Bedform Development Research Articles

Bedform migration and bedload transport on an intertidal shoal

The main objectives of the present study are to determine the time-dependent character of bedform dimensions and to assess the feasibility and reliability of dune-tracking techniques to estimate bedload transport in estuarine–marine conditions. A third additional aim is to evaluate the application of the Ribberink bedload transport formula for field conditions with a rippled bed. Field experiments were carried out on Spratt Sand, an intertidal shoal in the mouth of the Teign estuary, Devon, UK. In the framework of the COAST3D project, data of hydrodynamics, bedform development and sediment transport were collected over a period of a month. The presence of sediment on Spratt Sand turns out to be an important factor in the development of bedforms. As soon as sufficient sediment is available—a layer with a thickness of at least 0.20–0.30 m—ripples start to grow and develop into dunes. In general, an increase in bedform height is anticipated to result in an increase in bedload transport. However, the present study demonstrates that the development of dunes instead of ripples may lead to a reduction in migration rates, which, at the end, in reality causes a reduction in bedload transport. The presence of waves appears to be a critical factor to the applicability of the dune tracking technique. Dunes only start to migrate when wave orbital velocities exceed 0.3 m/s. Bedload transport by migrating dunes is generally in the order of 0–1.5 m 2 /s when orbital velocities remain below 0.7 m/s. Bedload transport increases by a factor 2 when wave orbital velocities reach values of 0.8–0.9 m/s. The increase in bedload transport is partly caused by the additional bed shear in the presence of waves. A second wave-generated mechanism is that stronger wave action reduces the effect of hiding which, under low wave–strong current conditions, suppresses sediment pick-up in the sand–gravel mixture. Application of the Ribberink bedload transport formula for conditions with a rippled bed and in case of unidirectional flows with superimposed oscillatory flows leads to a maximum difference between “measured” (dune-tracking) and computed bedload transport up to a factor 2. For low-energetic conditions, the difference is smaller and generally varies between 10% and 20%; for high-energetic conditions this figure amounts maximum 100%.

Initiation of Motion of Calcareous Sand

This paper examines the initiation of motion of four natural and five sieved calcareous sand samples in unidirectional flow. Flume experiments yield the sediment transport rate as a function of bed shear stress up to bed-form development. Reference-based criteria are supplemented by visual observations to determine the critical shear stress. The results are compared with published data for rounded and irregular particles in terms of the median sieve size and the corresponding nominal and equivalent diameters as functions of particle Reynolds number. The comparison shows that the critical shear stresses of the irregular particles are higher than the Shields curve in the hydraulically smooth flow regime and lower in the rough turbulent flow regime.

Coastal dune fields at the São Francisco River strandplain, northeastern Brazil: morphology and environmental controls

AbstractExtensive coastal dune fields occur on the Quaternary strandplain associated with the São Francisco River mouth. Two different generations of dunes are identified. One is inactive, already fixed by vegetation, comprising parabolic dunes. The other generation is active, bordering the present‐day shoreline and transgressing over the inactive dune field. Three morphological provinces in the active coastal dune fields are recognized. On the updrift side of the São Francisco River mouth, they are: (a) sand‐sheet with shrub coppice and shadow dunes; (b) isolated dunes of the barchan‐transversal type up to 5 m high, and interdune areas; and (c) a 23 m high compound dune, with superimposed small dunes. The same provinces are recognized on the downdrift side of the river mouth, with two important exceptions: the barchan‐transversal and compound dunes are replaced, respectively, by (i) zibar‐type dunes up to 5 m high, and (ii) a 19 m high precipitation dune, which is associated with numerous blowouts. The prevailing eastern winds from August to January favour the development of the aeolian bedforms and the migration of dunes. The shoreline orientation almost transversal to the winds and the great supply of fine‐grained sediments contribute to the formation of barchan‐transversal types and compound dunes in the updrift side. On the other hand, in the downdrift side the shoreline orientation is almost parallel to the prevailing winds. This fact, in association with a coarser grain size in the beachface, favours the formation of zibar‐type and precipitation dunes with numerous blowouts. The rate of migration of individual dunes is about 20 to 24 m per year. This study suggests that the aeolian sedimentation is a relatively recent phenomenon at the Quaternary strandplain of the São Francisco River. The first generation of dune fields initiated some time after 3000 years BP and the second generation originated some centuries ago. Copyright © 2004 John Wiley & Sons, Ltd.

