preservation of cross sets due to migration of current ripples over aggrading and non aggrading beds comparison of experimental data with theory an experimental study of the preservation of cross sets during the migration of current ripples under aggrading and non aggrading conditions was conducted in order to test the modified paola borgman theory for distribution of cross set thickness as a function of distribution of bed wave height in a series of flume experiments the geometry and migration characteristics of the ripples did not vary systematically with aggradation rate and are comparable to other flume and river data mean cross set thickness mean formative bed wave height is less than 0ae4 and mean cross set thickness mean bed wave height is less than 0ae53 in the present experiments the primary control of cross set thickness is the variability of ripple height aggradation rate accounts for only 1 7 of the total cross set thickness a two parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified paola borgman model this model underestimates cross set thickness because of its assumption that bed form height spreads evenly above and below the mean bed level which is not the case in reality mean cross set thickness is predicted quite well if the model constant is increased to 1ae3

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

polydisperse suspensions erosion deposition and flow capacity deposition from particle laden flows is often described in terms of the capacity and competence of the flow but robust definitions of these terms have proved elusive in this paper we provide a mathematical modelling framework within which erosion and deposition of polydisperse sediment and thus flow capacity and competence can be rigorously defined this framework explicitly captures the coupling between the suspension and an active layer of sediment at the top of the bed and is capable of describing both depositional and erosional flows over both erodible and non erodible beds crucially the capacity of a flow is shown to depend on the erosional and depositional history because these processes determine the composition of the active layer this dependence is explored within models of bidisperse and polydisperse suspensions it is further demonstrated that monodisperse representations of suspended sediment transport may severely underpredict actual flow capacity the polydisperse model is validated against recent experimental studies of the evolution of suspended material in waning turbulent flows and is used to demonstrate that loss of capacity is the principal driver of sediment deposition

experimental study on transient and steady state dynamics of bedforms in supply limited configuration the purpose of the present study is to investigate experimentally the development of bedforms in a configuration where the sediment supply is limited the experimental setup is a rectangular closed duct combining an innovative system to control the rate of sediment supply qin and a digitizing system to measure in real time the 3d bedform topography we carried out different sets of experiments with two sediment sizes 100 mm and 500 mm varying both the sediment supply and the water flow rate to obtain a total of 46 different configurations after a transient phase steady sub centimeter bedforms of various shapes have been observed barchans dunes straight transverse dune linguoid transverse dunes and bedload sheets height spacing migration speed and mean bed elevation of the equilibrium bedforms were measured for a given flow rate two regimes were identified with fine sediment i a monotonic increasing regime where the equilibrium bedform height and velocity increase with the sediment supply rate qin and ii an invariant regime for which both parameters are almost independent of qin for coarse sediment only the first regime is observed we interpret the saturation of height and velocity for fine sediment bedforms as the transition from a supply limited regime to a transport limited regime in which the bedload flux has reached its maximum value under the prevailing flow conditions we also demonstrate that all experiments can be rescaled if the migration speed and height of the bedforms are respectively divided and multiplied by the cube of the shear velocity this normalization is independent of grain size and of bedform morphology these experimental results provide a new quantification of the factors controlling equilibrium height and migration speed of bedforms in supply limited conditions against which theoretical and numerical models can be tested

development of bed features the concept of unification the opposite of branching theories used in biological sciences is introduced as a new avenue to investigate the behavior of bed features in alluvial channels under unidirectional flow the approach starts with the theoretical concepts initiated by exner according to which the features propagate at speeds inversely proportional to their heights this leads to coalescence and rearrangement of the general pattern of bed features numerical experiments show that an initial large number of arbitrary bed disturbances rapidly reduces to a small number if one feature reaches the maximum height for given flow conditions smaller features pass through it an initial uniform distribution of heights of disturbances slowly changes into a broad distribution of heights with some at the maximum height similar to what is observed in nature

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

conditions for formation of massive turbiditic sandstones by primary depositional processes it can be shown from bedform stability diagrams that each turbidity current regardless of whether its density is low or high or whether its history is simple e g surging flow or complex e g sustained flow should pass through a final waning phase in which flow conditions are prone to bedform development yet many thick deep marine sandstones of supposed turbidity current origin are totally devoid of tractional structures such as plane parallel lamination and ripple cross lamination tdure a mathematical model for the calculation of duration of sediment deposition from high density turbidity currents hdtcs is used to show that the absence of bouma tb and tc divisions cannot be explained by rapid temporal flow deceleration only the model calculations imply that discernible tractional structures are formed even at residence times within the upper stage plane bed and ripple stability regimes of several seconds and several tens of seconds respectively therefore other explanations have to be sought for the absence of lamination in turbiditic massive sandstones these include incompatible grain size post depositional removal or obstruction of lamination by bioturbation and liquefaction and erosion of laminated divisions followed by rapid burial

scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds scour holes often form in shallow flows over sand on the beach and in morphodynamic scale experiments of river reaches deltas and estuarine landscapes the scour holes are on average 2 cm deep and 5 cm long regardless of the flow depth and appear to occur under similar conditions as current ripples at low boundary reynolds numbers in fine sand and under relatively low sediment mobility in landscape experiments where the flow is only about 1 cm deep such scours may be unrealistically large have unnatural effects on channel formation bar pattern and stratigraphy this study tests the hypotheses that both scours and ripples occur in the same conditions and that the roughness added by sediment saltation explains the difference between the ripple dune transition and the clear water hydraulic smooth rough transition about 500 experiments are presented with a range of sediment types sediment mobility and obstructions to provoke scour holes or removal thereof to assess scour hole persistence most experiments confirm that ripples and scour holes both form in the ripple stability field in two different bedform stability diagrams the experiments also show that scours can be provoked by perturbations even below generalised sediment motion moreover the hydraulic smooth rough transition modified with saltation roughness depending on sediment mobility was similar in magnitude and in slope to ripple dune transitions given uncertainties in saltation relations the smooth rough transitions modified for movable bed are empirically equivalent to the ripple dune transitions these results are in agreement with the hypothesis that scours form by turbulence caused by localised flow separation under low boundary reynolds numbers and do not form under generalised flow separation over coarser particles and intense sediment saltation furthermore this suggests that ripples are a superposition of two independent forms periodic bedforms occurring in smooth and rough conditions plus aperiodic scours occurring only in hydraulic smooth conditions this article is protected by copyright all rights reserved

reversed sediment wave migration in the irish sea nw europe a reappraisal of the validity of geometry based predictive modelling and assumptions abstract the observation and prediction of the mobility of the seabed are in high demand with rapidly expanding offshore engineering developments and the increasing urgency to start mitigation against changing coastlines from repeatedly surveyed sediment wave fields in the irish sea average sediment wave migrations have been mapped from which broad scale net sediment transport directions can be derived the proximity of bathymetric highs and lows seems to redirect sediment migration pathways and increases migration rates by a factor 5 on average via current deflection and amplification a few hundred sediment waves were analysed geometrically in this study and compared to their migration behaviour revealing a lack of statistical correlation sediment wave lee slope directions asymmetry indices and other geometrical parameters appear poorly indicative of their degree of mobility in the irish sea a considerable amount of sediment waves counter intuitively migrate in the direction of their gentle stoss slope i e opposite to the expected sense of migration even when considering the limitations of the predominantly 2 dimensional sampling method these results fundamentally challenge established assumptions on the indicative meaning of sediment wave geometry they question the validity and robustness of sediment wave migratory predictions based on single geometrical readings that fail to represent the complexity of local hydrodynamics and partial sediment wave modifications until predictive tools for sediment wave migration are developed that can pass the test in all aquatic environments repeat surveying with sonar systems provides the only reliable way to record true natural variability

