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.     

Flow, sediment transport and bedform dynamics over the transition from dunes to upper-stage plane beds: implications for the formation of planar laminae

Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms^-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.     

Morphology, internal structure and mechanics of small longitudinal (seif) dunes in an aeolian horizon of the Proterozoic Dhandraul Quartzite, India

The excellently preserved metre-scale, linear bedforms in an aeolian horizon of the Proterozoic Dhandraul Quartzite, India, show oppositely dipping strata arranged in a zigzag pattern. The strata are dominantly of translatent type, deposited by along-crest migrating ripples preserved on the flanks of dunes. The bedforms thus may be interpreted in a morphodynamic sense as longitudinal (seif) dunes. In order to determine the regional palaeoflow pattern, the migration directions of ripples preserved at the top of sheet sandstones that are associated with the dune cross-strata and internally show subhorizontal translatent strata were measured. A directionally varying flow with a mean direction nearly parallel to the mean axial trend of the dunes is indicated. The kinematics of the dunes were thus largely the result of alternate operation of two oblique flow components, each of which was deflected at a dune crest into an along-crest flow on the downwind flank of the dune. The deflected flow formed along-crest migrating ripples, which in turn deposited climbing ripple strata. Alternate deposition on the two opposite flanks resulted in near vertical accretion of the dunes, as is indicated by the zigzag pattern of stratal arrangement.     

A field study of turbulence and sediment dynamics over subaqueous dunes with flow separation

This study examines flow, turbulence and sand suspension over large dunes in Canoe Pass, a distributary channel of the Fraser River delta, Canada. Dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30°. Velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter (OBS) probes. The general patterns of time-averaged velocity and sand suspension are consistent with previous studies, including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed, reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near-bed velocity and sand concentration along the stoss side of the dune. Frequency spectra of near-bed velocity and OBS records from leeside separation zones are composed of two distinct frequencies, providing field confirmation of scale relations based on flume experiments. The low-frequency spectral signal probably results from wake flapping and the high-frequency signal from vortex shedding. The wake-flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand. Predictions from a depth-scale Strouhal Law show good agreement with measured wake-flapping frequencies. Cross-correlations of OBS records reveal that turbulent sand suspension structures advect downstream at 23–25° above the horizontal. These advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field.     

The dynamics of turbulent, transitional and laminar clay-laden flow over a fixed current ripple

Most aqueous sedimentary environments contain varying concentrations of fine‐grained, often clay‐rich, sediment that is transported in suspension and may modify the properties of the flow and underlying mobile bed. This paper presents results from a series of laboratory experiments examining the mean and turbulent properties of clay‐laden (kaolinite) flows, of various volumetric sediment concentrations between 0·046% and 12·7%, moving over a fixed, idealized current ripple. As the kaolinite concentration was raised, with flow velocity and depth constant, four flow types were observed to occur: (i) turbulent flow, in which flow separation is dominant in the leeside of the ripple; (ii) turbulence‐enhanced transitional flow, in which turbulence in the leeside separation zone region is enhanced; (iii) turbulence‐attenuated transitional flow, in which turbulence along the separation zone shear layer and in the free flow above it becomes damped, eventually leading to a reduction in the size of the separation zone wake region; and (iv) laminar plug flow, in which turbulence is damped and flow is almost stagnant in the lee of the ripple. Such modulation of turbulence by increasing clay concentrations suggests that many paradigms of flow and bedform dynamics, which have been based on extensive past work in clear water flows, require revision. The present results highlight a need to fully characterize the boundary conditions for turbulence modulation as a function of clay type and applied flow conditions, and the effects of such flows on fully mobile cohesionless beds.     

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.     

Variations in wind velocity and sand transport on the windward flanks of desert sand dunes

The magnitudes of increases in wind velocity, or speed-up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed-up varied with dune shape and height. For linear dunes, speed-up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed-up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand-transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi-directional wind regime.     

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.     

