mobility of bed material in harris creek 1 the mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable the bed surface remained intact throughout the freshet and scour was limited to small areas of the bed observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature at low flows no size fraction was fully mobile at intermediate flows shear stress 27 pa sand was fully mobile while larger material remained partially mobile at high flows 43 pa the division between the full and partial mobility regimes occurred around 16 mm which remains finer than the median size of the subsurface material the largest material scarcely moves at mean annual flood magnetic tracers confirmed results obtained from the pit traps three distinct zones of mobility can be defined in fractional transport plots partial mobility full mobility and overpassing suspended critical shear stress for incipient motion varied over an order of magnitude a qualitatively similar result for bimodal sediment was reported by wilcock and mcardell 1993 incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface and a lower envelope that corresponds with our nonsimilar entrainment criterion

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

marginal bed load transport in a gravel bed stream sagehen creek california marginal bed load transport describes the condition when relatively few bed particles are moving at any time bed particles resting in the shallowest bed pockets will move when the dimensionless shear stress exceeds a value of about 0 020 as increases the number of bed particles moving increases significant motion of bed particles i e when a substantial fraction of the bed particles are moving occurs when exceeds a value of about 0 060 thus marginal bed load transport occurs over the domain 0 020 0 060 marginal bed load transport rates and associated hydraulic characteristics of sagehen creek a small mountain gravel bed stream were measured on 55 days at discharges ranging from slightly less than one half of the bank full discharge to more than 4 times the bank full discharge dimensionless shear stress varied from 0 032 to 0 042 and bed particles as large as the 80th percentile of the bed surface were transported the relation between reference dimensionless shear stress and relative particle protrusion for sagehen creek was determined by varying ri to obtain the best fit of the parker bed load function to the measured transport rates during the period of record water years 1954 1991 the mean annual quantity of bed load transported past the sagehen creek gage was 24 7 tons forty seven percent of all bed load transported during the 38 years of record occurred in just 6 years during 10 of the 38 years of record essentially no bed load was transported the median diameter of bed load was 26 mm compared to 58 mm in the surface bed material

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

equilibrium pool dimensions and sediment sorting patterns in coarse grained new england channels abstract pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat an understanding of this morphology is necessary for the successful design of a reconstructed or restored channel to better understand pool geometry and sorting characteristics of new england channels characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions characterizations of channel bed sediments were also performed at 34 of these locations data from 120 pools were used in least squares multiple regression analysis with 19 different independent variables to develop prediction equations for pool depth length and exit slope gradient these equations were then tested using the 25 remaining pools eighteen different geometric variables were then used to predict sediment 50 values in pool centers pool exit slopes and riffles for the subset of 34 samples the regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit slope width constriction gradient constriction width drainage area upstream channel width and the exit slope expansion ratio with an r 2 value of 0 53 similarly pool length is influenced by channel gradient location of the channel constriction pool width drainage area and constriction length with an r 2 value of 0 72 the gradient of pool exit slope is influenced by drainage area channel gradient and constriction gradient with an r 2 value of 0 34 however r 2 values for exit slope increased to 0 65 when sediment d 50 data were utilized using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero although moderate variations do exist a tukey kramer hsd comparison of the means for sediment 50 values indicates pool sediments are significantly coarser than pool exit slope and riffle sediments for the subset of 34 pools regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with r 2 values that range from 0 45 to 0 62 the results suggest that pool dimensions are related to basin wide variability and local hydraulic conditions the findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool riffle morphology

eolian ripples as examples of self organization in geomorphological systems abstract ripples arise spontaneoulsy from sandy beds subjected to winds sufficient to cause the saltation of surficial grains while the initially fastest growing wavelengths are 4 6 times the mean hop length of grains impelled downwind by long trajectory high energy saltating grains the ripple wavelengths developed over long periods can be several times this numerical simulations based upon techniques pioneered by p k haff and coworkers reveal that a complex process of ripple coalescence is responsible for this evolution of the ripple field small fast moving ripples catch up with and are absorbed by larger slower forms each such merger resulting in a growth of the mean wavelength and a decline in the relative dispersion of the wavelengths an heuristic argument is presented which accounts for the fast initial growth and subsequent slowing of growth of the mean wavelength as the ripple field evolves ripple cross sectional shape appears to be controlled in large part by a distrianment function which represents the declining likelihood of a grain stopping on a surface as it slopes more steeply downwind that the resulting asymptotic wavelength in the simulations depends strongly upon impact angle supports sharp s 1963 suggestion that the length of the shadow zone plays an important role in controlling the final ripple wavelength