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

bedforms and flow resistance of cohesive beds with and without biofilm coating abstractthe bedforms on a biofilm coated sediment biosediment bed are different from those on a noncohesive sediment bed under an identical flow condition and sediment particle size without biofi

transition from storm wave dominated outer shelf to gullied upper slope the mid pliocene orinoco shelf margin south trinidad shelf edge deltas are a key depositional environment for accreting sediment onto shelf margin clinoforms the moruga formation part of the palaeo orinoco shelf margin sedimentary prism of south east trinidad provides new insight into the incremental growth of a pliocene storm wave dominated shelf margin relatively little is known about the mechanisms of sand bypass from the shelf break area of margins and in particular from storm wave dominated margins which are generally characterized by drifting of sand along strike until meeting a canyon or channel the studied st hilaire siltstone and trinity hills sandstone succession is 260 m thick and demonstrates a continuous transition from gullied with turbidites uppermost slope upward to storm wave dominated delta front on the outermost shelf the basal upper slope deposits are dominantly mass transport deposited blocks as well as associated turbidites and debrites with common soft sediment deformed strata the overlying uppermost slope succession exhibits a spectacular set of gullies which are separated by abundant slump scar unconformities tops of rotational slides then filled with debris flow conglomerates and sandy turbidite beds with interbedded mudstones the top of the study succession on the outer shelf area contains repeated upward coarsening sandstone rich parasequences 2 to 15 m thick with abundant hummocky and swaley cross stratification clear evidence of storm swell and storm wave dominated conditions the observations suggest reconstruction of the unstable shelf margin as follows i the aggradational storm wave dominated shelf edge delta front became unstable and collapsed down the slope ii the excavated scars of the shelf margin became gullied but gradually healed aggraded by repeated infilling by debris flows and turbidites and then new gullying and further infilling and iii renewed storm wave dominated delta front prograded out across the healed outer shelf re establishing the newly stabilized shelf margin the moruga formation study along with only a few others in the literature confirms the sediment bypass ability of storm wave dominated reaches of shelf edges despite river dominated deltas being by far the most efficient shelf edge regime for sediment bypass at the shelf break this article is protected by copyright all rights reserved

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

morphology and dynamics of large subtidal dunes in bahia blanca estuary argentina abstract the purpose of this paper is to carry out a detailed analysis of subtidal dune morphology and temporal variability and to estimate dune migration rate to improve knowledge of this topic and so enhance the existing data on different marine environments in the world and especially in south america where this information is limited two swath bathymetry surveys were conducted across a dune field in the bahia blanca estuary argentina morphometric parameters and migration rate according to the dune type were analyzed the field is composed of large dunes exhibiting two morphological configurations which are differentiated into sinuous and barchan dunes the dunes studied are the largest of the estuary with heights and wavelengths greater than 5 m and 130 m respectively the crests of the large dunes are arranged with an orientation perpendicular to the axis of the channel from geometrical analysis of the parameters the dunes show a weakly positive correlation between dune height and wavelength as too between dune height and water depth no clear relationship was observed between maximum height and wavelength parameters with water depth across the estuary the bedforms migrate in the ebb direction with mean rate of 43 m year 1 comparison of our results with previous data shows that during three decades the western boundary of dune field has been displaced 900 m towards the outer estuary however the dune field configuration and distribution of diverse types of bedform appear to be relatively stable

bedforms and bed material transport pathways in the changjiang yangtze estuary abstract bed features were mapped using a thermal depth recorder and a side scan sonar in the changjiang yangtze estuary and flows were observed and bed surface materials were sampled to investigate bed material transport pathways distinct scales and dimensions of dunes were observed in terms of the classification scheme of ashley ashley g m 1990 classification of large scale sub aqueous bedforms a new look at an old problem journal of sedimentary research 61 160 172 forming a dune set in the flood season large dunes occurred predominately in the surveyed area of the estuarine zone and two dimensional dunes predominated in the dry season while three dimensional dunes prevailed in the flood season sharp lee sides of dunes have low angles indicating a significant suspended load transport for the bedform migration and the majority of them face down estuary reflecting the controlling process by the ebb current the bed material migrations in terms of relative maximum gross transport show a satisfactory agreement with the sonar mapping in the dry season the relative maximum gross transport as an independent estimator of migration direction can be used to effectively determine the bed material transport directions in the changjiang estuary

what sets the size of current ripples water flowing over sand in fluvial and marine settings often results in the formation of current ripples found in modern and ancient deposits on earth and mars ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records despite decades of observations of current ripples there is no universal scaling relation to predict their size or to distinguish them from dunes here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows results show that ripples are larger with more viscous fluids coarser grains smaller bed shear stresses and smaller specific gravity of sediment the scaling relation also segregates ripples from dunes highlighting a narrow regime of transitional bedforms that have morphologic properties and sediment transport conditions that overlap with both ripples and dunes our analysis shows that previous absolute size based definitions of ripples and dunes only hold for certain conditions such as water flows transporting siliciclastic grains on earth the new theory allows estimates of ripple sizes in foreign fluids and on other planets including meter scale ripples in methane flows on titan or in viscous brines on mars

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

an experimental study on the ripple dune transition flume experiments were conducted on different bed stages across the ripple dune transition as flow velocity increases an initially flat bed surface made of fairly uniform sandy material is gradually transformed into a two dimensional rippled bed with further increase in velocity two dimensional ripples are replaced by irregular linguoid ripples as the average velocity necessary for the ripple dune transition to occur is imposed on the bed surface these non equilibrium linguoid ripples are further transformed into larger two dimensional dunes for each of these stages across the transition a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated an acoustic doppler velocimeter was used to study the flow characteristics above each bed surface detailed profiles were used along a transect located in the middle of the channel results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields section view they clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition turbulence intensity and reynolds stresses gradually increase throughout the transition two dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed linguoid ripples induce three dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region a very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes the turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability when compared with the rippled beds further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis together with an emphasis on quadrant analysis copyright 2001 john wiley sons ltd

the impact of bedform migration on benthic oxygen fluxes permeable sediments are found wide spread in river beds and on continental shelves the transport of these sediments is forced by bottom water currents and leads to the formation of bedforms such as ripples and dunes the bottom water flow across the bedforms results in pressure gradients that drive pore water flow within the permeable sediment and enhance the supply of reactive substrates for biogeochemical processes this transport reaction system has been extensively studied for the case of stationary bedforms whereas bedform migration the most ubiquitous form of sediment transport has been often ignored to study the impact of sediment transport on pore water flow we incorporated an empirical model of bedform migration into a numerical transport reaction model for porous media using oxygen as reactive solute the modeled oxygen flux changes significantly as soon as the sediment divides into an upper mobile layer migrating bedform and a stationary layer underneath the bedform is increasingly flushed with oxic bottom water whereas pressure gradients and pore water flow reverse at increasing rate underneath the bedform this suppresses net pore water displacement and reduces the oxygen penetration depth up to 90 in effect the overall oxygen uptake decreases significantly with bedform migration although bottom water velocities increase this counterintuitive effect is systematically described for a range of different sediment types current velocities and respiration rates and should be considered in future studies

morphology and flow fields of three dimensional dunes rio parana argentina results from simultaneous multibeam echo sounding and acoustic doppler current profiling 1 most past studies of river dune dynamics have concentrated on two dimensional 2 d bed forms with constant heights and straight crest lines transverse to the flow and their associated turbulent flow structure this morphological simplification imposes inherent limitations on the interpretation and understanding of dune form and flow dynamics in natural channels where dune form is predominantly three dimensional for example studies over 2 d forms neglect the significant influence that lateral flows and secondary circulation may have on the flow structure and thus dune morphology this paper details a field study of a swath of 3 d dunes in the rio parana argentina a large 0 35 km wide 1 2 km long area of dunes was surveyed using a multibeam echo sounder mbes that provided high resolution 3 d detail of the river bed simultaneous with the mbes survey 3 d flow information was obtained with an acoustic doppler current profiler adcp revealing a complicated pattern of dune morphology and associated flow structure within the swath dune three dimensionality appears intimately connected to the morphology of the upstream dune with changes in crest line curvature and crest line bifurcations junctions significantly influencing the downstream dune form dunes with lobe or saddle shaped crest lines were found to have larger more structured regions of vertical velocity with smaller separation zones than more 2 d straight crested dunes these results represent the first integrated study of 3 d dune form and mean flow structure from the field and show several similarities to recent laboratory models of flow over 3 d dunes