浅水非线性波作用下沙纹床面底层流动特性试验研究

通过水槽试验研究浅水非线性波作用下沙纹床面底层流动特性,利用CCD图像技术观测分析非对称沙纹的形成和演化规律。利用声学多普勒测速仪(ADV)测量非对称沙纹底床上的流场,得到了不同波高、周期、水深条件下的沙纹峰顶和谷底断面的瞬时速度。试验结果分析表明,浅水非线性波作用下床面上形成非对称沙纹,其近底流速具有较强紊动特性,随着距床面距离的增大紊动强度逐渐减弱。在水流方向改变时,沙纹背部具有明显漩涡运动。沙纹背后形成的漩涡能起到维持沙纹的作用。浅水非线性波作用下,沙纹的形成原因主要是床面泥沙颗粒在非对称流动和床面近壁粘性底层中漩涡结构动力作用下,作受迫摆动、推移所致。     

Largescale ripples of the lower Wabash River

Largescale ripples in the meandering lower Wabash River of Illinois and Indiana, U.S.A., include scroll bars and three dunelike bed forms (dunes, sand waves, and transverse bars). Scroll bars are lobate crested, asymmetrical in stream-wise vertical profile, usually solitary, and oriented approximately normal to local channel strike. They form by passive flow expansion downchannel from locally emergent topographic highs, face and lie near inner banks of meander bends, enjoy a high preservation potential as leveelike ridges of ridge-and-swale topography, and migrate only during relatively low stream discharges, when water depth over bar crests is less than 0·5 m. Dunes correspond to dunes of the flow-regime classification and rarely are solitary or superimposed. Sand waves may be symmetrical or asymmetrical, are always superimposed by dunes, occur in depths greater than 4 m and in bed material coarser than 1 mm mean size, and develop at bankfull and flood flows. Transverse bars migrate in depths less than 5 m in straight reaches and near inner banks of bends, display crestal dunes, and correspond to the bars of Costello (1974) and to the sand waves of Boothroyd (1969). Hydrodynamic regimes of scroll bars and transverse bars differ from that of dunes. The omnipresence of dunes upon stoss-sides of sand waves confirms the existence of an equilibrium superimposition of dunelike largescale ripples. Depth-velocity-size diagrams appear to be a valid representation of empirical stability fields of dunelike largescale ripples in deep unsteady nonuniform aqueous flows. Stability fields of dunes and sand waves overlap greatly. Velocity profiles demonstrate an absence of leeside flow separation over dunes and an appearance (rare) over transverse bars only when the ratio of trough depth to crest depth exceeds two. Dune stratification displays (1) largescale trough cross-strata, (2) thinning of sets as bed-material size increases, and (3) an orientation within 20° of local channel strike. Transverse bars show avalanche sets up to 2 m thick, with reactivation surfaces. Scroll bars display thick avalanche sets separated by reactivation structures consisting of erratically oriented smallscale trough cross-strata. Avalanche sets of scroll bars and of transverse bars are oriented 50–150° from and within 50° of, respectively, local channel strike.     

Interaction between lateral sorting in river bends and vertical sorting in dunes

Sediment is sorted in river bends under the influence of gravity that pulls the heavier grains downslope and secondary flow that drags the finer grains upslope. Furthermore, when dunes are present, sediment is also sorted vertically at the dune lee side. However, sorting functions are poorly defined, since the relation to transverse bed slope and the interaction between lateral and vertical sorting is not yet understood for lack of data under controlled conditions. The objective of this study is to describe lateral sorting as a function of transverse bed slope and to gain an understanding of the interaction between lateral and vertical sorting in river bends. To this end, experiments were conducted with a poorly sorted sediment mixture in a rotating annular flume in which secondary flow intensity can be controlled separately from the main flow velocity, and therefore transverse bed slope towards the inner bend and dune dimensions can be systematically varied. Sediment samples were taken along cross-sections at the surface of dune troughs and dune crests, and over the entire depth at the location of dune crests (bulk samples), which enabled comparison of the relative contribution of vertical sorting by dunes to lateral sorting by the transverse bed slope. The data show that lateral sorting is always the dominant sorting mechanism in bends, and bulk samples showed minor effects of vertical sorting by dunes as long as all grain-size fractions are mobile. An empirical bend sorting model was fitted that redistributes the available sediment fractions over the cross-section as a function of transverse bed slope. Comparison with field data showed that the model accurately reproduces spatially-averaged trends in sorting at the bend apex in single-thread channels. The bend sorting model therefore provides a better definition of bend sorting with conservation of mass by size fraction and adds to current understanding of bend sorting. The implication for numerical modelling is that bend sorting mechanisms can be modelled independently of dunes, allowing the application of the active layer concept.     

