach which has been shown to accurately resolve most of the turbulent stresses produced by the thv to compute the erosion the exner equation is coupled to a new bed load transport model that directly incorporates the effect of the instantaneous flow field on sediment transport the morphodynamic model is integrated simultaneously with the flow equations using an arbitrary lagrangian eulerian method for moving boundaries even though the time rate of scour is slower compared to the observations the computed results exhibit essentially all the dynamics of erosion including the emergence of ripples reported in the experiments of dargahi 1990 the bed forms show similar velocities as reported in the experiments and are shown to be statistically similar to ripples measured in laboratory experiments and in nature to our knowledge this is the first three dimensional simulation to capture the ripple dynamics that evolve naturally from the nonlinear interactions between the flow and the bed

composition and origin of clay minerals in holocene sediments from the south eastern north sea the distribution of clay minerals varies systematically in sediments from holocene core material taken along a profile from the east frisian coast to the doggerbank the proportion of illite increases with distance from the coast at the expense of kaolinite whereas slight variations are seen in smectite and chlorite abundances the chemical composition changes and the k rb ratio and k ar isotopic age of illite increase seawards this trend results from progressive mixing processes of riverine detritus with pleistocene fluvioglacial material reworked during the holocene transgression however the clay fluvial flux only became dominant during the decreasing rate of sea level rise in the late holocene especially near the shore for example modern sediments in tidal flats contain 75 of river borne holocene supplied clay detritus whereas this amount is only 10 in modern marine sediments at the doggerbank

investigations on undercutting processes of cohesive river bank abstract river bank failure is the result of prolonged and repetitive erosion processes caused by the interplay between the flow turbulence with that of bank sediment these interactions of affecting factors become more complex when the river bank sediment is cohesive in nature the present study attempts to understand the erosion of cohesive river banks under controlled laboratory conditions starting from an uneroded bank to the state just before failure experimental runs are carried out over the cohesive river bank fabricated in a laboratory flume instantaneous real time transverse erosion depth within the bank is recorded using ultrasonic ranging system urs and the corresponding velocity data by acoustic doppler velocimeter adv the results from the experimental runs with varying flow characteristics and cohesive sediment properties reveal that the turbulent shear stress combined with velocity fluctuations close to the bank increases with the flow reynolds number the size of the dislodged cohesive aggregates from the cohesive bank depends on the shear stress fluctuations the transverse erosion depth as a function of time and flow reynolds number are the significant parameters on which fluctuations of shear stress depend these fluctuations in shear stress and flow velocity are high near the upstream and downstream sides of the undercuts resulting in a longitudinally elongated undercut zone

ripples on stream bed some 30 years ago john f kennedy laid the foundations for the description of the geometry of sand waves on stream beds by the potential flow model since then the analysis has been extensively refined and extended but no rigorous description of the small features called ripples exists the present study attempts a stock taking of the data and the various behavioral concepts with the aim of encouraging researchers to improve our knowledge of the physics of features called ripples various trends are identified from the data it is deduced that ripples act as roughness elements of the boundary the ripple troughs are filled with the energy consuming lee vortices the main flow is over the ripple crests and lee vortices and does not interact with the bed geometry the roughness layer of ripples and vortices is within the constant shear layer and is affected by the flow depth only as far as it affects the velocity distribution and alters the shear stress on the bed