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

bedform development in mixed sand mud the contrasting role of cohesive forces in flow and bed abstract the majority of subaqueous sediment on earth consists of mixtures of cohesive clay and cohesionless sand and silt but the role of cohesion on the development and stability of sedimentary bedforms is poorly understood the results of new laboratory flume experiments on bedform development in cohesive mixed sand mud beds are compared with the results of previous experiments in which cohesive forces in high concentration clay flows dominated bedform development even though both series of mixed sand mud experiments were conducted at similar flow velocities the textural and structural properties of the bedforms were sufficiently different to permit the designation of key criteria for identifying bedform generation under cohesive flows against bedform generation on cohesive substrates these criteria are essential for improving bedform size predictions in sediment transport modelling in modern sedimentary environments and for the reconstruction of depositional processes in the geological record the current ripples developing on the cohesive mixed sand mud beds with bed mud fractions of up to 18 were significantly smaller than equivalent bedforms in noncohesive sand moreover the bedform height showed a stronger inversely proportional relationship with initial bed mud fraction than the bedform wavelength this is in contrast with the bedforms developing under the cohesive clay flows which tend to increase in size with increasing suspended clay concentration until the flow turbulence is fully suppressed selective removal of clay from the mixed beds i e clay winnowing was found to be an important process with 82 100 clay entrained into suspension after 2 h of bedform development this winnowing process led to the development of a sand rich armouring layer this armouring layer is inferred to have protected the underlying mixed sand mud from prolonged erosion and in conjunction with strong cohesive forces in the bed may have caused the smaller size of the bedforms winnowing was less efficient for the bedforms developing under the cohesive clay flows where bedforms consisting of muddy sand were more characteristic the winnowed sand was also found to heal irregularly scoured topography thus reestablishing classic quasitriangular bedform shapes in cohesive flows the bedforms had more variable shapes and the healing process was confined to lower transitional plug flows in which strong turbulence is only present close to the sediment bed furthermore the bedforms on the cohesive beds tended to form angle of repose cross lamination whereas low angle cross lamination was more common in bedforms under cohesive flows in general terms erosional bedforms prevail when cohesive forces in the bed dominate bedform dynamics whereas depositional bedforms prevail when cohesive forces in the flow dominate bedform dynamics empirical relationships between the proportion of cohesive mud in the mixed sand mud bed and the development rate and size of the bedforms are defined for future use in field and laboratory studies

subaqueous barchans and plane beds from deposition of quartz silt abstractthe suspension flow of quartz silt geometric mean grain size of 4 15 m in unfavorable conditions for deposition is the motivation for the development of different bed morphologies particles deposit over a plane nonerodible surface and develop well defined barchan ripples barchanoids and plane beds in flume experiments bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport the final bed morphology is controlled by the suspended sediment concentration and running time of the experiment the average dimensions of the bedforms after 1 day experiments are 4 93 cm wide 10 33 cm long and 0 45 cm high cohesive plane beds appear after 2 day and 3 day experiments with very high sediment concentrations 22 5 kg m3 viscous effects are deemed relevant for the formation of the beds

flow and turbulence structure across the ripple dune transition an experiment under mobile bed conditions current knowledge of flow and turbulent processes acting across the sand bed continuum is still unable to unequivocally explain the mechanism s by which ripples become dunes understanding has been improved by comparative high resolution studies undertaken over fixed bedforms at different stages in the continuum however these studies both ignore the role of mobile sediment and do not examine flow structure during the actual transition from ripples to dunes the aims of the paper are i to describe flow and turbulence characteristics acting above mobile bedforms at several stages across the transition and ii to compare these data with those arising from experiments over fixed ripples and dunes laboratory experiments are presented that examine the turbulence structure across seven distinct stages of the transition from ripples to dunes single point acoustic doppler velocimeter sampling at three flow heights above a developing mobile boundary was undertaken time averaged statistics and the instantaneous quadrant record reveal distinct changes in flow structure either side of the change from ripples to dunes initially shear related high frequency vortex shedding dominates turbulence production this increases until two dimensional 2d dunes have formed thereafter turbulence intensities and reynolds stress decline and three dimensional dunes exhibit values found over 2d ripples this is the result of shear layer dampening which occurs when the topographically accelerated downstream velocity increases at a faster rate than flow depth activity at reattachment increases due to high velocity fluid imparting high mass and momentum transfer at the bed and or wake flapping suspended sediment may also play a role in turbulence dampening and bed erosion ejections dominate over sweeps in terms of event frequency but not magnitude strong relationships between inward interactions and sweeps and ejections and outward interactions suggest that mass and momentum exchanges are dependent upon activity in all four quadrants the results contradict the notion present in most physical models that larger bedforms exhibit most shear layer activity consequently an improved model for the ripple dune transition is proposed

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

a new phase diagram for combined flow bedforms abstract combined flows which involve a combination of unidirectional and wave induced oscillatory flow are omnipresent in coastal and lacustrine environments despite the extensive progress made in understanding the bedforms generated by such flows in past studies there still remains a wide range of unexplored flow conditions where the bedform geometry and hence consequent sedimentary structures have not been explored especially for strong unidirectional flows greater than 0 30 m s 1 and intermediate oscillation periods between 2 and 8 seconds to address this gap in our knowledge the stability of bedforms in a 0 25 mm diameter sand bed was studied under pure unidirectional pure oscillatory and combined flow conditions with oscillation periods of 4 5 and 6 s the maximum orbital velocity uo was varied from 0 00 to 1 00 m s 1 while the unidirectional component uu was varied from 0 to 0 50 m s 1 the experimental data collected under unidirectional flows stronger than 0 30 m s 1 allows expansion of current understanding of the bed configurations for current dominated combined flows where the phase boundary between combined flow bedforms and current ripples was uncertain under these flow conditions ten distinctive bedform states can be recognized no motion nm 2d symmetric ripples 2d sr 3d symmetric ripples 3d sr 3d symmetric dunes 3d sr 3d asymmetric ripples 3d ar 3d quasi asymmetric ripples 3d qar 3d asymmetric dunes 3d ad 3d current ripples 3d cr 3d current dunes 3d cd and upper stage plane bed uspb each of these bedform stages is described quantified and characterized in dimensional phase diagrams a complete re evaluation of the nomenclature for combined flow bedforms is proposed which includes their planform and cross sectional geometry in order to better represent the bed morphologies this new nomenclature also unifies past research on bedforms in both unidirectional and oscillatory flows and thus presents a new synopsis of bedforms developed under such flows one of the main changes proposed that allows integration with the nomenclature used in unidirectional flows is the reclassification of large ripples as dunes furthermore the introduction of the planform and cross sectional geometries as properties by which to classify bedforms leads to the definition of a stable phase space for two dimensional symmetrical ripples and three dimensional quasi asymmetrical ripples these new data and analysis allow proposition of a new unified phase diagram for combined flows

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

origin of hysteresis in bed form response to unsteady flows 1 field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge this lagged response produces hysteresis in the relationship between water discharge and bed form geometry to understand these phenomena we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge for an abrupt rise in discharge we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities growth rate slowed as bed forms approached equilibrium with the higher discharge regime after an abrupt discharge drop bed form decay occurred through formation of smaller secondary bed forms in equilibrium with the lower discharge which cannibalized the original relict features we present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes based on equilibrium bed form heights lengths and celerities at low and high flows for rising discharge the model assumes that all bed form collisions result in irreversible merger due to a dispersion of initial celerities for falling discharge we derive a diffusion model for the decay of relict high stage features our models predict the form and time scale of experimental bed form adjustments additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled and measured bed form adjustment time we show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods

return of the fan that never was westphalian turbidite systems in the variscan culm basin bude formation south west england thirty ve years ago harold reading stated 1963 p 69 the precise depositional environment of the bude sandstones is not easy to establish appropriately two of his former students are still arguing whether the environment was a storm affected shelf higgs 1986a 1987 1991 or a deep water fan burne 1969 1995 the paper by burne 1995 questions my shelf interpretation prompting this discussion burne and i agree that deposition took place offshore in a large westphalian lake with occasional marine connections based on faunal aspects and on the lack of evidence for emergence goldring seilacher 1971 lake bude of higgs 1986b 1994 we also agree that the majority of the sandstone beds are turbidites but whereas i argued for river fed turbidites on a shelf with storms accounting for both catastrophic rainfall hence sandy under ows and accompanying waves burne 1995 invokes slump generated turbidites on a deep water fan below storm wavebase the tectonic setting was the northern passive margin of a foreland basin higgs 1991 burne 1995 burne 1995 argues that all of the sedimentary structures interpreted by higgs 1991 as indicating shallow water deposition above storm wavebase can also occur in deep water turbidite settings these structures are quasi symmetrical ripples irregular cross lamination hummocky cross strati cation multidirectional tool marks and mud draped scours rare symmetrical wave ripples occur in the bude formation although burne 1995 p 130 points out that no convincing illustration of them has been published to date his italics in the following discussion i shall sequentially address these and other contentious issues raised by burne 1995 my intentions are threefold 1 to question the basis of burne s fan model 2 to reiterate evidence for my lake shelf model and 3 to explain that the shelf origin has been masked for decades by certain unusual facies characteristics including slurried and slumped beds formed in situ as seismites but widely misinterpreted as debris ows and slumps the shelf vs fan debate is not only interesting academically but also important economically because the easily accessible cliffs of bude are a magni cent natural laboratory in which to gather data for petroleum reservoir models