苦草对水流结构的影响研究

为研究沉水植被对水流结构的影响,以苦草为对象,利用实验室水槽实验研究了含淹没水生植被水流时均流速、雷诺应力及紊动能的垂向分布特征。受苦草冠层的影响,时均流速在冠顶以上呈对数分布,且随着流量的增加,冠层倾伏高度降低,对数剖面愈加明显;冠层内部,由于冠层阻流面积在垂向分布上的差异,冠层内时均流速出现逆梯度分布,且在床面附近出现局部流速最大值。雷诺应力在冠顶附近达到最大值,并向水面与床底方向逐渐减小;受逆流速梯度的影响,冠层内部雷诺应力出现负值以及局部最大值。雷诺应力产生的剪切紊动使得紊动能在冠顶处最大,并向水面与床底进行垂向传输;受紊动传输距离的限制,冠层底部以叶片后产生的尾流紊动为主,紊动能较小。     

Toward a model for airflow on the lee side of aeolian dunes

The interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back-flow eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. The interior is an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is the wake, which consists of two layers. The upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is the internal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee-face causes flow deceleration.     

溢流堰与竖缝组合式鱼道紊流结构试验研究

利用声学多普勒测速仪（ADV）实测了凹口溢流堰与同侧布置竖缝组合式鱼道的三维瞬时流速,应用射流力学和紊流统计理论剖析了鱼道水池内的时均流速、紊动强度、雷诺应力、相关函数、紊动尺度等紊流结构。试验研究结果表明:溢流堰与竖缝组合式鱼道池内水流流态呈现出复杂的三维紊流结构;同竖缝相比,溢流堰对紊动强度和雷诺应力的变化有着更为显著的影响,尤其是鱼道池内表层区域;堰流区涡旋的相关性比竖缝壁面射流区好,并且堰流区存在较大尺度的涡旋结构,而竖缝壁面射流区的涡旋尺度则较小。相比单一式鱼道,组合式鱼道紊流结构更加复杂,研究结果可为优化鱼道设计、修复鱼类生境提供参考。     

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.     

Microbial influence on erosion,grain transport and bedform genesis in sandy substrates under unidirectional flow

Interpreting the physical dynamics of ancient environments requires an understanding of how current‐generated sedimentary structures, such as ripples and dunes, are created. Traditional interpretations of these structures are based on experimental flume studies of unconsolidated quartz sand, in which stepwise increases in flow velocity yield a suite of sedimentary structures analogous to those found in the rock record. Yet cyanobacteria, which were excluded from these studies, are pervasive in wet sandy environments and secrete sufficient extracellular polysaccharides to inhibit grain movement and markedly change the conditions under which sedimentary structures form. Here, the results of flume experiments using cyanobacteria‐inoculated quartz sand are reported which demonstrate that microbes strongly influence the behaviour of unconsolidated sand. In medium sand, thin (ca 0·1 to 0·5 mm thick) microbial communities growing at the sediment–water interface can nearly double the flow velocity required to produce the traditional sequence of ripple→dune→plane‐bed lamination bedforms. In some cases, these thin film‐like microbial communities can inhibit the growth of ripples or dunes entirely, and instead bed shear stresses result in flip‐over and rip‐up structures. Thicker (ca≥1 mm thick) microbial mats mediate terracing of erosional edges; they also, foster transport of multi‐grain aggregates and yield a bedform progression consisting of flip‐overs→roll‐ups→rip‐ups of bound sand.     

The dynamic characteristics and migration of a pyramid dune

The results of wind tunnel experiments and field observations show that when the intersection angle between airflow direction and dune crest (ridge) line is > 30°, a reverse vortex is formed. Because of the convergence of sand streams from the windward and lee slopes at the crest, sand accumulates in the crestal region, causing vertical growth. Nevertheless, studies also show that the common asymmetry of the two slopes of a dune may significantly influence the evolution of arms of a pyramid dune. The migration rates of pyramid dunes are mediated by the interplay of their arms moving transversely and the vertical growth in response to the variations in wind regimes. Comparing the effects of airflow transverse to a given arm with longitudinal airflow, it is indicated that the transverse airflow is more significant in controlling the arms of pyramid dunes. The whole body of the studied pyramid dune, particularly the upper quarter section, migrated SE direction during the monitoring period. The patterns of wind erosion and deposition change alternately with seasonal variations in wind directions. The W, NE and SE sides undergo constant erosion, deposition and both erosion and deposition, respectively. The results of long-term monitoring of a pyramid dune show that southerly winds, resulting from a local circulation, markedly affect the transverse migration of the whole pyramid dune.     

Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA

The geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day-long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three-dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted. Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate. Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant.