Two-dimensional Dunes Research Articles

Behavioral patterns of turbulent kinetic energy budget and quadrant analysis for flows over bedform under the influence of downward seepage

Seepage is one of the important factors involved in natural flow conditions, contributing to changes in flow turbulence patterns and morphological changes due to the transport of sediments. This transport of sediment particles influences the development of fluvial bedforms in any river channel. However, previous research on fluvial dynamics has not considered the influence of seepage on the flow field over the fluvial bedforms. The present experimental research aims to explore the behavioral patterns of turbulent kinetic energy, the turbulent kinetic energy (TKE) budget, and quadrant analysis for flows over two-dimensional dune shaped bedforms in the absence and presence of downward seepage. Results from the study illustrate that at the measurement locations on the initial and lee side sections of a dune, the TKE increases with the introduction of downward seepage, leading to an increase in turbulence production near the bed-surface region. The flow energy under both no seepage and seepage conditions contributes mainly to the turbulent production. Turbulence diffusion and dissipation rates have been found to decline in the near-bed region of the initial and lee side sections of the dune. However, turbulent production has been found to be significantly higher in the presence of downward seepage than under the no seepage condition. Similarly, turbulent kinetic energy flux increases in the streamwise direction, while it reduces in the vertical direction at initial sections and lee side sections of the dune under seepage conditions. However, at the middle sections and crest portion of the dune, opposite behavioral patterns are observed for all the aforementioned turbulent entities. Quadrant analysis reveals that the sweep and ejection event increases while inward and outward interaction reduces in the near bed zone. Although the contribution of both sweep and ejection events increases in the presence of downward seepage, sweep events have clear dominance in the near bed region, suggesting the possibility of a higher rate and amount of sediment transportation than under the no seepage condition.

Large Eddy Simulation of particle-laden flow over dunes

The particle-laden turbulent flow over a two-dimensional dune bathymetry is studied by means of Large Eddy Simulation (LES). The 4-way coupling regime for particle–particle and particle-flow interactions is applied and an appropriate empirical formula is used to estimate the bedload transport. The flow conditions correspond to river flow in laboratory scale at a Reynolds number equal to 17,500 with respect to the maximum water depth and bulk velocity. The dune-shaped bed is introduced within a Cartesian grid by means of the Immersed Boundary Method. A particle cloud consisting of spherical particles is released within the carrier fluid under the influence of the Stokes drag and gravity forces. Three different types of particles are examined: acrylic, glass and steel particles. The particle volume fraction is significant and a hard-sphere model is deployed to account for particle–particle collisions in a 4-way coupling regime. Closure parameters for the collision model have been adopted for each type of the examined particles, based on experimental works in literature. Results focus on the effect of the particle inertia on their kinematics, the velocity field and the shear stress along the dune bed which modifies bedload transport. Using the numerical simulations presented, it can be observed that when larger particle-to-fluid density ratios were examined, larger slip velocities between the fluid and particle phases were recorded.

Free surface flow over two-dimensional dunes under different flow regimes

To explore the influence of different flow regimes on fluid dynamics over dunes after the construction of the Three Gorges Dam, a model is applied by utilizing a large eddy simulation, immersed boundary method and level set method to address the turbulence, dune morphology and free water surface, respectively. Seven simulations with incrementally increased Froude numbers are simulated based on previous experimental work. The vertical profiles of the non-dimensional double-averaged streamwise velocity agree well with the experimental data. The profiles display an inflection caused by the effects of the dunes within the form-induced sublayer, where the individual profiles are reduced by wake flow. As the Froude number increases, the flow regime transitions from subcritical flow to supercritical flow, and the mean streamwise velocity and the shear velocity increase. Additionally, the reattachment location of the recirculation zone downstream of the crest moves upwards, while the location of the highest water elevation moves downwards, approaching the crest of the dune. The fluctuation in the free water surface increases as the Froude number increases.