preservation of autogenic processes and allogenic forcings in set scale aeolian architecture i numerical experiments a reduced complexity aeolian dune stratification model is developed and applied to explore the role of dune morphodynamics in the creation of synthetic sections of aeolian stratigraphy originating from three sets of environmental forcing 1 steady wind transport capacity 2 steady bed aggradation and variable wind transport capacity and 3 steady wind transport capacity and bed aggradation in each scenario the forward motion of initial highly disorganized dunes generates a significant record exclusively containing autogenic signals that arise from early dune growth deformation and merger however continued dune growth scours deeply and shreds all records of early dunes afterward dunes self organize into quasi stable groups forward motion of dune groups create truncate and amalgamate sets and co sets of cross strata quickly forming a second significantly more robust stratigraphic record which preserves a comingling of signals sourced from ongoing autogenic processes and each scenario s specific set of environmental forcings the allogenic boundary conditions of the sand sea although the importance of self organization on modeled aeolian stratification is clear in the few presented scenarios self organization may be throttled via variability within environmental forcings as thoroughly documented within a companion paper cardenas et al this issue therefore additional work is warranted as this numerical experiment only begins to sample possible sets of environmental forcing boundary conditions and initial conditions geomorphic responses and consequential preservation possible within the presented surface stratigraphic dune modeling framework

an anomalous high velocity layer at shallow crustal depths in the narmada zone india summary the narmada zone in central india is a zone of weakness that separates the region of vindhyan meso neoproterozoic deposition to the north from gondwana permo carboniferous lower cretaceous deposits to the south the reinterpretation of analogue seismic refraction data acquired during the early 1980s using 2 d raytracing techniques reveals a basement velocity 5 8 6 0 km s 1 topography suggesting that the narmada zone bounded by the narmada north and narmada south faults is a region of basement uplift a layer of anomalously high velocity 6 5 6 7 km s 1 at depths between 1 5 and 9 0 km appears to be present in the entire region within the narmada zone this layer occurs at shallower depths than outside the narmada zone at two places within the narmada zone this layer is at a depth of about 1 5 km this layer cannot be considered as the top of the lower crust because in this case it should have produced large positive gravity anomalies at the shallowest parts instead these parts correspond to bouguer gravity lows furthermore lower crust at such shallow depths has not been reported from any other part of the indian shield therefore this layer is likely to represent the top of a high velocity mafic body that has different thicknesses in different places

instability theory of sand ripples formed by turbulent shear flows abstracta theory of turbulent shear flow over a sand bed is developed addressing the instability principle of the fluid granular bed interface leading to the formation of ripples the reynolds averaged navier stokes rans equations and the time averaged continuity equation are analyzed using a 1 7 power law of the time averaged streamwise velocity and treating the curvilinear streamlines by the boussinesq approximation the integration of the rans equations leads to a governing dynamical equation of flow over a mobile bed a near bed flow layer of 3 5 times the ripple height is considered being affected by the ripples the dynamical equation of the mobile sand bed is based on the exner s sediment continuity equation in conjunction with the meyer peter and muller bed load transport formula as modified to account for the effect of local bed slope attributable to bed forms the coupled dynamical equations are then analyzed to estimate the parameters for the instability that results in the formation of ripp

a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the development of bedforms under unidirectional oscillatory and combined flows results from temporal changes in sediment transport flow and morphological response in such flows the bedform characteristics for example height wavelength and shape change over time from their initiation to equilibrium with the imposed conditions even if the flow conditions remain unchanged these variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record hence understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record this article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional purely oscillatory and combined flow conditions which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples the present results yield a unified model for bedform development and equilibrium under unidirectional oscillatory and combined flows the experimental results show that the processes of bedform genesis and growth are common to all types of flows and can be characterized into four stages i incipient bedforms ii growing bedforms iii stabilizing bedforms and iv fully developed bedforms furthermore the development path of bedform growth exhibits the same general trend for different flow types for example unidirectional oscillatory and combined flows bedform size for example small versus large ripples bedform shape for example symmetrical or rounded bedform planform geometry for example two dimensional versus three dimensional flow velocities and sediment grain sizes the equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition

duration of deposition from decelerating high density turbidity currents using recent advances in the stability analysis of current ripples a new model for the calculation of duration of sediment deposition from decelerating high density turbidity currents is proposed the model named tdure refines a duration model proposed by allen 1991 j sediment petrol 61 291 295 by calculating the accumulation time of the rippled tc division separately from the massive and plane parallel laminated tab divisions in bouma type turbidites tdure consists of three modules in the first module the accumulation time for theta and tb divisions is approximated by assuming a linear decrease in sedimentation rate with height in the turbidite as in the original model a straight line is fitted through inferred sedimentation rates at the tab and tbc boundaries in the second module angle of climb of ripples thickness of the tc division and grain size distribution are used in empirical relationships between ripple migration rate and a grain related mobility parameter to estimate the accumulation time of the tc division in the third module the expected development of ripple size across the tc division is calculated using empirical relationships between rate of development of ripple height and grain related mobility parameter and subsequently compared with the observed development of ripple size in the turbidite in this way the accuracy of the accumulation time calculated in module 2 can be verified independently tdure was tested using bouma type turbidites from the doheny channel capistrano fm ca usa and the flysch di motta calabria italy accumulation times of 12 and 10 75 min for the tc division and 19 and 16 min for the tabc sequence were calculated for the doheny channel and the flysch di motta turbidites respectively although module 3 underestimates the rate of development of current ripples near the tbcboundary in both beds ripple size at the tcd boundary is calculated accurately the underestimation of development rate may be caused by differences between flow conditions in experimental flumes on which the model calculations are based and turbiditycurrent dynamics q 2000 elsevier science b v all rights reserved

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

ripple formation induced by biogenic mounds abstract bedload and ripple formation at subcritical flows were observed during laboratory studies involving the mound forming head down deposit feeding polychaete cistenides gouldii flume experiments were designed to investigate the conditions resulting in sediment transport within and immediately downstream of biogenic mound assemblages experiments focused on the inputs of mound population density mound height and the presence absence of cistenides the activities of cistenides may generate sediment transport in two ways via particle ejection during feeding or via scour generated by their mounds the first set of experiments were conducted to determine which process was predominantly responsible for ripple formation mounds were required for ripple formation but ripples formed only at the two higher mound densities 150 and 570 per m 2 the second set of experiments used mounds of different densities and heights to investigate density effects one of three final bed states was always attained 1 mounds became and generated migrating ripples 2 mounds changed into ripples which did not migrate beyond the test area called stationary ripples or 3 mounds remained separate the division between migrating ripple and no ripple cases was used to develop a threshold similar to that established by southard and dingler southard j b dingler j r 1971 flume study of ripple propagation behind mounds on flat sand beds sedimentology 16 pp 251 263 for ripple propagation behind isolated mounds our threshold was higher due to the coarser sediments used in this study stationary ripples exhibited a local equilibrium where despite a two fold range of initial mound separation resulting ripples had similar wavelengths this stationary ripple wavelength was smaller than the equilibrium wavelength found for migrating ripples generated from a flat bed the ability of biogenic structures to generate ripples has implications in studies of near bed processes ripples seen in bottom photographs and stratigraphy may not be an absolute indicator of supercritical flows additionally when the topography within patches of mound building worms changes the bed geometry generating interfacial flow and bottom drag is ripples instead of mounds estimates of reworking due to ripple migration greatly outweigh rates of biological reworking observed in the field all of these consequences reinforce the importance of sediment transport induced by biogenic structures

wave ripple development on mixed clay sand substrates effects of clay winnowing and armoring based on bed form experiments in a large scale flume we demonstrate that the rate of development of wave ripples on a mixed sand clay bed under regular waves is significantly lower than on a pure sand bed even at clay fractions as low as 4 2 and that this rate of development decreases exponentially from 4 2 to 7 4 clay these experiments also showed that despite the slow growth of the bed forms in the mixed sand clay the equilibrium length and height of the wave ripples were independent of the initial bed clay fraction given that the ripple crests were composed of pure sand at the end of all the experiments that started with well mixed sand clay it is inferred that the clay was removed from the bed during the development of the wave ripples through winnowing into the water column and possibly also by sieving into the subsurface where the final clay fractions were found to be higher than the initial clay fractions these clay removal processes are interpreted to have facilitated the wave ripples to reach equilibrium lengths and heights that are similar to those in pure sand clay carrying pore flow initiated by pressure gradients between the wave ripple troughs and crests might also have contributed to the accumulation of clay in the sediment below the wave ripples the formation of the clay enriched armoring layer in the substrate is likely to further reduce erosion rates and could influence the dispersion of nutrients and pollutants in coastal seas

pore pressure in a wind swept rippled bed below the suspension threshold toward elucidating how a wavy porous sand bed perturbs a turbulent flow above its surface we record pressure within a permeable material resembling the region just below desert ripples contrasting these delicate measurements with earlier studies on similar impermeable surfaces we run separate tests in a wind tunnel on two sinusoidal porous ripples with aspect ratio of half crest to trough amplitude to wavelength of 3 and 6 for the smaller ratio pore pressure is a function of streamwise distance with a single delayed harmonic decaying exponentially with depth and proportional to wind speed squared the resulting pressure on the porous surface is nearly identical to that on a similar impermeable wave pore pressure variations at the larger aspect ratio are greater and more complicated consistent with the regime map of kuzan et al 1989 the flow separates creating a depression at crests unlike flows on impermeable waves the porous rippled bed diffuses the depression upstream reduces surface pressure gradients and gives rise to a slip velocity thus affecting the turbulent boundary layer pressure gradients within the porous material also generate body forces rising with wind speed squared and ripple aspect ratio partially counteracting gravity around crests thereby facilitating the onset of erosion particularly on ripples of high aspect ratio armored with large surface grains by establishing how pore pressure gradients scale with ripple aspect ratio and wind speed our measurements quantify the internal seepage flow that draws dust and humidity beneath the porous surface

a unified model of ripples and dunes in water and planetary environments subaqueous and aeolian bedforms are ubiquitous on earth and other planetary environments however it is still unclear which hydrodynamic mechanisms lead to the observed variety of morphologies of self organized natural patterns such as ripples dunes or compound bedforms here we present simulations with a coupled hydrodynamic and sediment transport model that resolve the initial and mature stages of subaqueous and aeolian bedform evolution in the limit of large flow thickness we identify two types of bedforms consistent with subaqueous ripples and dunes and separated by a gap in wavelength this gap is explained in terms of an anomalous hydrodynamic response in the structure of the inner boundary layer that leads to a shift of the position of the maximum shear stress from upstream to downstream of the crest this anomaly gradually disappears when the bed becomes hydrodynamically rough by also considering the effect of the spatial relaxation of sediment transport we provide a new unifying framework to compare ripples and dunes in planetary environments to their terrestrial counterparts terrestrial and planetary subaqueous and aeolian ripples and dunes can be compared in a unified framework according to simulations with a hydrodynamic and sediment transport model

directional petrological characterisation of deep marine sandstones using grain fabric and permeability anisotropy methodologies theory application and suggestions for integration abstract different methods for quantifying preferred grain orientation or grain shape fabric in sandstones are reviewed and their value for the reconstruction of palaeocurrent direction and preferred direction of pore fluid flow is reassessed critically based on existing knowledge from literature and new grain fabric data it is shown that petrographic analysis has distinct merits because grain fabric can be determined with low equipment costs and population statistics are based on the orientation of individual grains several recommendations are given how to standardise procedures for acquisition of grain fabric through microscopy these recommendations include 1 the careful selection of cross section orientations to acquire apparent long grain axes that are representative of true long grain axes 2 the preferred measurement of the coarsest grains in cross section and 3 a new statistically founded method to select a subset of grains that is representative of the total population of grains in a sandstone sample magnetic techniques that use anisotropy of magnetic susceptibility ams as a proxy for grain orientation require specialised equipment and are more restricted in statistical evaluation but these methods benefit greatly from being able to measure a large number of grains in a three dimensional space in a short amount of time and with a lower sensitivity to user bias of the two magnetic techniques discussed in this paper the new enhanced ams method in which rock magnetic properties are enhanced by introducing magnetite into the pores and as precipitates on grain surfaces is regarded to be an improvement over the traditional natural ams method in which only the natural assemblage of magnetite particles present in the rock is analysed enhanced ams measures the preferred orientation of essentially all grains in a rock sample thus providing the shape fabric of a hydrodynamically more accurate representation of framework grains than natural ams through a small technical modification the enhanced ams method can be used to obtain a magnetic cast of the pore network of a rock sample so that the resulting ams is controlled by the shape anisotropy of the individual pores it is commonly assumed that long axes of sand grains in sedimentary deposits parallel the direction of the flows that formed the deposits this concept is challenged by a new systematic study of grain fabric in deep marine turbidites it indeed appears that grains in ripple cross laminated divisions of turbidites dominantly attain a preferred orientation parallel to the palaeoflow direction but grains in plane parallel laminated divisions have an additional flow perpendicular mode which suggests a contribution of rolling grain transport three different fabric modes occur in massive structureless sandstones and massive divisions in turbidites apart from flow aligned and flow perpendicular modes there is a statistically significant but hydrodynamically enigmatic flow oblique mode with an average azimuthal deviation of 40 this large variation of fabric types suggests that a similar variation exists in preferred paths of pore fluid flow within turbidites controlled to at least some degree by depositional process this is confirmed by independent laboratory measurements of permeability anisotropy on samples for which grain fabric data are available and thus defines grain fabric analysis as a viable alternative to traditional permeametry techniques

physical scale modelling of jokulhlaups glacial outburst floods with contrasting hydrograph shapes jokulhlaups glacial outburst floods are hazards in many glaciated environments jokulhlaups generated by different trigger mechanisms have different hydrograph shapes field evidence indicates that jokulhlaups with different hydrograph shapes produce different impacts however little attention has been paid to understanding the controls on jokulhlaups with differing hydrograph shapes and their subsequent impacts historically flume studies have been used to investigate fluvial processes under controlled laboratory conditions nevertheless few flume studies have replicated scaled hydrographs and none have attempted to model field jokulhlaups this study documents the technical procedure by which field prototype jokulhlaups were replicated using physical scale modelling techniques an exponentially rising jokulhlaup characterized by a prolonged rising stage and a rapid falling stage and a linearly rising jokulhlaup characterized by a rapid rising stage and a prolonged falling stage the parameters of water discharge time discharge acceleration and deceleration rate sediment size and sediment discharge were scaled to a 1 20 ratio better sorted sediment more uniform downstream sediment thickness and more gradual aggradation and deposition rates resulted from the prolonged rising stage of the exponentially rising hydrograph more time was available during the prolonged rising stage of the exponentially rising hydrograph for bedload sheets to migrate and for sand ripples to develop rapid rising stage deposition during the linearly rising hydrograph produced thick proximal deposits limited bedload sheet migration massive sand sheets and structureless and poorly sorted sediments during the prolonged falling of the linearly rising hydrograph sufficient time was available to support sand ripple development the study highlights that jokulhlaup hydrograph shape is important for controlling rates of sediment acquisition transport deposition erosion and bedform development the outcomes of these experimental investigations indicate that modelling of high magnitude floods is feasible in a laboratory flume this technical approach has proved very beneficial for developing a greater understanding of flood impacts and the processes that operate during jokulhlaups copyright 2006 john wiley sons ltd