a conceptual model for meander initiation in bedload dominated streams a simple analytic model is presented relating local sediment transport capacity to variance in the transverse shear stress distribution in a stream channel the model is used to develop a physically based conceptual model for the initiation of meandering in straight bedload dominated streams as a result of a feedback mechanism the feedback maximizes the cross sectional shear stress variance and in order to achieve stability ultimately minimizes the energy slope at repeated locations along the channel subject to steady state mass flux and the stability of the channel boundary these locations develop into pools in a fully developed meandering channel they represent attractor states wherein sediment continuity is satisfied using the least possible energy expenditure per unit length of channel however since the cross sectional geometry of a pool and the adjacent bar is asymmetric these attractor states are only conditionally stable requiring strong curvature induced secondary circulation to maintain their asymmetry between two successive pools a stream occupies a metastable higher energy state corresponding to a riffle that requires greater energy expenditure per unit length of channel to transport the same volume of sediment the model we present links processes at the scale of a channel width to adjustments of the channel sinuosity and slope at the scale of a channel reach we argue that the reach scale extremal hypotheses employed by rational regime models are mathematical formalisms that permit a one dimensional theory to describe the three dimensional dynamics producing stream morphology our model is consistent with the results from stream table experiments with respect to both the rate of development of meandering and the characteristics of the equilibrium channel morphology copyright 2006 john wiley sons ltd

modeling of spatial lag in bed load transport processes and its effect on dune morphology in the present study two bed load transport models are introduced in an existing idealized dune model these allow for the modeling of the spatial lag between the sediment transport rate and bed shear stress along dune surfaces this lag is an important factor in determining transitions between bedform regimes results of the original dune model using an equilibrium transport formula are compared with 1 a new model version that directly models spatial lag with a relaxation equation and 2 a new model version including pick up and deposition processes both bed load models use mean particle step length as an important parameter which is varied to assess which value is appropriate for the dune regime laboratory experiments are simulated with the model this shows that the results are best with the pick up and deposition model version combined with a step length of 25 times the particle diameter it is furthermore shown that in principle the model is also able to wash out fully grown dunes by increasing the step length parameter see more at http ascelibrary org doi 10 1061 28asce 29hy 1943 7900 0001254 sthash to4wr5cu dpuf

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

the influence of grain size on the velocity and sediment concentration profiles and depositional record of turbidity currents cross stratification formed by migrating dunes is ubiquitous in fluvial channel deposits yet cross stratification specifically formed by dunes is comparatively uncommon in deep marine channel deposits subjected to similarly unidirectional flows the reasons for which is the subject of much debate in part this is related to the difficulty associated with acquiring high resolution data sets natural turbidity currents are highly destructive whereas physical models are hampered by the shortcomings of conventional laboratory instrumentation consequently the internal structure of turbidity currents is poorly understood thus hindering the ability to identify the linkages between flow properties and depositional characteristics in this study we use a medical grade computed tomography scanner coupled with a three dimensional ultrasonic doppler velocity profiler to characterize the flow field and depositional morphology across a range of grain sizes d 50 70 330 m and sediment concentrations 5 17 5 by mass results show that the development of angular bedforms is suppressed in all flows with particle concentrations greater than 9 5 by mass but notably also in all flows with particles 230 m and sediment concentrations down to 5 the lowest slurry densities used in these experiments by mass this suggests a first order control on the response of the bed by the makeup of the overriding current more fundamentally we propose that grain size controls the character of near bed density stratification which is a requisite condition to generate the hydrodynamic instabilities that create the initial bed defects which then lead to the development of angular bedforms like ripples and dunes

the stability of ripples and dunes abstract the stability of ripples and dunes in flows of finite depth are investigated for the linear small amplitude case and also for the more realistic finite amplitude situation for the former perturbation approach is taken leading to the general result that without the inclusion of some lag between the sediment transport rate and the boundary shear stress all wavelengths are unstable this results from an upstream shift in the stress relative to the bedform leading to deposition at the crest by assuming a realistic lag based on the mechanics of bed load transport a fastest growing wave at ripple scales results similarly if suspended sediment were included longer lags result and a dune scale instability could result some investigators have invoked a gravitational effect arguing that sediment transport is enhanced downslope and retarded upslope this creates an effective downstream shift in the transport rate and some argue that both ripple and dune instabilities result however a realistic evaluation of this effect shows that it is much too small to be effective naturally occurring bedforms are of finite amplitude typically they are asymmetrical often causing flow separation this highly non linear process creates a vastly different flow environment from that assumed in the initial stability case a wake like flow structure develops downstream of the separation zone and an internal boundary layer develops beneath the complex interaction of these two different flow regimes as well as the effect induced by the topography of the bedform itself produces a shear stress field that dictates the stability of the feature itself