Conditional spatially averaged turbulence and dispersion characteristics in flow over two-dimensional dunes

Conditional turbulence and dispersion characteristics in flow over a series of two-dimensional dunes are analyzed by applying the spatial averaging methodology. To this end, the flow data measured over an array of points on the vertical central plane were used. The manifestation of the wake-interference flow over dunes, including the boundary layer diffusion, flow separation, and flow reattachment, is demonstrated by overlapping the vorticity contours on the velocity vector diagram. The vertical distributions of the spatially averaged (SA) streamwise velocity, Reynolds shear and normal stresses, dispersive shear and normal stresses, turbulent kinetic energy (TKE) fluxes, and dispersive kinetic energy (DKE) fluxes are analyzed. It is recognized that the dominance of the conditional SA streamwise velocity associated with the temporal sweep events causes the flow to accelerate temporally. The conditional SA Reynolds stresses and TKE fluxes associated with the temporal ejection events are dominant above the dune crest, whereas those associated with the temporal sweep events are the governing mechanism below the crest. An examination of the conditional dispersive normal stresses and the DKE fluxes reveals that within the upper portion of the roughness sublayer, the dispersive ejection events are the key flow dispersion mechanism, whereas near the dune trough, the dispersive outward interaction events prevail. Furthermore, the temporal ejection events are found to be more persistent, but less frequent, than the temporal sweep events. Besides, the frequencies of the occurrences of the temporal bursting events are higher below the crest than those above it, suggesting the flow to be more turbulent below the crest.

Morphological Characteristics of Low-angle Dunes on a Tidal Ridge, the Jiangsu Macrotidal Coast, China

Wang, L.; Yu, Q.; Zhang, Y., and Gao, S., 2020. Morphological characteristics of low-angle dunes on a tidal ridge, the Jiangsu macrotidal coast, China. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 717-721. Coconut Creek (Florida), ISSN 0749-0208.Subaqueous dunes are typical flow-transverse rhythmic bedforms in fluvial, intertidal and marine environments. The low-angle dunes (LADs), commonly associated with a leeside angle of less than 15°, differ in terms of the formation mechanism from the high-angle dunes (HADs). Here, multibeam echosounder exploration and seafloor surficial sediment sampling were carried out on a tidal sand ridge off the Jiangsu coast, China, a macrotidal area with the spring tidal range exceeding 6 m. The spatial distribution of the dune morphological parameters, including wave height, wavelength, asymmetry, and lee-side angle, are analyzed in association with the water depth and seafloor sediment. The following results are obtained. (1) Two-dimensional linear dunes are superimposed on the tidal ridge, and a total of 3599 dunes detected are all LADs with an average maximum leeside angle of 5.46°. (2) The spatial distribution of the dune parameters is closely related to the topography of the ridge-swath system. The asymmetries and migration directions of the dunes on the northern and southern slopes of the tidal ridge are opposite, indicating a clockwise residual sand circulation, while the ridge represents the transition zone with relatively symmetrical dunes. (3) Water depth is the controlling factor for the dune geometry in shallow water places. In areas where water depth is below 7.5 m and the seafloor sediments are composed of pure sand, the scales of LADs are proportional to the water depth. (4) Bed sediment composition becomes important for the deeper water areas. For water depth more than 7.5 m, the ratio of cohesive particles (< 8 µm) of bed sediment, which increases with the water depth, inhibits the growth of the LADs. These observations reveal the behavior of LADs in association with sediment particle size and water depth.

Hydrodynamics of flow over two-dimensional dunes

The turbulence characteristics in flow over and within the interface of two-dimensional dunes are investigated experimentally. Besides the spatial flow and turbulence quantities, their double-averaged profiles are also analyzed. The flow over dunes is recognized to be a wake-interference flow, where the decelerated flow at the immediate downstream of the crest causes the kolk-boil effect. The flow reattachment can be explained from the perspective of the Coandă effect. The inner boundary layer edge follows the locus of the inflection points of velocity profiles having a velocity defect. The Reynolds shear stress profiles attain their respective peaks along this locus. In addition, the dispersive shear stress initiates from the edge of the form-induced sublayer being negative, indicating a spatially decelerated flow. The third-order correlations reveal that an inrush of rapidly moving fluid streaks coupled with a downward-downstream Reynolds stress diffusion prevails within the interfacial sublayer, while an arrival of slowly moving fluid streaks coupled with an upward-upstream stress diffusion governs the flow zone above the crest. The turbulent kinetic energy (TKE) flux results corroborate the similar findings. Concerning the TKE budget, the dispersive kinetic energy diffusion is found to be substantial within the roughness sublayer. The budget terms exhibit their respective peaks near the crest. The production rate is greater than the dissipation rate. However, the TKE diffusion and pressure energy diffusion rates are negative in the interfacial sublayer. The bursting analysis endorses that the sweeps and ejections govern within the interfacial sublayer and the flow zone above the crest, respectively.