styles of interaction between aeolian fluvial and shallow marine environments in the pennsylvanian to permian lower cutler beds south east utah usa the pennsylvanian to permian lower cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the paradox foreland basin fill exposed in and around the canyonlands region of south east utah aeolian facies comprise i sets and compound cosets of trough cross bedded dune sandstone dominated by grain flow and translatent wind ripple strata ii interdune strata characterized by sandstone siltstone and mudstone interbeds with wind ripple wavy and horizontal planar laminated strata resulting from accumulation on a range of dry damp or wet substrate types in the flats and hollows between migrating dunes and iii extensive near flat lying wind rippled sandsheet strata fluvial facies comprise channel fill sandstones lag conglomerates and finer grained overbank sheet flood deposits shallow marine facies comprise carbonate ramp limestones tidal sand ridges and bioturbated marine mudstones during episodes of sand sea construction and accumulation compound transverse dunes migrated primarily to the south and south east whereas south westerly flowing fluvial systems periodically punctuated the dune fields from the north east several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds these surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo water table during episodes of aeolian quiescence fluvial systems became more widespread forming unconfined braid plains that fed sediment to a coastline that lay to the south west and which ran approximately north west to south east for at least 200 km shallow marine systems repeatedly transgressed across the broad low relief coastal plain on at least 10 separate occasions resulting in the systematic preservation of units of marine limestone and calcarenite between units of non marine aeolian and fluvial strata to form a series of depositional cycles the top of the lower cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north eastward directed marine transgression into the basin prior to the onset of exclusively non marine sedimentation of the overlying cedar mesa sandstone styles of interaction between aeolian fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small scale autocyclic behaviour of competing coeval depositional systems and larger scale allocyclic changes that record system response to longer term interdependent variations in climatic and eustatic controlling mechanisms the architectural relationships and system interactions observed in the lower cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea level and that these two external controls probably underwent cyclical change in harmony with each other in the paradox basin during late pennsylvanian and permian times this observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio eustatic driving mechanism

the development of small scale bedforms in tidal environments an empirical model for unsteady flow and its applications dimensions and plan morphology of current ripples are generally considered to vary with flow velocity and grain size recently however it has been shown that for sand of d50 0 095 and 0 238 mm the equilibrium dimensions are identical at all velocities within the stability field of ripples and that the plan form of equilibrium ripples is linguoid on this basis an empirical unsteady flow model has been developed and tested with flume experiments in order to predict ripple development in natural depositional environments the model includes the development of washed out ripples and upper stage plane bed the unsteady flow model explains the development and preservation of small scale bedforms in various tidal environments more accurately than previous models such bedforms can serve therefore as indicators of prevailing hydrodynamic conditions

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

can liquefied debris flows deposit clean sand over large areas of sea floor field evidence from the marnoso arenacea formation italian apennines the marnoso arenacea formation in the italian apennines is the only ancient rock sequence where individual submarine sediment density flow deposits have been mapped out in detail for over 100 km bed correlations provide new insight into how submarine flows deposit sand because bed architecture and sandstone shape provide an independent test of depositional process models this test is important because it can be difficult or impossible to infer depositional process unambiguously from characteristics seen at just one outcrop especially for massive clean sandstone intervals whose origin has been controversial beds have three different types of geometries facies tracts in downflow oriented transects facies tracts 1 and 2 contain clean graded and ungraded massive sandstone deposited incrementally by turbidity currents and these intervals taper relatively gradually downflow mud rich sand deposited by cohesive debris flow occurs in the distal part of facies tract 2 facies tract 3 contains clean sandstone with a distinctive swirly fabric formed by patches of coarser and better sorted grains that most likely records pervasive liquefaction this type of clean sandstone can extend for up to 30 km before pinching out relatively abruptly this abrupt pinch out suggests that this clean sand was deposited by debris flow in some beds there are downflow transitions from turbidite sandstone into clean debrite sandstone suggesting that debris flows formed by transformation from high density turbidity currents however outsize clasts in one particular debrite are too large and dense to have been carried by an initial turbidity current suggesting that this debris flow ran out for at least 15 km field data indicate that liquefied debris flows can sometimes deposit clean sand over large 10 to 30 km expanses of sea floor and that these clean debrite sand layers can terminate abruptly

new evidence and interpretation of facies provenance and geochemistry of late triassic early cretaceous tethyan deep water passive margin related sedimentary rocks ayios photios group sw cyprus in the context of eastern mediterranean geodynamics abstract the late triassic early cretaceous deep sea sedimentary succession exposed in western cyprus is used here to help understand the wider context of continental break up and seafloor spreading in the easternmost mediterranean region the restored sequence begins with late triassic base of slope sandstone turbidites and mudrocks and fines upwards into mudrocks distal calciturbidites and metalliferous sediments after a hiatus late jurassic early cretaceous non calcareous mudstones and radiolarites accumulated beneath the carbonate compensation depth together with redeposited shallow water derived carbonates including calcirudites the succession culminates in late jurassic early cretaceous very thickly bedded and highly mature quartzarenites that accumulated by mass flow processes the fine grained siliciclastic sediments are terrigenous in origin as indicated by whole rock trace element and ree analysis fe mn and trace metal enrichment in the late triassic represents hydrothermal input related to submarine volcanism that was probably triggered by the nearby onset of seafloor spreading the late triassic sandstones were derived from a low to intermediate grade metamorphic source terrane the late jurassic early cretaceous sandstones reflect extreme physical and chemical maturity in the source area followed by transport and mass flow to a deep water base of slope setting regional facies variation and locally available palaeocurrent data suggest sediment supply originally from a continental block no more than tens of km to the west or north similar successions are exposed in w turkey represented by the antalya complex in the light of alternatives the sedimentary succession is restored as the base of slope apron of a rifted microcontinental block adjacent to the northern margin of the s neotethys the overall succession indicates unstable passive margin development after regional latest triassic early jurassic continental break up with implications for plate reconstructions and hydrocarbon exploration

quantitative analysis of submarine flow deposit shape in the marnoso arenacea formation what is the signature of hindered settling from dense near bed layers abstract submarine sediment density flows are one of the volumetrically most important processes for sediment transport across earth the sediment concentration of flows that reach the deep ocean has never been measured directly and understanding these long runout flows remains a major challenge the miocene marnoso arenacea formation in the italian apennines is the only ancient sequence where individual submarine sediment density flow deposits single beds have been mapped out for more than 100 km down flow here we document the external shape and internal architecture of thirty two individual beds that record flow evolution and can be compared to deposit shapes in mathematical or experimental models the large number of beds allows modes of flow behavior to be identified larger volume turbidites are typically dominated by massive ta or planar laminated tb sandstone intervals that have a broad thickness maximum this shape is important because it suggests that massive and planar laminated sandstones record hindered settling from dense near bed layers which have high 10 by volume sediment concentrations previously some authors have inferred that planar laminated sandstones tb are deposited mainly by dilute flows the position of the broad thickness maximum moves basinward as the volume of sand in the flow increases this is consistent with mathematical modeling that suggests the position of the thickness maximum depends on flow thickness flow speed and sediment settling velocity as well as sediment concentration variations in seawater entrainment rate and local changes in seafloor gradient smaller volume turbidite sandstone intervals are finer grained and dominated by ripple cross lamination tc and have a near exponential decay in thickness that is consistent with deposition from a dilute sediment suspension the rate of near exponential thinning is controlled by sandstone volume in contrast turbidite mudstone intervals show an approximately linear increase in thickness with distance flows that entered the basin in opposite directions produced turbidite mudstone intervals that thicken towards the same location indicating that muddy turbidity currents can drain back over long distances to basinal lows

the south westland basin seismic stratigraphy basin geometry and evolution of a foreland basin within the southern alps collision zone new zealand abstract this paper develops further the case for a foreland basin origin of south westland basin located adjacent to the southern alps mountain belt geohistory analyses show middle miocene initiation of subsidence in the basin with marked increases at 5 6 ma five seismic reflection horizons including basement middle miocene top awarua limestone top miocene mid pliocene ppb and mid pleistocene ppa have been mapped through the grid of seismic data a series of five back stripped structure contour maps taken together with five isopach maps show that prior to the middle miocene subsidence and sedimentation occurred mainly along the rifted continental margin of the challenger plateau facing the tasman sea subsequently it shifted to a foredeep trending parallel to the southern alps and located northwest of them through the late miocene recent this depocentre has progressively widened and the loci of thickest sediment accumulation have moved northwestward most prominently during the late pliocene and pleistocene with the progradation of a shelf slope complex at the northern end of the basin the shelf slope break is currently located over the forebulge which appears not to have migrated significantly probably because the mountain belt is not advancing significantly northwestwards modelling of the lithospheric flexure of the basement surface normal to the trend of the basin establishes values of 3 1 to 9 8 10 20 n m for the flexural rigidity of the australia plate this is at the very low end of rigidities for plates and 1 2 orders of magnitude less than for the australia plate beneath the taranaki basin maps of tectonic subsidence where the influence of sediment loading is removed also clearly identify the source of the loading as lying within or beneath the mountain belt the basin fill shows a stratigraphic architecture typical of underfilled ancient peripheral foreland basins this comprises transgressive basal unconformity thin limestone slope depth mudstone flysch sequence and regressive prograding shelf slope complex followed by molasse deposits components in addition the inner margin of the basin has been inverted as a result of becoming involved in the mountain building as revealed earlier by fission track thermochronological data the timing and degree of inversion fits well with the geometrical and stratigraphic development of the basin that the inversion zone and the coastal plain underlain by molasse deposits are narrow and most of the basin is beneath the sea highlights this as an underfilled active foreland basin the basin is geodynamically part of the southern alps collision zone

