The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D₅₀ of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D₅₀ of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D₅₀ < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.     

Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu-Natal shelf, south-east Africa

Subaqueous dunes are formed on the KwaZulu-Natal outer-shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of ? 35 to ? 70 m. The net sediment transport direction is south, but short-period reversals form northward-migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large-scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of ? 60, ? 47 and ? 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.     

Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition

A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.     

Flow and sediment transport over large subaqueous dunes: Fraser River, Canada

Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows.     

Aeolian dune interactions and dune‐field pattern formation: White Sands Dune Field,New Mexico

Pattern formation is a fundamental aspect of self‐organization in fields of bedforms. Time‐series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off‐centre collision; (vi) defect creation; and (vii) dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off‐centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of dunes, crest length, defect density, crest spacing and dune height), dune migration rates, and the type and frequency of dune interactions within a 3500 m box transect from the upwind margin to the core of the dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo‐lake topography. Pattern evolution is most strongly coupled to the pattern parameters of dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the dunes become further apart and have fewer defects.     

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. Lee-side sediments are often finer than the stoss-slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly 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.     

A consideration of the dune:antidune transition in fine gravel

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

An empirical model of aeolian dune lee-face airflow

Airflow data, gathered over dunes ranging from 60-m tall complex-crescentic dunes to 2-m tall simplecrescentic dunes, were used to develop an empirical model of dune lee-face airflow for straight-crested dunes. The nature of lee-face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee-face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low-speed (0–3O% of crestal speed) back-eddy flow, typically occur leeward of steep-sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment. As each of the three types of lee-face flow is defined by a range of wind speeds and directions, the nature of lee-face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross-strata.     

Evolution of a carbonate delta generated by gateway‐funnelling of episodic currents

Cool‐water carbonate sedimentation has dominated Mediterranean shelves since the Early Pliocene. Skeletal sand and gravel herein consist of remains of heterozoan organisms, which are susceptible to reworking due to weak early cementation in non‐tropical waters. This study documents the Lower Pleistocene carbonate wedge of Favignana Island (Italy), which prograded from a 5   km wide passage between two palaeo‐islands into a perpendicular, 10 to 15   km wide strait between the palaeo‐islands at one side and Sicily at the other during the Emilian highstand (1·6   Ma to 1·1   Ma). The clinoformed carbonate wedge, which is 50   m thick and 6   km long, formed by east/south‐east progradation of a platform on the submarine sill by currents that were funnelled between the two palaeo‐islands. Platform‐slope clinoforms evolved from initial aggradation (thin and low‐angle) into a progradation phase (thick and high‐angle). Both clinoform types are characterized by a bimodal facies stacking pattern defined by sedimentary structures created by: (i) subaqueous dunes associated with dilute subcritical currents; and (ii) upper‐flow‐regime bedforms associated with sediment‐laden supercritical turbidity currents. Focusing of episodic currents on the platform by funnelling between the islands controlled the downstream formation of a sediment body, here named carbonate delta. The carbonate delta interfingers with subaqueous dune deposits formed in the perpendicular strait. This study uses a reconstruction of bedform dynamics to unravel the evolution of this gateway‐related carbonate accumulation.     

Formation and preservation of open-framework gravel strata in unidirectional flows

Open‐framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed‐load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee‐side of unit bars depend on grain‐size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross‐stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound‐bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits.     

Bedform spurs: a result of a trailing helical vortex wake

The formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein. Bedform spurs are formed by helical vortices that trail from the lee surface of oblique segments of bedform crest lines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. The geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross‐stream Strouhal number. When present, spur‐bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska, and flume experiments described in previous studies demonstrate that this trailing helical vortex‐mediated sediment transport is a mechanism for bedform deformation, interactions and transitions between two‐dimensional and three‐dimensional bedforms.     

Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows

For more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6-m-diameter sand-covered turntable on the floor of a 4-m-wide flume. In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time-averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s^-1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°. One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction. These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform-normal transport (the orientation at which the sum of the bedform-normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.     

Catastrophic flooding of an aeolian dune field: Jurassic Entrada and Todilto Formations, Ghost Ranch, New Mexico, USA

Surveyed outcrops of the Middle Jurassic Entrada Sandstone at Ghost Ranch, New Mexico, show the unusual occurrence of preserved aeolian dune palaeotopography buried beneath subaqueous strata. The preserved dune remnants have relief up to 35 m, trend NNW, and show internal scalloped cross-strata dipping to the WSW, with small sets occurring as both topsets and bottomsets. Outcrop data are best satisfied in computer models by 50 m high, sinuous bedforms that migrated to the WSW, while the sinuosity migrated alongcrest to the NNW. Superimposed small dunes occurred upon the stoss slope, and at the basal lee of the main bedform where they migrated alongslope to the NNW. Remnant dune palaeotopography is buried by onlapping, subaqueous, largely structureless sandstones believed to be derived by mass wasting of the upper portions of the dunes and deposited as sediment-gravity flows that infilled between the dunes. Preservation of dune palaeotopography beneath mass-flow deposits, with no evidence for gradually rising water, argues that flooding of the Entrada dune field was geologically instantaneous. The thickness and lithology of the overlying Todilto Formation conform to slight remnant palaeotopography on the Entrada surface. The Todilto is a laminated limestone and thinnest over remnant dune crestal areas, but thickens and increases in gypsum content downslope until it abruptly yields to a gypsum mound positioned over a remnant interdune hollow. The Todilto laminations are interpreted as seasonal varves deposited below wave base in a density-stratified water body. The flooding event that gave rise to the controversial Todilto water body occurred during Entrada time, with Todilto deposition occurring within an already substantial water body.     