Flow Structure in a Shallow Mixing Layer Developing over 2-D Dunes

A high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel over two-dimensional (2D) dunes. The study discusses the vertical non-uniformity in the mixing layer and provides a quantitative characterization of the growth of the large-scale quasi 2D coherent structures with the distance from the splitter plate. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a very rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness. Results show that in streamwise sections situated after 100D (D is the channel depth) from the splitter plate, the width of the mixing layer close to the free surface stays constant. The tilting of the mixing layer interface toward the low speed stream is observed as the free surface is approached in all vertical sections.

Detached eddy simulation of shallow mixing layer development between parallel streams

Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a smooth flat bed and medium size dunes. The study discusses the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale quasi two-dimensional (2D) coherent structures with the distance from the splitter plate. Results show that in streamwise sections situated between 75D (D is the channel depth) and 150D from the splitter plate, the width of the mixing layer close to the free surface is 20–30% more than the width in the near-bed region. This is mostly because of the tilting of the mixing layer interface on the low-speed side toward the low speed stream as the free surface is approached. Power spectra of the horizontal velocity components near the free surface show the presence of a −3 subrange, corresponding to inverse energy cascade in two-dimensional turbulence, at streamwise locations situated more than 10D from the splitter plate, consistent with the presence of large-scale quasi 2D horizontal eddies and the transfer of energy (inverse energy cascade) from the smaller scales toward these eddies. Consistent with visualizations of the mass transport of a passive scalar within the mixing layer, close to the free surface, the estimated streamwise length of the quasi 2D mixing layer eddies is about 2.5–3.0 times larger than the local width of the mixing layer. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a much more rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness.

Turbulence characteristics in free-surface flow over two-dimensional dunes

The turbulent structure of open-channel flows over two-dimensional dunes is investigated numerically using large-eddy simulation (LES), in order to improve our understanding of the interaction between the dune-generated turbulence and the free surface dynamics. The filtered Navier–Stokes equations in the LES model have been discretised using the finite volume method, with a dynamic sub-grid model being employed for the unresolved scales of turbulence. The partial cell treatment has been implemented in a Cartesian grid form to deal with the dune topography. Both the volume of fluid method and rigid lid approach have been employed in the numerical framework to investigate the effects of the free surface treatment on the flow characteristics. The numerical model predicted mean flow velocities, turbulence intensities and Reynolds stresses have been compared with experimental measurements published in the literature, with a detailed analysis being undertaken to assess the accuracy of the model results and the effects of the free surface treatment on the velocity and turbulence predictions. The instantaneous flow structure has been investigated, with emphasis being focused on the free surface dynamics and coherent structures.

Distribution of subtidal sedimentary bedforms in a macrotidal setting: The Bay of Fundy, Atlantic Canada

The Bay of Fundy, Canada, a large macrotidal embayment with the World's highest recorded tides, was mapped using multibeam sonar systems. High-resolution imagery of seafloor terrain and backscatter strength, combined with geophysical and sampling data, reveal for the first time the morphology, architecture, and spatial relationships of a spectrum of bedforms: (1) flow-transverse bedforms occur as both discrete large two-dimensional dunes and as three-dimensional dunes in sand sheets; (2) flow-parallel bedforms are numerous straight ridges described by others as horse mussel bioherms; (3) sets of banner banks that flank prominent headlands and major shoals. The suite of bedforms developed during the Holocene, as tidal energy increased due to the bay approaching resonance. We consider the evolution of these bedforms, their migration potential and how they may place limitations on future in-stream tidal power development in the Bay of Fundy.

Dune bedforms produced by dilute pyroclastic density currents from the August 2006 eruption of Tungurahua volcano, Ecuador