alluvial eolian and lacustrine sedimentology of a paleoproterozoic half graben baker lake basin nunavut canada the northeast trending baker lake sub basin was a volcanically active half graben during deposition of ca 1 85 1 76 ga baker lake group drainage was oriented along transverse and axial directions with flow to playa lake and deeper perennial lacustrine depocentres basin marginal streamflow dominated alluvial fans were concentrated along the southern margin and provided sediment from archean crystalline basement rocks these fed transverse gravel and sand bed braided streams alluvial dynamics were characterized by channel aggradation and abandonment abandoned channel belts were sites of floodplain and eolian deposition basin axial braided streams fed northeast and southwest to a depocentre near christopher island where eolian playa and lacustrine environments were intimately linked felsic minette flows were initially erupted from localized centres contemporaneous sedimentary deposits typically contain minor volcaniclastic components that increase in abundance basinward voluminous and widespread younger minette flows prograded outward from volcanic centres contributing significant additional basin infill

role of latitudinal shift and climate change in evolution of red and yellow palaeosols of the himalayas implications for early oligocene seasonality and mid miocene enhanced precipitation

evolution of gravel surfaces in a sediment recirculating flume the assumption of the equilibrium state of gravel surfaces in flume experiments under feeding or recirculating conditions is generally justified by three equilibrium criteria based on sediment transport slopes and bed features when these parametersbecome stable an experiment is expected to reach equilibrium this equilibrium state however is based on a one dimensional 1d model the exner equation which may not truly reflect the equilibrium state of the system considering the complex flow and sediment processes in this paper the evolutionary process of a gravel surface is investigated based on a large scale recirculating flume experiment the performances of the three equilibrium criteria are evaluated first and then the evolution of the bed morphology is studied the key findings include the following 1 the sediment transport rate slopes of water and bed surfaces and 1d morphological features reach equilibrium roughly simultaneously 2 two dimensional 2d morphology continually evolves after these characteristics reach equilibrium which is confirmed by the characteristics of the sediment transport process and 3 the results from a numerical simulation suggest that a much longer time is required to reach an equilibrium state our results suggest that sufficient experimental time is required to investigate the equilibrium morphological characteristics of gravel surfaces which is much longer than the equilibrium time reflected by the 1d equilibrium criteria

the transition between sheet like lobe and basin plain turbidites in the hecho basin south central pyrenees spain genetic facies analysis based on bed by bed correlations from sheet like lobes to the basin plain in some hecho turbidite systems demonstrates that at least 50 of the flows building the sheet like lobes kept moving downcurrent to the closely related basin plain and underwent flow transformations interpreted to have resulted from interaction with topography at the basin margin s that gave rise to specific facies these facies form a new facies tract that replaces the fine grained group of turbidite facies very fine sand to mud and characterizes the basin plain beds beds in the sheet like lobes evolve downcurrent in a way that is predictable by the existing turbidite facies tract models whereas 36 of basin plain beds which account for 78 of the basin plain volume do not the latter have deposits from high density turbidity currents at their bases and typically complete basin floor coverage the new facies tract developed when the flows obliquely encountered the southern foreland margin ramp at the ramp the lower sand laden and high density part of the larger flows was deflected evolving downcurrent along the ramp trend the upper part of the flow more dilute and thicker was reflected from the foreland margin as a train of declining undular bores moving hydraulic jumps subsequent reflections generated against the flanking margins in the closed basin led to ponding which resulted in an overall sheet like stacking pattern across sheet like lobes and the basin plain and is the diagnostic feature of the distal element in the lower sand rich stages of the turbidite systems calcilutites on top of beds interpreted until now as hemipelagites show field evidence of having a turbiditic origin hemiturbidites thus forming the facies capping the new facies tract

bed configurations in steady unidirectional water flows part 3 effects of temperature and gravity abstract a change in bed configuration caused by a change in water temperature without any change in flow and sediment variables in a unidirectional flow is analyzed in the context of the dimensionless depth velocity size diagram discussed in part 2 bed states that differ only in water temperature are termed temperature related states these are attainable from any given state solely by a change in water temperature such a set of bed states forms a curve in the dimensionless depth velocity size diagram this curve a straight line in a log log log plot is termed a temperature change line to predict the change in bed state consequent upon a change in water temperature alone one finds the temperature related state lying along the temperature change line passing through the ori inal bed state in the dimensionless depth velocity size diagram three examples from the literature are analyzed in this way to show how such changes in bed configuration can be put into the context of existing stability relationships among the various bed phases without however addressing the general dynamical problem of why those relationships are as they are the effect of the acceleration of gravity on the bed configuration is also examined from the standpoint of the dimensionless depth velocity size diagram the results show that in a hypothetical open channel water flow on mars a given transition between two adjacent bed phases with increasing flow velocity e g ripples to dunes would take place at a lower velocity on mars than on earth by a factor of 0 74

response of sand ripples to change in oscillatory flow ripples take time to evolve to a new equilibrium state in response to a change in wave generated oscillatory flow the paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled full scale laboratory conditions the experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow in general ripples evolve through a combination of two main processes i from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of slide and merge ii from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of split and merge the experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size the time to reach equilibrium depends strongly on the mobility number and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed this equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution which gives a reasonably good overall agreement with the measurements the model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes

back flow ripples in troughs downstream of unit bars formation preservation and value for interpreting flow conditions back flow ripples are bedforms created within the lee side eddy of a larger bedform with migration directions opposed or oblique to that of the host bedform in the flume experiments described in this article back flow ripples formed in the trough downstream of a unit bar and changed with mean flow velocity varying from small incipient back flow ripples at low velocities to well formed back flow ripples with greater velocity to rapidly migrating transient back flow ripples formed at the greatest velocities tested in these experiments back flow ripples formed at much lower mean back flow velocities than predicted from previously published descriptions this lower threshold mean back flow velocity is attributed to the pattern of velocity variation within the lee side eddy of the host bedform the back flow velocity variations are attributed to vortex shedding from the separation zone wake flapping and increases in the size of and turbulent intensity within the flow separation eddy controlled by the passage of superimposed bedforms approaching the crest of the bar short duration high velocity packets whatever their cause may form back flow ripples if they exceed the minimum bed shear stress for ripple generation for long enough or if much faster may wash them out variation in back flow ripple cross lamination has been observed in the rock record and by comparison with flume observations the preserved back flow ripple morphology may be useful for interpreting formative flow and sediment transport dynamics

bed load transport and boundary roughness changes as competing causes of hysteresis in the relationship between river discharge and seismic amplitude recorded near a steep mountain stream hysteresis in the relationship between bedload transport and river stage is a well documented phenomenon with multiple known causes consequently numerous studies have interpreted hysteresis in the relationship between seismic ground motion near rivers and some measure of flow strength i e discharge or stage as the signature of bedload transport here we test this hypothesis in the erlenbach stream swiss prealps using a metric to quantitatively compare hysteresis in seismic data with hysteresis recorded by geophones attached beneath steel plates within the streambed a well calibrated proxy for direct sediment transport measurements we find that while both the geophones and seismometers demonstrate hysteresis the magnitude and direction of hysteresis are not significantly correlated between these data indicating that the seismic signal at this site is primarily reflecting hysteresis in processes other than sediment transport seismic hysteresis also does not correlate significantly with the magnitude of sediment transport recorded by the geophones contrary to previous studies assumptions we suggest that hydrologic sources and changes in water turbulence for instance due to evolving boundary conditions at the bed rather than changes in sediment transport rates may sometimes contribute to or even dominate the hysteresis observed in seismic amplitudes near steep mountain rivers

