length of flow separation over dunes dimensional analysis was used to define the length of the flow saparation for the case of two dimensional uniform flow over dunes which was expressed in terms of bed form geometry and the sand size of the bed material experiments were conducted in a rectangular flume using artificial dunes and a flow visualization method to measure the flow separation length when dunes are flat length of the flow separation is affected by the sand grain roughness when dunes are steep the length of the flow separation is due mainly to the bed form geometry and is relatively constant the reverse rectangular steps can be considered as dunes of infinite length and the length of the flow separation for such steps will always be greater than that obtained for dunes of finite length

general threshold for sediment transport induced by mounds abstract predicting the generation of ripples downstream of bluff obstacles such as sediment mounds is critical to estimating current speed from bedforms and predicting interfacial exchanges of solutes and fine particles herein observations from previous studies of sediment transport downstream of bed roughness on flat sand beds are condensed into a single threshold for the generation of ripples this threshold is based on two dimensionless numbers one describing the impact of the roughness on near bed turbulence and another comparing the bed shear stress to the threshold for sediment motion results from additional flume experiments confirmed this threshold for sediment mounds of both fine and coarse sands measurements of the velocity field near sediment mounds revealed changes in the vortices shed from bed roughness depending on the flow and roughness scales when vortices begin to coalesce the magnitude of velocity fluctuations downstream of the mound increases even though mean conditions are below shields threshold sediment is transported when the local bed shear stress due to vortex shedding exceeds the critical shear stress for initiation of sediment motion including the critical shear stress in the threshold for ripple generation makes it applicable to more complex boundaries such as biologically bound and heterogeneous sediments the threshold for ripple generation and the mechanisms behind it describe a potentially important sediment transport process

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

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

closed conduit bed form initiation and development nonintrusive measurement of closed conduit erodible bed development was undertaken for 12 experiments of ranges of flow strengths and sediment solids sizes analogous to open channel flows wavelets on the sediment bed of a closed conduit are instigated by discontinuities in the bed with wavelet lengths for laminar and turbulent open channel and closed conduit flows given by 175d0 75 where and sediment size d are in millimeters for closed conduit flows ripples and dunes grow from these wavelets at rates increasing with increasing flow strength and utilizing the mechanisms of bed form coalescence and throughpassing to limiting lengths heights steepnesses and bed friction factors that are approximately maintained or possibly decrease thereafter limitation of free surface deformation results in increased rates of bed wave development for closed conduit flows in comparison to open channel flows measured results indicate that equilibrium closed conduit ripple and dune magnitudes can be pre

in situ acoustic measurements of marine gravel threshold and transport measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles the turbulence in the bottom boundary layer was measured using two electromagnetic current meters and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material visual estimates of bedload were also obtained with an underwater tv camera the acoustic technique unlike a conventional bedload sampler has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected this has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions the results of the study show that of the turbulent bursting events which contribute towards the reynolds stress only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material the measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process

time as an independent variable for current ripples developing towards linguoid equilibrium morphology flume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non equilibrium bedforms the time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity in the transitional stage from current ripples to upper stage plane bed i e washed out ripple stage only the equilibrium wavelength remains constant whereas equilibrium height rapidly decreases to zero our observations imply that bed roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with caution