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

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

spatial autoregressive structure in meander evolution the migration of a river bend depends in part on the high flow velocities that characteristically impinge on its outside bank recent models have treated this in terms of a spatial convolution whereby local bend migration is mathematically a weighted aggregate of up stream curvature and bed topography the convolution model can be tested using river migration data after it is discretized and recast into a finite autoregressive form published isochrones marking former positions of bends on the beatton river canada support the hypothesis that rates of bend migration follow a convolutional relation in addition a comparison of the underlying flow model with published flume experiments involving constant curvature bends illustrates how it predicts the near bank depth averaged velocity associated with a forced bar topography in absence of free bars the autoregressive form of the model is equivalent to a stochastic linear difference equation this allows bend curvature to be treated as a random process cast in the frequency domain the convolution model predicts that big bends tend to grow at the expense of little bends and curvature irregularities in complex trains there exists no tendency for preferential growth of an intermediate bend size the model also predicts the well known shift of maximum migration rates to positions down stream of curvature apexes and implies that the magnitude of this shift increases with decreasing bend size predicted shifts compare well with published measured shifts on the nishnabotna river iowa the sensitivity of the meandering process to initial bend geometries and entrance flow conditions ensures that diverse bend shapes arise along freely migrating rivers independently of factors such as unsteady flow and nonuniform erodibility no single geometrical form serves as an asymptotic evolutionary state for individual bends

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

virtual rate and mean distance of travel of individual clasts in gravel bed channels travel distances in gravel bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge mean distance of movement irrespective of grain size is weakly correlated with stream power especially near the threshold of movement there may be several reasons for the weak correlation including variable effects of bed structure varying magnitudes of sediment mobilizing events and sampling problems grain size itself is of marginal significance the virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period better results are obtained by relating the virtual rate of travel to the first peak of the flow event only this implies that the initial seeding of the tagged particles dominates the observations nonetheless an underlying general relation is indicated by the data which are derived from a wide range of flow regime types

evolution of coarse gravel bed forms field measurements at flood stage field measurements to investigate the origin and growth of mesoscale gravel bed forms in deep flows were made in the north fork toutle river washington sonar observations of the gravel streambed at a stationary point were recorded during two storm flows in december 1989 and january 1990 with concurrent bed load sampling and continuous velocity measurements mean diameter of bed load was about 3 cm flow depths were 1 4 2 4 m and bed shear stresses were 2 5 times the critical stress of mean bed load diameter as computed from the depth slope product these records document the hydrodynamic conditions under which dunelike coarse gravel bed forms were observed coarse gravel dunes height 20 cm length 6 15 m evolved more than 24 hours after peak stage primarily by accretion as inferred from bed form changes revealed in dual sonar records dune heights increased to 40 cm as mean trough elevation rose about 50 cm over several hours smaller dunes wavelength 1 3 m transitional from bed load sheets migrated on the backs of the large dunes the superposed dunes finally became indistinguishable from the large dunes which diminished in height by increasing the mean level of troughs gravel deposition occurred at the observation point in conjunction with migration of gravel dunes the direct comparison of known bed form regimes and gravel bar facies provides alternative interpretations of gravelly deposits

estimating local bed shear stress from velocity observations replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements the bed shear velocity u can be estimated within 3 using the depth averaged velocity in the vertically averaged logarithmic velocity profile to be accurate this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure estimates of u made using a single near bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation accuracy of this method requires appropriate flow conditions only near the bed so it may be applied in a wider range of flow conditions including spatially variable flow estimates of u from the slope of the near bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roughness different methods for estimating roe should be useful in judging the precision of estimates made under similar conditions these estimates of precision together with considerations of convenience and model accuracy provide the basis for select ing the most appropriate or advantageous method for different conditions and purposes

modelling gravel bed topography from sediment transport a simple queueing model which generates bed topography consistent with the mechanics of gravel motion is presented the equations on which it is based are derived from the application of simple theoretical ideas and an analysis of published flume data meland and norrman 1966 francis 1973 abbott and francis 1977 the simulation produces an extremely variable bed topography in which at least two scales of bedform may be identified features of 5 10 grain wavelengths similar to pebble clusters are superimposed on other forms whose wavelength is up to 30 40 grains and which resemble step pools systems and antidunes sediment transport shows many of the characteristics found in the field and the model is therefore thought to provide a useful starting point from which to examine the interaction between flow sediment transport and bedforms in gravel bed and cobble streams