A series of pyroclastic density currents were generated at Tungurahua volcano (Ecuador) during a period of heightened activity in August 2006. Dense pyroclastic flows were confined to valleys of the drainage network, while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross-stratified ash exposed on the surface. Here, the description, measurement, and classification of more than 300 dune bedforms are presented. Four types of dune bedforms are identified with respect to their shape, internal structure, and geometry (length, width, thickness, stoss and lee face angles, and stoss face length). (1) “Elongate dune bedforms” have smooth shapes and are longer (in the flow direction) than wide or thick. Internal stratification consists of stoss-constructional, thick lensoidal layers of massive and coarse-grained material, alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests. (2) “Transverse dune bedforms” show linear crests perpendicular to the flow direction, with equivalent lengths and widths. Internally, these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration. Steep truncations of the bedsets are visible on the stoss side only. (3) “Lunate dune bedforms” display a barchanoidal shape and have stratification patterns similar to those of the transverse ones. Finally, (4) “two-dimensional dune bedforms” are much wider than long, exhibit linear crests and are organized into trains. Elongate dune bedforms are found exclusively in proximal deposition zones. Transverse, lunate, and two-dimensional dune bedforms are found in distal ash bodies. The type of dune bedform developed varies spatially within an ash body, transverse dune bedforms occurring primarily at the onset of deposition zones, transitioning to lunate dune bedforms in intermediate zones, and two-dimensional dune bedforms exclusively on the lateral and distal edges of the deposits. The latter are also found where flows moved upslope. Elongate dune bedforms were deposited from flows with both granular-based and tractional flow boundaries that possessed high capacity and competence. They may have formed in a subcritical context by the blocking of material on the stoss side. We do not interpret them as antidune or “chute-and-pool” structures. The dimensions and cross-stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate, but we rule out their interpretation as “antidunes”. A similar conclusion holds for lunate dune bedforms, whose curved shape results from a sedimentation rate dependent on the thickness of the bedform. Finally, two-dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport; i.e., in areas where currents were able to spread laterally in low velocity zones. We suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension. This produced differential draping with no further reworking. We propose the name “regressive climbing dunes” for structures produced by this process. A rapid decrease in current velocity, possibly triggered by hydraulic jumps affecting the entire parent flows, is inferred to explain their deposition. This process can in principle hold for any kind of particulate density current.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-013-0762-x) contains supplementary material, which is available to authorized users.

Large-eddy simulation of turbulent open-channel flow over three-dimensional dunes

A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow over three-dimensional (3D) dunes. The governing equations have been discretized using the finite volume method, with the partial cell treatment being implemented in a Cartesian grid form to deal with the 3D dune topography. The simulated free surface elevations, mean flow velocities and Reynolds shear stress distributions have been compared with experimental measurements published in the literature. Relatively close agreement has been obtained between the two sets of results. The predicted mean velocity field and the associated turbulence structure are significantly different from those observed for flows over two-dimensional dunes. The effects of dune three-dimensionality are reflected in spanwise variations of mean flow fields, secondary currents and different distributions of vertical profiles of the double-averaged velocity. Furthermore, large-scale vortical structures, such as spanwise rollers and hairpin-like structures, are predicted in the simulations, with most of them being generated in the concave regions of the 3D dunes.

Conditional statistics of Reynolds shear stress in flow over 2D dunes in the presence of surface waves

This study presents the analysis of the velocity fluctuations to describe the conditional statistics of Reynolds shear stress in flow over two-dimensional dunes in the presence of surface waves of varying frequency. The flow velocity measurements over the dunes are made using a 16-MHz 3D acoustic Doppler velocimeter. The joint probability distributions of the normalized stream-wise and vertical velocity fluctuations at different vertical locations are calculated in the trough region of a selected dune in quasi-steady region of the flow. Third-order moments of the stream-wise and vertical velocity components over one dune length are also calculated throughout the flow depth for understanding the effect of surface waves on relative contributions to the Reynolds shear stress due to the four quadrant events. The structure of instantaneous Reynolds stresses is analysed using quadrant analysis technique. It has been shown that the contributions of second and fourth quadrant events to the Reynolds shear stress increase with increase in the frequency of surface waves. In fact, the largest contribution to turbulent stresses comes from the second quadrant. The cumulant discard method is applied to describe the statistical properties of the covariance term u′w′. Conditional statistics and conditional sampling are used to compare the experimental and theoretical relative contributions to the Reynolds shear stress from the four quadrant events. Copyright © 2013 John Wiley & Sons, Ltd.

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

AbstractWe 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.

Three-dimensional river bed forms

AbstractThe linear stability of a uniform flow in an infinitely wide erodible channel is investigated with respect to disturbances of the bed that are periodic in both the transverse and the longitudinal directions. A rotational flow and sediment transport model, originally developed to study the formation of two-dimensional dunes and antidunes, is straightforwardly extended to cover variations in the lateral direction. Sediment is assumed to be transported as bed load, disregarding the role of suspension. Following a standard linearization procedure, a dispersion relationship is obtained that expresses the growth rate and the celerity of the sand wave as a function of the streamwise and spanwise wavenumbers and of the relevant flow and sediment parameters. Regions of instabilities in the space of the parameters are found, which can be associated with bed forms of different kinds, spanning from dunes and antidunes to alternate bars. Therefore, the present theory allows for a unified view of the formation of two- and three-dimensional bed forms in rivers in terms of the relevant flow and sediment parameters.