flume experiments on lower flow regime bed forms in coarse sand thirty five runs were made in a recirculating channel 11 5 m long and 0 92 m wide with four sizes of very well sorted coarse sand to study geometry migration and hydraulics of bed configurations in equilibrium with steady uniform open channel flows at flow depths of about 15 cm equilibrium energy slope and sediment transport rate were measured as well bed phases kinds of bed configurations studied were ripples low flow velocity flat beds with sand transport and dunes identical with megaripples current velocities in none of the runs were high enough for development of an upper flow regime flat bed a plot of mean flow velocity versus sand size for all the runs shows that at low flow velocities the ripple field narrows with increasing sand size and pinches out at about 0 7 mm the field for flatbed transport narrows with decreasing sand size and pinches out at about 0 5 mm just above the ripple field at high flow velocities dunes are the stable phase for all sand sizes studied dune configurations are divided into two subphases two dimensional dunes 2d dunes with fairly straight continuous even crests and no strong localized scour in troughs are formed at relatively low flow velocities three dimensional dunes 3d dunes with strongly sinuous discontinuous uneven crests and strong localized scour pits in troughs are formed at relatively high flow velocities 3d dunes tend to be higher than 2d dunes have larger height spacing ratios and show less variability in height spacing and migration rate dunes can be viewed as kinematic shock waves differences between 2d dunes and 3d dunes lie in the differing importance of shock wave coupling and of sand transport in bed form troughs in both geometry and hydraulic ralationships the 2d dunes and 3d dunes in our flume runs correspond to the relatively regular megaripples at low flow velocities and relatively irregular megaripples at high flow velocities described by other authors from intertidal environments

a flume experiment on the development of subaqueous fine gravel dunes from a lower stage plane bed 1 determining the hydraulic conditions whereby gravel dunes first develop in subaqueous environments is fundamental as their presence may influence engineering solutions designed to maintain bed stability in addition estimates of the flow conditions associated with preserved gravel bed forms in sedimentary sequences are useful for reconstructing the depositional environments and geometries of for example oil and gas bearing geological strata consequently a series of experiments considered dune initiation in these experiments defects and latterly incipient dunes developed from lower stage plane gravel beds during near threshold conditions of motion crit 1 0 1 016 and long periods of marginal bed load transport rates the three dimensional defects were almost imperceptible positive ovoid features with heights of one or two grain diameters and lengths and spans of a few decimeters after 17 hours of flow incipient low amplitude simple two dimensional dunes developed from the defects with heights ranging between 0 029 and 0 055 m nonequilibrium wavelengths of 1 4 m and spans of 0 6 0 9 m continued development over several days with crit ratios of around 1 3 resulted in near equilibrium two dimensional dunes with wavelengths averaging 2 6 3 5 m and spans equal to the flume width 4 m the inception of incipient dunes could be predicted using bulk flow models however this approach was not suitable for the prediction of defect development near bed turbulence in the form of small scale sweep events of limited breadth controls the initiation of defects but larger scale coherent turbulent structures in the outer flow are related to dune development significantly both defects and incipient dunes can exist at the same time which indicates that the effects of sweeps on the bed morphology persist at the same time as larger scale turbulent structures are beginning to effect sediment transport

subaqueous gravel dunes abstract gravel dunes are rarely reported owing to comparative rarity of occurrence and generic confusion with antidunes and classes of gravel bars a compilation of data from the literature for sediments with a median size greater than 2 mm shows that dunes have been developed in the laboratory in median grain sizes up to 28 6 mm for field conditions there are data for grain sizes up to 60 mm equilibrium and non equilibrium gravel dunes range in length from less than 0 6 m to greater than 100 m heights range from less than 0 1 m to 16 m height and length data for the steepest three dimensional 3 d gravel dunes are consistent with the h l function reported by ashley 1990 for equilibrium dunes developed in sand for a broad range of dune length 3 d gravel dunes are steeper than two dimensional 2 d dunes but for l 100 m and h 8 m there are too few data to draw any conclusion ashley s h l function does not apply to a broad range of 2 d gravel dunes a separate well defined function is proposed to describe the h l relationship for 2 d gravel dunes of maximum steepness there are few data on the hydrodynamic development of gravel dunes as a guide gravel dunes develop for a range of froude numbers up to 0 75 given that the nondimensional mean bed shear stress exceeds circa 0 1 dunes reach their maximum height at 0 25 and have reduced in height when 0 3 froude numbers of 0 84 and above lead to crestal flattening dune diminution and eventual replacement by antidunes or upper stage plane beds however owing to lag effects transitional bedforms have been recorded for froude numbers near the critical value of 1 the development of gravel dune steepness from lower stage plane bed through to upper stage plane bed is well described in only one case by dinehart 1992a fredsoe s 1975 function for equilibrium dune steepness applies reasonably well to dinehart s steepest dunes allen s 1984 bedform existence field based on and a characteristic grain size of the bed sediment has been extended from 5 mm to include bedstocks as coarse as 33 mm