transition from foreland to piggyback basin deposition plio pleistocene upper siwalik group shinghar range nw pakistan plio pleistocene synorogenic deposits of the upper siwalik group in the shinghar range trans indus salt ranges of north western pakistan record the transition from foreland basin to piggyback basin deposition on the hangingwall of the salt range thrust the siwalik and upper siwalik groups are over 4 km thick in the shinghar range the lower 3 km consists of the miocene siwalik group which was deposited by a south flowing foreland trunk stream the palaeo indus river the upper 1 5 km consists of the upper siwalik group which is herein divided into three members the lowest member includes deposits of the south flowing palaeo indus river and is distinguished from the underlying siwalik group by the first appearance of conglomerate the transition from the lower member to the middle member is interpreted as recording uplift on the salt range thrust as the salt range thrust was active the palaeo indus river was bifurcated to the east and west around the embryonic shinghar range and overbank and lacustrine deposition occurred represented by the middle member when the shinghar range achieved significant topography the upper member was deposited by streams transporting gravel and sand that flowed north and west out of the range and into a piggyback basin that formed on the hangingwall of the salt range thrust new and previously published palaeomagnetic stratigraphy and fission track ages from volcaniclastic deposits within the upper siwalik group provide tight constraints on the chronology of sedimentary facies transitions and timing of uplift of the shinghar range the integration of sedimentological and geochronological data indicates that motion on the salt range thrust and repositioning of the indus river began at 1 0 ma

intermittent turbulent events over sandy current ripples a motion picture analysis of flume experiments abstract a series of laboratory flume experiments was conducted in order to elucidate sediment transport processes over sandy current ripples instantaneous cinematography capable of recording up to 30 frames per second was employed to trace the coherent trajectories of the sediment particles and flow analysis of channel side view images suggests that sediment behaviour over a fully developed current ripple can be categorized as distinct stages on both stoss and lee sides near the reattachment point on the stoss side sediment particles exhibit the following intermittent but coherent motions 1 impact of sediment laden fluid on the reattachment point 2 entrainment of particles 3 spiral like turbulent motion and 4 downstream dissipation of particles the sequential sediment behaviour is called micro scale kolk which is a key mechanism responsible for turbulent sediment entrainment over current ripples the salient features of boil were not observed in the crest and lee side region sequential images show the following characteristics 1 detachment of particles and fluid at the crest 2 entrainment and shielding of particles 3 avalanching and rolling of free shear layer and 4 downstream dissipation and recreation of a lee face segregation of heavy minerals was accomplished by successive avalanches along the lee slope and mixing vortices in the separation cell

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

on the transition between 2d and 3d dunes sediment transport in sand bedded alluvial channels is strongly conditioned by bedforms the planimetric morphology of which can be either two or threedimensional experiments were undertaken to examine the processes that transform the bed configuration from two dimensional 2d dunes to threedimensional 3d dunes a narrowly graded 500 lm size sand was subjected to a 0ae15 m deep non varying mean flow ranging from 0ae30 to 0ae55 m sec 1 in a 1 m wide flume changes in the planimetric configuration of the bed were monitored using a high resolution video camera that produced a series of 10 sec time lapsed digital images image analysis was used to define a critical value of the non dimensional span sinuosity of the bedform crestlines that divides 2d forms from 3d forms significant variation in the non dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random images also reveal that once 2d bedforms are established minor transient excesses or deficiencies of sand are passed from one bedform to another the bedform field appears capable of absorbing a small number of such defects but as the number grows with time the resulting morphological perturbations produce a transition in bed state to 3d forms that continue to evolve but are pattern stable the 3d pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment which leads to bifurcation the experiments demonstrate that 2d bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator

morphology sedimentology and palaeohydraulic significance of large gravel dunes altai mountains siberia coarse gravel bedforms which resulted from pleistocene glacial outburst floods are identified as subaqueous dunes comparison of the morphology of these fossil structures with modern dunes shows that the form of two dimensional 2 d transverse dunes and 3 d cuspate and lunate dunes developed in coarse gravels is comparable with sand dune morphology within lesser scale geophysical flows the similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform internal structure indicates that flow over 2 d dunes was relatively uniform but over 3 d bedforms flow was locally variable flow separation and complex streaming of flow occurred over the steepest 3 d dunes cross beds are thin and few approach the angle of repose consequently most dunes did not migrate primarily by avalanching but by stoss entrained gravel transported over the crests rolling down and depositing on the lee slopes the lee formed when flood power was waning some dunes were siightly planed down during falling stage because lee side cross beds tend to be steeper than the angle of the preserved lee slope however silt rich caps indicate that any height reduction was contemporary with the final deposition of foresets post flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs lee side sediments are often finer than the stoss slope sediments which indicates

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

large eddy simulation of three dimensional dunes in a steady unidirectional flow part 1 turbulence statistics we performed large eddy simulations of flow over a series of three dimensional dunes at laboratory scale reynolds number based on the average channel depth and streamwise velocity was 18 900 using the lagrangian dynamic eddy viscosity subgrid scale model the bedform three dimensionality was imposed by shifting a standard two dimensional dune shape in the streamwise direction according to a sine wave the statistics of the flow are discussed in 10 cases with in phase and staggered crestlines different deformation amplitudes and wavelengths the results are validated qualitatively against experiments the three dimensional separation of flow at the crestline alters the distribution of wall pressure which in turn may cause secondary flow across the stream which directs low momentum fluid near the bed toward the lobe the most downstream point on the crestline and high momentum fluid near the top surface toward the saddle the most upstream point on the crestline the mean flow is characterized by a pair of counter rotating streamwise vortices with core radius of the order of the flow depth however for wavelengths smaller than the flow depth the secondary flow exists only near the bed and the mean flow away from the bed resembles the two dimensional case staggering the crestlines alters the secondary motion the fastest flow occurs between the lobe and the saddle planes and two pairs of streamwise vortices appear a strong one centred about the lobe and a weaker one coming from the previous dune centred around the saddle the distribution of the wall stress and the focal points of separation and attachment on the bed are discussed the sensitivity of the average reattachment length depends on the induced secondary flow the streamwise and spanwise components of the channel resistance the skin friction and the form drag and the contribution of the form drag to the total resistance are also studied three dimensionality of the bed increases the drag in the channel the form drag contributes more than in the two dimensional case to the resistance except for the staggered crest case turbulent kinetic energy is increased in the separated shear layer by the introduction of three dimensionality but its value normalized by the plane averaged wall stress is lower than in the corresponding two dimensional dunes the upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer respectively affecting the turbulent kinetic energy

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 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

bed topography and sand transport responses to a step change in discharge and water depth abstractat base flow conditions ephemeral streams with mobile beds may inherit bed topography caused by previous higher flow events resulting in bed topography that is not in equilibrium with flow conditions major flow events can generate large bedforms that result in hydrodynamics and sediment transport that are not in equilibrium with subsequent lower flow conditions to simulate a large storm runoff driven stream flow followed by lower base flow conditions changes in sand bed topography and sand transport caused by a simultaneous reduction in discharge and flow depth in a laboratory flume with an equilibrium sand bed and steady approximately uniform flow were investigated bed topography was evaluated using second order structure functions semivariograms and statistical analyses sediment transport responded rapidly to a near instantaneous negative step in mean flow depth and discharge the relative contributions of reduced bedform height and celerity to the decreased transport rate are quantif

unit bar architecture in a highly variable fluvial discharge regime examples from the burdekin river australia unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes unit bars formed within the highly variable discharge burdekin river in queensland australia were examined over three field campaigns between 2015 and 2017 these bars had complex internal structures dominated by co sets of cross stratified and planar stratified sets the cross stratified sets tended to down climb the development of complex internal structures was primarily a result of three processes i superimposed bedforms reworking the unit bar avalanche face ii variable discharge triggering reactivation surfaces and iii changes in bar growth direction induced by stage change internal structures varied along the length and across the width of unit bars for the former down climbing cross stratified sets tended to pass into single planar cross stratified deposits at the downstream end of emergent bars such variation related to changes in fluvial conditions whilst bars were active a hierarchy of six categories of fluvial unsteadiness is proposed with these discussed in relation to their effects on unit bar and dune internal structure across deposit variation was caused by changes in superimposed bedform and bar character along bar crests such changes related to the three dimensionality of the channel and bar geometry when bars were active variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform for example dune deposits formed in the same river this is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width in regimes where unit bar deposits are well preserved their architectural variability is a potential aid to their identification this complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development