Theory for flow resistance caused by submerged roughness elements

Herein, a phenomenological theory that unifies the flow resistance caused by a variety of submerged bluff bodies, including cylindrical vegetation, two-dimensional dunes and the roughness elements studied in Schlichting's 1936 experiments, is presented. For the different flow obstructions, scaling relations that relate the equivalent roughness length to the obstructions’ spatial dimensions are derived, by adopting the concepts of the turbulent energy cascade and the momentum thickness in the wake of a bluff body. In the derivation, it is hypothesized that the equivalent roughness height represents the volumetric average of the prevailing turbulence length scale that is present in wakes of the roughness elements. A comparison with experimental literature data indicates that the derived scaling relations capture the dominant dependencies of the equivalent roughness length. These expressions have important engineering applications in large-scale flows where employment of more detailed flow-modelling techniques is not feasible.

Age and sedimentological features of fluvial series in the Toruń Basin and the Drwęca Valley (Poland)

Abstract The deposits of the Toruń Basin are dominated by a few-metre thick sand series which fill up buried valley-like depressions. In many cases they underlie the Weichselian till which builds up the ice marginal streamway (pradolina) terraces or they are exposed at the basin slopes. As the results of the geological and sedimentological studies, as well as of the dating of the deposits at the sites in the Toruń Basin indicate, the deposits include two fluvial series accumulated before the advancement of the Leszno Phase ice sheet, i.e. in Middle Weichselian and at the beginning of Late Weichselian. The oldest fluvial series connected with the Saalian Glaciation was found at the mouth section of the Drwęca Valley. The fluvial system of the Toruń Basin during Middle Weichselian and at the beginning of Late Weichselian developed in two phases of the sand-bed braided river. During the first one the river channel were dominated by large mid-riverbed sandbars, while during the second phase the water flow was smaller and, as a result, low transverse sandbars and two-dimensional dunes developed. Other active river channel also showed low-energy flows, more intensive meandering than in the case of the braided rivers, as well as sandy side-bars. Analysis of the rounding and frosting of the quartz grains indicate that the studied series of the Weichselian sandy deposits represent alluvia of a river which were fed from two diverse sources. The first one might have represented the alluvia of a warm river which transformed its load, while the other one might have mainly carried the underlying Quaternary deposits.

Large-eddy simulation of two-dimensional dunes in a steady, unidirectional flow

We performed large-eddy simulations of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results are validated by comparison with simulations and experiments in the literature. The flow separates at the dune crest, generating a shear layer that plays a crucial role in the transport of momentum and energy, and the generation of coherent structures. The turbulent kinetic energy budgets show the importance of the turbulent transport and mean-flow advection in the bulk flow above the shear layer. In the recirculation zone and in the attached boundary layers production and dissipation are the most important terms. Large, coherent structures of various types can be observed. Spanwise vortices are generated in the separated shear layer due to the Kelvin–Helmholtz instability; as they are advected, they undergo lateral instabilities and develop into horseshoe-like structures, are tilted downward, and finally reach the surface. The ejection that occurs between the legs of the vortex creates the upwelling and downdrafting events on the free surface known as “boils.” Near-wall turbulence, after the reattachment point, is affected by large streamwise Taylor–Görtler vortices generated on the concave part of the stoss side, which affect the distribution of the near-wall streaks.

Subaqueous Dunes of the Upper Slope of the Fraser River Delta (British Columbia, Canada)

Abstract Multibeam sonar data provide new high-resolution characterization of morphological and sedimentological features on the upper slope of the Roberts Bank section of the Fraser River delta. Subaqueous dunes are more abundant and broadly distributed over the slope than previously documented and include large asymmetric two-dimensional dunes (sinuous crested and straight crested), large asymmetric three-dimensional dunes (high relief, low relief) and irregularly-spaced dunes. The two-dimensional and three-dimensional dunes fall within the typical range of subaqueous dune heights and wavelengths and show dimensions that do not universally scale to water depth but are probably scaled to the dynamic conditions. The irregularly-spaced dunes fall outside the typical scaling, having excessively low height to wavelength ratios. Current data indicate that these irregularly-spaced dunes form under conditions where the ratio of suspended to bedload transport is too high to maintain fully-scaled bedforms. The or...

Bedforms and bed material transport pathways in the Changjiang (Yangtze) Estuary

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.