Conditions for formation of massive turbiditic sandstones by primary depositional processes

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

Measurement and prediction of wave‐generated suborbital ripples

Experiments in a large‐scale wave flume using regular and irregular waves with periods between 4 s and 6 s and heights between 0.2 m and 1.55 m have examined the formation of wave‐generated ripples using sediment beds composed of four grain sizes (D50 = 0.349 mm, 0.329 mm, 0.220 mm, and 0.162 mm) in a water depth of approximately 4 m. Estimates of wave‐generated ripple height,η, and wavelength, λ, were obtained using zero down‐crossing analyses of bed profiles measured by acoustic means along a 4‐m transect normal to the ripple crests. Further information pertaining to λ was obtained from bed images obtained using scanning sonar. The analyses reported here focus on the sequence bedforms that evolved in response to stepwise increases and decreases in wave height. Results show that ripples for the most part are suborbital in nature and do not conform well to empirical equations used frequently to predict η and λ values in the field. On the basis of the present data, two new equations for prediction of η and λ are obtained and their use in field situations where hydrodynamic and sedimentary conditions favor development of suborbital bedforms is recommended.

Longitudinal velocity patterns and bed morphology interaction in a rill

AbstractPresent‐day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills. The latter are difficult to collect with traditional techniques due to the very low water depths and the relatively high flow velocities in rills. The objectives of this paper were to investigate the feasibility of miniaturized acoustic Doppler velocimeter (mADV) measurements in rill flow and to explore longitudinal variations in flow velocities and their relationship with rill bed morphology. Detailed data on longitudinal flow velocity were required to achieve these objectives.A 1·8 m long rill was formed freely in a flume at 5° slope and 0·001 m3 s−1 discharge. Rill topography was characterized by an alternation of steps and pools. The flume surface was then fixed to preserve rill roughness. A topographical scanning of the entire flume surface was made. Velocity was measured with a mADV along the rill, and at different depths. Flow depth in a longitudinal direction was also measured using an elevation gauge.A strong relationship exists between rill topography and flow hydraulics. Over steps, flow was unidirectional and rapidly accelerating until a threshold Froude number (Fn) value between 1·3 and 1·7 was reached and a hydraulic jump occurred leading to the formation of a pool. In the pool, the flow pattern was multidirectional and complex. The flow was subcritical when leaving the pool and accelerated over the next step until the threshold Froude number value was again reached. Energy loss in the rill was concentrated in the pools, mainly due to the action of a hydraulic jump. This mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution. Copyright © 2004 John Wiley & Sons, Ltd.

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS 14C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.

Time and space scales of bedform evolution in sandy nearshore and inner shelf environments

Recent experiments in wave-forced nearshore and inner shelf environments have demonstrated through acoustic and optical imaging techniques a remarkable range of bedform patterns in mobile sandy sediments at sub-10 m horizontal scales. The observed bedstates differ both in the level of complexity of their spatial patterns, and in the time scales of their response to changes in hydrodynamic forcing conditions. Yet, the (re)occurrence of these different bed states at a given location can be quite repeatable and, once formed, the migration of the bedforms characterizing a particular state can in some instances be predicted reasonably well from the local hydrodynamics. These results provide a basis against which the present capability for predicting bedstate evolution in wave-forced mobile bed environments can be assessed. It is argued that, with the possible exception of a few simple cases, current models are able to predict neither the level of complexity in the observed characteristic patterns, nor the time evolution of the bed given the prior history of the forcing. Nevertheless, the repeatability of bedstate occurrence, and the reasonably good predictions of migration velocity and shear stress at the bed, together suggest that such a predictive capability may be achievable in the not-too-distant future.

Bottom-current sediment waves, sediment drifts and contourites in the northern Rockall Trough

Four types of large-scale sediment bedform occur in the northeast Rockall Trough: broad sheeted drifts, elongate drifts, sediment waves and thin contourite sheets. A large sheeted drift occupies the northern basin; its crest can be traced for 40 km perpendicular to the bottom-current flow direction. Elongate drifts occur adjacent to steep areas of continental slope, separated from the break of slope by a moat. Drift crests are oriented alongslope, parallel to bottom-current flow. Elongate drifts are asymmetric, with the steeper flank facing upslope; sedimentation rates are higher on the downslope flank. Sediment waves, with a wavelength of 1–2 km and wave height up to 20 m, occur locally on the flanks of sheeted and elongate drifts; larger waves (wavelength of 3–4 km and wave height of 50–100 m) occur on the flank of the Wyville–Thomson Ridge. Wave crests are oblique to bathymetric contours, to drift crests and to bottom-current flow. The association of sediment waves and drifts, their orientation oblique to regional slopes, and the presence of contourite sediments in cores, indicate that these are bottom-current rather than turbidity-current waves. A contourite sheet, typically 10–25 cm in thickness and composed of well sorted sand, covers the present-day seafloor along the lower slope. Bedform evolution through time has involved a complex interaction between bottom currents of variable intensity, sea level change and sediment input. The present sea level highstand is characterised by minimal sediment input and redistribution of sediments by strong bottom currents, giving active bedform growth and contourite development. Variability of sediment input is the main factor affecting sedimentation during sea level lowstands. Where input was high, as when ice sheets reached the shelf edge, downslope sediment transport dominated, overwhelming any bottom currents that were active. When input was low, such as during the last lowstand, some evidence for active currents can be observed, although these were less energetic than those during highstands.

Geometry, migration and evolution of small-scale bedforms generated by regular and irregular waves

Ripples generated by both regular and irregular waves have been experimentally investigated. In particular, their geometry at the equilibrium (i.e. height, length and steepness) has been analyzed and compared with semi-empirical predictive models. It has been observed that in the investigated range no distinction between ripples generated by regular or irregular waves is detectable, even though the equilibrium configuration is reached through different mechanisms. Moreover, it has been found that ripples migrate as soon as they appear and their velocity of migration qualitatively agrees with theories based on stability analyses recently proposed. Starting from results in literature, new semi-empirical formulae are provided for calculation of equilibrium characteristics of ripples.

Vers l’intégration des structures turbulentes de l’écoulement dans la dynamique d’un cours d’eau à lit de graviers

La compréhension de la dynamique des cours d'eau nécessite la connaissance des interactions qui lient l'écoulement, le transport des sédiments et le développement des formes sur le lit. Ceci implique la caractérisation des structures individuelles présentes dans l'écoulement mais aussi une prise en compte de leur organisation spatiotemporelle dans la section du cours d'eau. Trois structures turbulentes sont fréquemment utilisées pour décrire les écoulements sur lit de graviers : les structures d'éjection au lit, les structures d'échappement à l'aval d'obstacles protubérants et les structures à grande échelle se développant sur toute la tranche de l'écoulement. Il est possible de décrire les mécanismes de formation de ces structures de même que d'envisager leurs interactions dans un écoulement turbulent. Par exemple, les structures à grande échelle changent la dynamique des zones de recirculation à l'aval d'obstacle, ce qui donne naissance à différentes manifestations des structures d'échappement. Il est cependant plus difficile de regrouper ces structures à l'échelle d'une section de rivière. À partir des mécanismes de formation des structures et de leurs interactions, nous illustrons la complexité de l'organisation d'un écoulement à l'échelle d'une section de rivière. Bien que spéculative, cette illustration fait ressortir (1) l'étroite relation entre les échelles de rugosité du lit et les échelles des structures qui organisent l'écoulement ; (2) la présence de zones de production et de dissipation des structures ; et (3) l'anisotropie structurale de l'organisation de l'écoulement caractérisée par des interactions complexes entre les différentes structures. Cet essai souligne que les interactions entre les structures sont tout aussi importante que les structures ellesmêmes dans notre compréhension de l'organisation de l'écoulement.

Evolution of subglacial bedforms along a paleo‐ice stream, Antarctic Peninsula continental shelf

Geophysical data from the Antarctic Peninsula continental shelf reveal streamlined subglacial bedforms in a cross‐shelf trough. Bedforms exhibit progressive elongation with distance along the trough, and record flow of a paleo‐ice stream from the Antarctic Peninsula Ice Sheet during the last glacial maximum. Downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf. This, in turn, is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf. Although streaming flow operated across both substrates, the highest flow velocities occurred over the soft bed. Spatial variation in the inferred nature of fast‐flow, from sliding to subglacial sediment deformation and/or ploughing, was also lithologically controlled. These data highlight the control of subglacial geology on ice‐stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo‐ice streams.