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

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

partial transport of a sand gravel sediment grains of a single size within a mixed size bed are entrained over a range of flows within this range some grains exposed on the bed surface are active entrained at least once over the duration of a transport event while the remaining surface grains are immobile a condition we define as partial transport we demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand gravel mixture the active proportion of the bed surface increases with bed shear stress 0 at a given 0 90 of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed mobilization of grains in a size fraction increases from 10 to 90 over a range of 0 of a factor of 2 the bounds of this range increase with grain size di so that at a given 0 sizes over a range of a factor of 4 are in a state of partial transport fractional transport rates are independent of di for fully mobilized fractions and decrease rapidly with di for partially mobile fractions partial transport is associated with substantial transport rates of finer fully mobile sizes limits both the rate and size distribution of grain exchange with the bed subsurface and may be the dominant transport condition in many gravel bed rivers

the properties of bed sediments in pools and riffles abstract the size and shape characteristics of the surface bed sediments in a sequence of four pools and four riffles were investigated in a small cobble bed stream the riffle sediments are both coarser and better sorted than the pool sediments these differences are explained in terms of reversal of the bottom velocity shear stress hierarchy between pools and riffles at high flow riffle sediments are also less spherical than pool sediments but the differences in shape are entirely size dependent thus it appears that sorting occurs by size but not by shape as sediments are deposited in pools and riffles

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

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

simulation of individual particle movement in a gravel streambed a two dimensional simulation model of travel distances of individual particles in a gravel bed river is presented the model is based on a number of rules which include particle size entrainment trajectory distance of movement and entrapment particle interactions are controlled by resistance fields defined about each obstacle and critical elevation defined in the model resistance fields particle dropping and critical elevation rules control particle interactions the interaction rules cause the particles to develop pebble clusters stone cells and transverse structures transverse ribs the simulated travel distances of individual particles are consistent with reported field results individual particle travel distances were simulated using two different models one without interactions between the individual particles and the stationary bed and one with interactions the case without interactions demonstrates the random nature of sediment transport and narrow ranges of travel distances wider ranges of travel distances similar to those for natural situations were obtained for the cases with interactions the more intense the interaction between the mobile stones and the stationary ones the wider the range of distances of travel for a given particle size modelling the mean travel distance yielded a result similar to that published previously which was based on empirical data well developed bed surface structures were obtained for relatively poorly sorted sediment with intense interactions between particles transverse structures developed when relatively large particles were allowed to move copyright 2002 john wiley sons ltd

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

grain size segregation and stratigraphy in aeolian ripples modelled with a cellular automaton aeolian ripples are distinguished by coarse grained crests finegrained troughs and thin veneers of coarse grains on the upwind stoss slopes migration of these sorted bedforms during periods of net deposition results in a characteristic inversely graded stratigraphy that distinguishes aeolian sandstones from those of fluvial origin1 here we investigate the formation and evolution of aeolian ripples using a cellular automaton model of a sandbed which incorporates two grain sizes and is subject to the impacts of episodically hopping saltating grains using this model we identify the physical processes responsible for both the spatial sorting and the stratigraphic signature of the ripples high energy saltating grains are found to eject small grains preferentially from the impact site leading to a coarsening of the heavily bombarded stoss slopes coarse grains do not in general leap far enough to escape the shadow zones on the downwind lee slopes and therefore tend to accumulate and cycle around the ripple crests small grains on the other hand are ejected at higher velocities enabling them to hop further incorporating net deposition into the model results in a stratigraphy closely resembling thin planar laminae pinstriping found in aeolian sandstones the inverse grading of grain sizes is a direct consequence of their different hop lengths

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