Preservation of cross-strata due to the migration of subaqueous dunes: an experimental investigation

This experimental investigation examined the controls on the geometry of cross‐sets formed by subaqueous dunes. A range of steady, unidirectional flow conditions spanning the field of dune existence was investigated, and aggradation rate ranged from 0 mm s^?1 to 0·014 mm s^?1. Data from an ultrasonic depth profiler consist of high‐resolution temporal and spatial series of bed profiles from which dune height and length, migration rate and the depth of trough scour were measured. Cross‐set thickness and length were measured from sediment peels. The size and shape of dunes from an equilibrium assemblage change continuously. Individual dunes commonly increase in height by trough scouring and, occasionally, by being caught‐up by the upstream dune. Both types of behaviour occur suddenly and irregularly in time and, hence, do not appear to depend on dunes further upstream. However, dune climbing or flattening is a typical response of dunes that disappear under the influence of the upstream dune. All types of behaviour occur at any flow velocity or aggradation rate. Successive dune‐trough trajectories, defined by dunes showing various behaviours, affect the geometry of the preserved cross‐sets. Mean cross‐set thickness/mean dune height averages 0·33 (±0·7), and mean cross‐set length/mean dune length averages 0·49 (±0·08), and both show no systematic variation with aggradation rate or flow velocity. Mean cross‐set thickness/mean cross‐set length tends to decrease with increasing flow velocity and Froude number, therefore allowing a qualitative estimation of flow conditions. Quantitative analysis of the temporal changes in the geometry and migration rate of individual dunes allows the development of a two‐dimensional stochastic model of dune migration and formation of cross‐sets. Computer realizations produced stacks of cross‐sets of comparable shape and thickness to laboratory flume observations, indicating a good empirical understanding of the variability of dune‐trough trajectories. However, interactions among dunes and aggradation rates of the order of 10^?2 mm s^?1 should be considered in future improved models.     

An empirical model of subcritical bedform migration

The ability to predict bedform migration in rivers is critical for estimating bed material load, yet there is no relation for predicting bedform migration (downstream translation) that covers the full range of conditions under which subcritical bedforms develop. Here, the relation between bedform migration rates and transport stage is explored using a field and several flume data sets. Transport stage is defined as the non‐dimensional Shields stress divided by its value at the threshold for sediment entrainment. Statistically significant positive correlations between both ripple and dune migration rates and transport stage are found. Stratification of the data by the flow depth to grain‐size ratio improved the amount of variability in migration rates that was explained by transport stage to ca 70%. As transport stage increases for a given depth to grain‐size ratio, migration rates increase. For a given transport stage, the migration rate increases as the flow depth to grain‐size ratio gets smaller. In coarser sediment, bedforms move faster than in finer sediment at the same transport stage. Normalization of dune migration rates by the settling velocity of bed sediment partially collapses the data. Given the large amount of variability that arises from combining data sets from different sources, using different equipment, the partial collapse is remarkable and warrants further testing in the laboratory and field.     

Stratigraphic architecture resulting from dune interactions: White Sands Dune Field,New Mexico

Aeolian dune interactions provide the dynamics for field‐scale pattern emergence and evolution within a set of boundary conditions. Although morphologies for a spectrum of dune interactions are recognized, associated stratigraphic architectures are unknown and have probably been misidentified in the rock record. A unique data set for the White Sands Dune Field in New Mexico (USA) allowed for a detailed analysis in which the morphological evolution of defect and bedform repulsion interactions is chronicled over a decadal time‐series of images and coupled with the resulting stratigraphic architecture, documented from cross‐strata exposed in interdune areas and ground‐penetrating radar imaging of dune interiors. Defect and bedform repulsions represent a class of interactions in which the faster‐migrating dune termination or defect (defect repulsion), or pair of defects (bedform repulsion), collides with the target dune downwind. Results document that during the collision, the defect(s) of the impactor dune recombine(s) with a segment of the target dune, and the redundant target dune segment is ejected as a parabolic‐shaped ejecta dune. The ejecta dune assumes a more barchanoid shape as it migrates downwind. The interaction architecture consists of lateral truncation of the target set by an interaction bounding surface. Defect cross‐strata tangentially approach the surface in plan‐view, and downlap onto the surface in cross‐section. The orientation of the defect cross‐strata is at an acute angle to the trend of the interaction surface. Orientations of the defect cross‐strata, which represent the defect approach angle, and the target dune cross‐strata, which represent the general dune migration direction, diverge at a high angle. Defect cross‐strata typically consist of wind‐ripple laminae, in contrast to the target set that may house grainflow cross‐strata. In the transport direction, the erosional interaction surface curves to become subparallel to subjacent and superjacent cross‐strata where the defect and target unify into a single lee face.     

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.     

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 three-dimensional. Experiments were undertaken to examine the processes that transform the bed configuration from two-dimensional (2D) dunes to three-dimensional (3D) dunes. A narrowly graded, 500 μm size sand was subjected to a 0·15 m deep, non-varying mean flow ranging from 0·30 to 0·55 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.     

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 10⁵ m³ of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s^?1. At least 300–400 m³ of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.