Stochastic Fluvial Hydraulics

The purpose of this issue of the Journal of Hydraulic Engineering ~JHE! is to report on the latest developments in the field of ‘‘stochastic hydraulics.’’ The effort is the result of the ASCE Task Committee on Probabilistic Methods and was coordinated by Dr. Athanasios ~Thanos! Papanicolaou of the Albrook Hydraulics Laboratory at Washington State University. In particular, this volume is dedicated to the memory of Professor Ben Chie Yen of the University of Illinois, one of the primary contributors to the Task Committee and a world-renowned authority in the area of stochastic hydraulics and hydrology. In his quite determined way, Professor Yen contributed greatly to the robust development of the field of river hydraulics and hydrology, especially through his pioneering work in meandering channels in 1965. To honor Professor Yen, this volume starts with his forum entitled Stochastic Inference To Sediment Transport and Fluvial Hydraulics, which eloquently discusses the concept of stochastic processes in sediment transport applications and provides a thorough recollection of past and recent work in that area of research. Although the emphasis of this special issue is on stochastic methods in sediment transport and fluvial hydraulics, the peerreviewed articles selected for publication cover a wide range of stochastic methods in water resources engineering. The issue has gathered several original and innovative papers that describe different stochastic tools that could be useful in hydraulic and environmental engineering studies. The papers can be classified as follows: 1. Stochastic Incipient Motion Criteria: • Probability of Sediment Pick-up under a Log-Normal Distribution • Stochastic Incipient Motion Criterion for Spheres under Various Bed Packing Conditions 2. Bedform Development and Saltation processes

Discrete Particle Model for Analyzing Bedform Development

A discrete element model is used to simulate the emergence and evolution in time of wind ripples with the use of simple rules for the motion of individual sediment particles. It is shown, by analyzing the probabilities for the particles to be entrained and deposited over the bed implied by the selected rules, that the same model can be used to simulate the development of underwater bedforms. The results of the model show that the particle-transport processes implied by the simple rules used in the model can represent many different complex aspects of the physics of bedforms. The results of the model are compared with experimental results on underwater ripples obtained by the authors, and a surprisingly good agreement is found in terms of the main dynamical properties of the bedforms, such as growth rate of the amplitude, evolution of the steepness, growth saturation, and bedform celerity. Even though the simulated bedforms do not reproduce the exact geometry of their physical counterparts, the richness of the strongly nonlinear processes that this simple model is able to simulate makes this kind of approach useful to explain many of the observed features of underwater bedforms, which are otherwise very difficult to address theoretically.

Aeolian Sand Transport on a Ridge and Runnel Beach: Preliminary Results from Leffrinckoucke Beach, Northern France

The aeolian transport conditions on a macro-tidal ridge and runnel beach in northern France were monitored over two 30-minute experiments, representing respectively conditions of onshore and offshore to shore-parallel wind flow. Several anemometers, a portable weather station, acoustic grain impact counters (saltiphones), temperature and ground moisture probes, and aeolian sand traps were deployed in order to determine the effects of topographic, moisture and bedform variations on aeolian sand transport. The instruments were deployed across a ridge/runnel/ridge system on the mid- to upper beach, and on the upper beach terrace linking this ridge and runnel system to the dune front. The results show that the near-ground wind velocities are slowed by the pronounced ridge and runnel topography of the upper beach. The ridge and runnel system segments the fetch, whatever the wind direction. The experiment involving an onshore wind was associated with negligible rates of sand trapping in spite of suitable wind conditions. Significant trapping occurred during the offshore to shore-parallel wind experiment, but this was limited to the upper beach terrace and upper ridge and runnel, and there was little downwind transport below this upper ridge/runnel/ridge set. These preliminary experiments and the field observations suggest that the important degree of wave-tidal bedform development over the beach surface and the high moisture levels in the runnels and sometimes on the ridges, both common characteristics of ridge and runnel beaches, tend to limit sand mobilisation. It is tentatively suggested from this data set that this ridge and runnel beach is characterised by a moderate and balanced exchange of sand with the dune front.