paleoecology and proliferation of the bivalve chondrodonta joannae choffat in the upper cenomanian upper cretaceous adriatic carbonate platform of istria croatia abstract chondrodonta joannae choffat is a morphologically variable oyster like bivalve with a predominately calcitic shell an exceptional exposure of c joannae bearing strata of late cenomanian age crops out along the seaside in northern istria croatia and permits a taphonomical and functional analysis in order to define the life habit and growth strategies of this bivalve the c joannae population from the studied succession is characterized by highly elongated large and curved shells reaching about 50 cm in height and 5 cm in length this shell shape is typical of the club like bivalve morphotype which was adapted to soft bottom substrates with high sediment accumulation the shell is slightly inequivalve and characterized by a reduced body cavity a few centimetres high and a dorsal region up to 10 times longer the shell opening mechanism was mostly based on the resilium located between the chondrophores which protrude in the body cavity the abandoned dorsal cavity is filled by a calcite hinge plate the ventral edge of which acted as fulcrum for the valve flexibility in the hinge plate the function of chondrophores changed they acted as a hinge to keep tightly interlocked the valves which considerably emerged above the sediment water interface the individuals were arranged in low shrub type congregations which produced low relief mounds the functional morphology and taphonomic signature suggest that c joannae individuals collected food at a greater distance from the bottom with respect to the co occurring rudists we speculate that the c joannae proliferation could be related to a late cenomanian phase of environmental instability predating the oae2 with fluctuating climatic conditions and ocean fertility

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

turbulent flow and drag over fixed two and three dimensional dunes 1 detailed measurements of turbulent flow were obtained over a fixed flat bed two dimensional 2 d dunes and four types of three dimensional 3 d dune morphologies including 1 full width saddles 2 full width lobes 3 sinuous crests and 4 irregular shaped crests the time averaged turbulence structure over the fixed flat bed was dynamically similar to flow over a flat bed with active sediment transport the flow field over 2 d dunes conforms with previous observations of flow over mobile and fixed bed forms bed form three dimensionality significantly altered the flow field observed over the 2 d dunes lobe shaped dune crest lines curved crests with topographic highs that bowed downstream enhanced the level of turbulence producing a better defined wake structure and more vigorous mixing in the separation cell than observed over 2 d dunes saddle shaped dune crest lines curved crests with topographic highs that bowed upstream diminished the level of turbulence suppressing a well defined wake structure and mixing in the flow separation cell flow over sinuous crested bed forms with lobes and saddles was dynamically similar to flow over full width bed forms development of in line paired saddles and lobes in the downstream direction appears to be controlled by near bed velocity gradients which were larger over saddles and smaller over lobes these gradients control local boundary shear stress patterns that should promote scour and high transport rates over saddles and deposition on lobes two dimensional and 3 d bed forms with the same height and length offered different levels of flow resistance the flow observations support the idea that saddles decrease flow resistance and lobes increase flow resistance relative to a 2 d dune bed form crests composed of irregularly spaced lobes and saddles suppressed turbulent flow structure development and reduced drag by 20 below levels for 2 d or sinuous crested dunes the results suggest that bed form crest shape needs to be accounted for in flow resistance calculations and that a single drag coefficient is inadequate where the bed can evolve through 2 d 3 d and irregular 3 d bed form states

rhomboid beach pattern a laboratory investigation 1 the formation of beach rhomboid pattern by swash is investigated experimentally this centimeter scale structure is classically interpreted as the mark of stationary gravity waves generated by obstacles in supercritical flows however thanks to the use of water based fluids of various viscosity our experiments show that a rhomboid pattern can develop in subcritical flows its angle is primarily a function of the froude number as suggested by woodford 1935 but our data do not support his classical model nor do they support any of the existing theories the slowness of the rhombus motion indicates that it is not simply the mark of a hydraulic phenomenon but rather results from the coupling between the water flow and sediment transport