sedimentation and tectonic geomorphology adjacent to major active and inactive normal faults southern greece the active psatha skinos normal fault is uplifting a contrasting variety of lithologies in its footwall including mesozoic limestones ophiolitic peridotites and poorly consolidated plio pleistocene sediments of the inactive megara basin in the latter basin linear marginal uplands border the sedimentary fill and are interpreted as degraded and segmented scarp lines which exhibit features such as facetted spurs drainage domains are defined as groups of drainage basins sharing a common tectono geological substrate contrasting domains in the footwall of the active fault have given rise to spectacular differences in drainage basin growth and form in the uplifting footwall raised coastal terrace levels are present at about 10 and 40 m the latter have yielded corals dated at between 90 126 ka and have enabled us to calculate uplift and erosional rates extrapolation of these rates indicate that some 350 m of footwall relief has been produced over approximately 1 ma most of the sediment eroded from the uplifting footwall is presumed to have been re sedimented into the alkyonides gulf basin plain during this time the prominent footwall unconformity so produced forms a fine modern analogue to features seen in ancient syn rift tilt blocks such as the viking graben footwall uplift has also backtilted the former depositional surface of the megara basin and caused reversal of flow in its axial drainage domain a variety of depositional environments are documented in the subsiding hangingwall to the coastal fault including syn quake talus cones alluvial fans intra fan marshes and swamps truncated fans transgressive barriers and spits

numerical model of turbulent flow over sand dune a two dimensional numerical model based on solution of the reynolds averaged navier stokes equations and the k turbulence model which takes into account sand grain roughness is developed to describe the flow in a fixed dune bed channel the model predicts the velocity and turbulence fields as well as the pressure and friction distributions along the dune the details of the flow in the separation eddy are calculated the model predictions are in general agreement with existing detailed experimental data in a rectangular channel with two dimensional dunes of typical but regular shape the calculated pressure and friction distributions enable determination of the resistance components of the channel without further empiricism however compared to a well known and representative semiempirical engineering formula for the prediction of resistance of natural dune bed channels the two dimensional numerical model predicts significantly different contributions of friction and pressure to the total resista

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

an experimental study of turbulent flow over a low angle dune 1 many large sand bed alluvial channels are dominated by dunes that possess low angle lee sides often 10 which play a critical role in the transportation of sediment and generation of significant bed form roughness despite the fact that these low angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee study of flow associated with low angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms results from a detailed scale model experimental study of flow over a low angle dune which is based on a prototype dune from the fraser river canada present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms two dimensional laser doppler anemometer measurements over a low angle dune maximum lower lee side slope 14 show that dune morphology exerts a dominant control on the turbulent flow causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above the scale model confirms that permanent flow separation does not occur over low angle dunes but instead is replaced by a small region here 7 of the dune wavelength in length of intermittent flow reversal which may be present for up to 4 of the time shear layers generated along this small zone of decelerated and or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes turbulence production associated with low angle dunes is dominated by eddies generated along this shear layer which produce highly variable horizontal and vertical velocities and large reynolds stresses in this region these results show that macroturbulence associated with low angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low angle dunes in natural sand bed rivers these results highlight the need to consider the temporal evolution and intermittency of shear layer behavior often very near the bed when interpreting the generation of macroturbulence and dispersal of sediment associated with low angle dunes