Predicting channel patterns

The proposed distinction between meandering and braided river channel patterns, on the basis of bankfull specific stream power and bed material size, is analysed and rejected. Only by using regime-based estimates of channel widths (rather than actual widths) has discrimination been achieved, and it is argued that this procedure is unacceptable. An alternative is to explore the patterning processes underlying the marked pattern scatter on bankfull stream power/bed material size plots. Of the five sets of patterning processes, large-scale bedform development and stability is seen as especially important for meandering and braiding. For gravel-bed rivers, bedforms developed at around or above bankfull stage appear important for pattern generation, with braiding relating to higher excess shear stress and Froude number. There seems to be an upper threshold to both meandering and braiding which is achieved at extreme discharges and steep gradients, as on steep alluvial fans, rather than for the rivers with available flow data here considered. For sand-bed rivers with greater excess shear stress, the equivalent upper plane bed threshold may occur below bankfull, with bed material mobility and bedform modification occurring over a wider range of sub-bankfull discharges. Sand-bed channel margin outlines appear to be less perturbed by bedform effects than gravel bed planforms, and they may have naturally straight or sinuous planforms. Bedform relief may nevertheless lead to some being designated as braided when viewed at low flows. It is concluded that the use of a single-stage stream power measure and bed material size alone is unlikely to achieve meandering/braiding discrimination.

Evolution of small scale regular patterns generated by waves propagating over a sandy bottom

In this paper the evolution of a sandy bottom subject to a wave generated flow has been analyzed using an image acquisition technique. In particular, ripple formation was observed starting from a flat bed till a stable configuration was attained. The experimental findings showed that rolling grain ripples never appeared as a stable configuration but only as a transition toward the equilibrium, represented by vortex ripples. The latter stage was reached after about 100 cycles if lightweight sediments were used, or about 400 cycles if quartz sediments were adopted. It was also observed that when the bedforms appear their wavelength is much smaller (about one half) than that at the equilibrium stage, this result being in contrast with most of the analytical models on bedform evolution. Finally, it was observed that ripples migrate as soon as they appear. The measured velocity of migration seems to confirm, from a qualitative point of view, the theoretical findings obtained for rolling grain ripples by Blondeaux et al. [Eur. J. Mech. B/Fluids 19, 285 (2000)].

Experimental study of wind directional variability in the vicinity of a model valley

This paper reports the results of a preliminary study examining the effects of valley presence on airflow direction and focuses on valleys situated in areas of low relief. Wind tunnel experiments are used to visualise near-surface airflow over a series of models representing valleys oriented at different angles to mean airflow. Results indicate that the presence of a valley has no significant influence on airflow direction when the approach angle of the wind is perpendicular to the valley strike. However, any wind approach-angle less than 90° is found to cause marked deflection of airflow to become more parallel to the valley. Analogies can be drawn between the surface streamlines recorded on the negative terrain of the valley models and those around positive terrain such as hills. A discussion of air pressure perturbation over both positive and negative terrain concludes that valley structures can have a significant effect on overlying synoptic windflow. The nature and magnitude of this effect is dependent upon a complex range of environmental parameters including valley morphometry, wind speed and the thermal stability of the airflow. The findings outlined have implications for the development of aeolian bedforms and processes of aeolian sedimentation in the vicinity of valleys.

SMITH, N. D. &amp; ROGERS, J. (eds) 1999. Fluvial Sedimentology VI. Proceedings of the Sixth International Conference on Fluvial Sedimentology, held 22–26 September 1997, Cape Town. International Association of Sedimentologists, Special Publication no. 28. x+478 pp. Oxford: Blackwell Science. Price £55.00 (paperback). ISBN 0 632 05354 2.

This book is a collection of papers arising from the 6th International Conference on Fluvial Sedimentology, held in Cape Town in 1997. It is the latest in a series of volumes from these four-yearly conferences and continues the wide-ranging approach evident in previous volumes. The book is organized into five sections, which cover progressively larger timescales and lengthscales of observation. The first section (‘Sediment transport and bedforms’) contains four case studies documenting processes of sediment transport and resulting bedform evolution in a range of modern rivers and estuaries. This process-based approach is continued in …

Turbulence–particle interactions and their implications for sediment transport and bedform mechanics under unidirectional current: some recent developments

In spite of significant developments made in turbulent boundary layer research over the years, our understanding of particle interactions with organized flow configurations (coherent structures) and their implications for mechanics of bedform development is far from completion. Researchers have made sincere efforts to obtain fundamental understanding of bedform mechanics through theoretical and experimental studies supplemented by field observations. A brief outline emphasising the nature of recent advancements and their consequences in sediment transport and bedform mechanics in aqueous environment under unidirectional current is given in this paper.