a unifying model for planform straightness of ripples and dunes in air and water abstract geologists physicists and mathematicians have studied ripples and dunes for more than a century but despite considerable effort no general model has been proposed to explain perhaps the most fundamental property of their morphology why are some bedforms straight continuous parallel and uniform in planform geometry i e two dimensional whereas others are irregular three dimensional here we argue that physical coupling along the crest of a bedform is required to produce straight crests and that along crest flow and sand transport provide effective physical mechanisms for that coupling ripples and dunes with the straightest and most continuous crests include longitudinal and oblique dunes in unidirectional flows wave ripples dunes in reversing flows wind ripples and ripples migrating along a slope at first glance these bedforms appear quite different ripples and dunes air and water transverse oblique and longitudinal orientations relative to the net sand transport direction but they all have one property in common a process that increases the amount of along crest sand transport that lengthens and straightens their crests relative to the across crest transport that makes them migrate and take the more typical and more three dimensional planform geometry in unidirectional flows that produce straight bedforms along crest transport of sand is caused by along crest flow non transverse bedform orientation gravitational transport along an inclined crest or ballistic splash in air bedforms in reversing flows tend to be straighter than their unidirectional counterparts because reverse transport across the bedform crest reduces the net across crest transport that causes the more typical irregular geometry relative to the along crest transport that smoothes and straightens planform geometry

a classification scheme for turbulence based on the velocity intermittency structure with an application to near wall flow and with implications for bed load transport kolmogorov s classic theory for turbulence assumed an independence between velocity increments and the value for the velocity itself however recent work has called this assumption in to question which has implications for the structure of atmospheric oceanic and fluvial flows here we propose a conceptually simple analytical framework for studying velocity intermittency coupling that is similar in essence to the popular quadrant analysis method for studying near wall flows however we study the dominant longitudinal velocity component along with a measure of the roughness of the signal given mathematically by its series of holder exponents thus we permit a possible dependence between velocity and intermittency we compare boundary layer data obtained in a wind tunnel to turbulent jets and wake flows these flow classes all have distinct characteristics which cause them to be readily distinguished using our technique and the results are robust to changes in flow reynolds numbers classification of environmental flows is then possible based on their similarities to the idealized flow classes and we demonstrate this using laboratory data for flow in a parallel channel confluence our results have clear implications for sediment transport in a range of geophysical applications as they suggest that the recently proposed impulse based methods for studying bed load transport are particularly relevant in domains such as gravel bed river flows where the boundary layer is disrupted and wake interactions predominate

on the entrainment of sediment and initiation of bed defects insights from recent developments within turbulent boundary layer research sediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds results of visualization experiments suggest that these sweep impacts are grouped and may define patches of entrainment upon a mobile bed that may be wider than an individual sweep impact additionally the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks these observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed these features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripples

nature of deformation of sandy bed forms 1 we explore a stochastic component of topographic evolution of sandy river beds and its relationship to bed material flux the behavior of trains of mobile bed forms can be decomposed into two independent constituents translation and deformation translation is the mean downstream migration of the bed at velocity that defines the lagrangian reference frame of the bed deformation is the sum of all changes to the bed s topographic profile measured from within the bed s moving reference frame the occurrence of deformation leads to exponential decorrelation of bed topography that is in dynamic equilibrium with flow conditions for the field and laboratory data sets used correlation decays to 0 5 by the time the bed translates 40 and 360 of the mean bed form length respectively proportions of bed material flux responsible for translation and deformation can be straightforwardly calculated translation flux is measured using the traditional bed form bed load equation deformation flux is determined by excess topographic change scaled by the ratio of horizontal sediment velocity to fall velocity deformation represents the sediment exchanged between bed load and suspended load because deformation is a stochastic process with zero mean the apparent rate of deformation decreases as a function of time interval between bed surveys for the field case deformation accounts for 40 of bed material flux while it is only 1 of the flux in the laboratory