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

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

the role of sediment specific gravity and availability on cluster evolution the objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics spacing and size to isolate the effects of sediment availability and specific gravity on cluster evolution mono sized spheres were used to simulate the cluster evolutionary cycle overall twelve experimental runs were carried out in the laboratory flume six of these tests were performed by using glass spheres specific gravity sg 2 58 and the other six by employing an equal combination of glass and teflon spheres sg 2 12 of the same diameter to evaluate the role of specific gravity on cluster evolution the three sediment availability conditions that were investigated here simulated isolated gravel elements pool riffle sequences and densely packed gravel bed an advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area the results of this study showed that 1 sediment availability affects the architecture and size of cluster microforms and 2 clusters consisting of mono sized sediments start disintegrating at twice the incipient conditions by performing complementary tests for the isolated gravel elements case it was found that the evolutionary cycle of individual clusters could be described as follows in order of increasing stress no cluster two particle cluster comet triangle rhomboid break up copyright 2003 john wiley sons ltd

the morphodynamics of fluvial sand dunes in the river rhine near mainz germany ii hydrodynamics and sediment transport the dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply limited reach of the river rhine during daylong surveys suspended sediment concentration bedload transport rate water depth flow velocity turbulence intensity near bed shear stress and water temperature were recorded over individual isolated dunes this paper considers the hydrodynamic environment and sediment transport over the dunes a companion paper details the sedimentology and morphology of the dunes flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes and gravels are rarely entrained by in bank discharges unsteady and non logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes and the near bed flow demonstrates evidence of large scale coherent low frequency flow structures these may reflect stacked sequences of separated boundary layers generated by secondary dunes however the low amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides bed roughness and near bed shear stress commonly increase steadily over the stoss of dunes but may decrease near the crest especially where a crestal platform exists that is devoid of secondary bedforms bed roughness scales with the physical size of bed roughness elements however variability in roughness lengths is large owing to the composite nature of bed roughness bedload transport over stoss slopes is spatially variable but often shows an increase with increasing bed shear stress over the stoss well formed wakes only develop downstream of lee slopes which are close to the angle of repose otherwise separation is weak and suspension and settling of fine sediments is of little consequence to dune evolution wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side which may be related to vortex shedding from the dune however wake current speeds are extremely low and variable in direction reverse flow is not sustained and no retrogressive bedforms occur in the lee of large dunes near bed shear stress and bed roughness are usually low within the wake reflecting low current speeds weak wake flow reattachment occurs at a variable distance downstream up to several times the duneform height a two dimensional numerical model for flow over dune topography calibrated using average parameter values provided a reasonable description of the flow upstream and over the backs of large dunes as far as the crestal region wake flow could not be modelled however the temporal and spatial complexity of natural three dimensional flow over the dunes resulted in variance in parameter estimates this variance precluded modelling of flow and bedload transport consequently it was not possible to model dune evolution in a deterministic sense

introduction to special section on marine sand wave and river dune dynamics marine sand waves and river dunes are rhythmic patterns formed at the interface between a sandy bed and turbulent flows driven by tidal and river currents their origin development and dynamics are far from being fully understood to enhance the discussion on this topic we organized the international conference marine sand wave and river dune dynamics marid in april 2004 in enschede netherlands scientific papers that originated from the marid conference are presented in this special section this introduction places these papers in the context of recent progress

a flume study on the development and equilibrium morphology of current ripples in very fine sand an empirical model is constructed for the development and equilibrium dimensions of small scale unidirectional bedforms in sand with a median grain size of 0 095 mm based on a series of steady flow experiments in a flume current ripples always attain a linguoid plan morphology with constant average height 13 1 mm and wavelength 115 7 mm provided that sufficient time is allowed for their formation the development pattern of these ripples on a flat bed is independent of flow velocity and involves four stages 1 incipient ripples 2 straight and sinuous ripples 3 non equilibrium linguoid ripples and 4 equilibrium linguoid ripples straight and sinuous ripples are non equilibrium bedforms at all flow velocities the time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours at flow velocities where washed ripples are stable the equilibrium wavelength is similar to that of equilibrium linguoid ripples but the equilibrium height rapidly decreases from 13 1 mm to zero towards upper stage plane bed conditions the results of the flume experiments correspond reasonably well with those of previous studies provided that various complicating factors such as different experimental methods different sediment characteristics shallow flow depths and non equilibrium runs are accounted for

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