Bedform Types Research Articles

An experimental study on the ripple–dune transition

AbstractFlume experiments were conducted on different bed stages across the ripple–dune transition. As flow velocity increases, an initially flat bed surface (made of fairly uniform sandy material) is gradually transformed into a two‐dimensional rippled bed. With further increase in velocity, two‐dimensional ripples are replaced by irregular, linguoid ripples. As the average velocity necessary for the ripple–dune transition to occur is imposed on the bed surface, these non‐equilibrium linguoid ripples are further transformed into larger, two‐dimensional dunes. For each of these stages across the transition, a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated. An acoustic Doppler velocimeter was used to study the flow characteristics above each bed surface. Detailed profiles were used along a transect located in the middle of the channel. Results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields (section view). They clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition. Turbulence intensity and Reynolds stresses gradually increase throughout the transition. Two‐dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed. Linguoid ripples induce three‐dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region. A very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes. The turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability (when compared with the rippled beds). Further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis, together with an emphasis on quadrant analysis. Copyright © 2001 John Wiley & Sons, Ltd.

A New Type of Bedform Produced by Backfilling Processes in a Submarine Channel, Late Miocene, Tabernas-Sorbas Basin, SE Spain

A New Type of Bedform Produced by Backfilling Processes in a Submarine Channel, Late Miocene, Tabernas-Sorbas Basin, SE Spain

The influence of bedforms on flow and sediment transport over intertidal mudflats

A range of bedforms of different types are found at a range of scales on intertidal mudflats. The different types include (a) channels, creeks and gullies, (b) ridge–runnel systems, (c) ripples and other micro-topography, and (d) cliffs. The main features of these bedform types are illustrated with examples found at a number of NW European mudflat sites studied during the EC INTRMUD project. Some comments are made on their environment of formation and observations of bed sediment properties and flow and suspended sediment processes are presented. Finally, the influence of bedforms on the hydrodynamics, shallow water tidal flow and wave propagation, and sediment processes are discussed.

Velocity characteristics along a small step-pool channel

This paper summarizes measurements of velocity along three reaches of a small mountain channel with step–pool bedforms. A one-dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph. Measurement cross-sections were located upstream from a bed-step, at the step lip, downstream from the step, and in a uniform-gradient run. Data analyses focused on characteristics of velocity profiles, and on correlations between velocity characteristics and the potential control variables bedform type, reach gradient and flow depth. To test the hypothesis that velocity characteristics are related to channel bedform types, ANOVA and ANCOVA tests were performed for the average velocity and coefficient of variation of point velocity data. Results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge. Velocity became more variable as stage increased, particularly at low-gradient reaches with less variable bed roughness. Velocity profiles suggest that locations immediately downstream from bed-steps are dominated by wake turbulence from mid-profile shear layers. Locations immediately upstream from steps, at step lips, and in runs are dominated by bed-generated turbulence. Adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation, whereas favourable pressure gradients at steps are suppressing turbulence. The bed-generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake-generated turbulence and form drag of step–pool bedforms. Copyright © 2000 John Wiley & Sons, Ltd.

The hydraulic resistance of sand streambeds under steady flow conditions

Laboratory and river data has been analyzed in terms of applied power principles to determine the relationships between absolute roughness and bed deformation under steady state sediment transport conditions. It has been found that the full spectrum of bed conditions can be represented by simple relationships within a single system. As the bed forms grow, the unit stream power being applied along the bed decreases. The process of deformation continues until equilibrium is reached. Dynamic equilibrium is reached with the average rate of deposition of particles equal to the rate of re-suspension. By expressing the applied power in terms of the size of the boundary eddies i.e. the bed form sizes and comparing this with the power required to suspend the sediment particles, the link is obtained between absolute roughness and particle characteristics. Transition from lower regime conditions to upper regime conditions is initiated when the applied (laminar) power along the bed becomes greater than that which is required to maintain turbulent flow. Flow along the bed becomes fully turbulent, the (large) bed forms are rapidly broken down and fully turbulent conditions prevail until the applied turbulent power again becomes greater than that which is required for a laminar boundary layer. From this point onwards upper regime bed forms develop and equilibrium is again reached when the eddies that fit in with the bed forms grow large enough for the rate of entrainment of particles to be equal to the rate of deposition. A single diagram has been developed for determining flow resistance under all flow conditions as well as for identifying bedform types.

Side-scan sonar mapping of bedforms in the middle Tay Estuary, Scotland

Side-scan sonar, augmented with echo-sounding and sediment sampling, has been employed to examine and map the distribution of sediments, bedform types and bedform geometries in the middle reaches of the macrotidal Tay Estuary of eastern Scotland. Much of this dynamic sector is characterised by mobile, medium to slightly gravelly sands. The most abundant bedforms are small to medium dunes (wavelength 2-10m, height up to 0.5m) of varying sinuosity and superposition the asymmetries of which are indicative of the dominance of flood currents and the marine derivation of sands. However, previously unrecognised abrupt, non-gradational changes in bedform morphology and sediment type occur both with and without apparent bathymetric control. A hitherto unreported acoustic signature, referred to as 'patchy discontinuous', was recorded in many areas of channels. This is attributed to the patchy colonisation of mussels preferentially on the gentle stoss slopes of dunes, with resultant sediment stabilisation, whilst uncolonised sediments are exposed on the steeper lee slopes.

Landform modification by palaeo-ice streams in east-central Ireland

AbstractIn eastern Ireland, subglacial bedforms including drumlins and Rogen moraines were modified by headward erosion along two ice streams which had overlapping flow tracks. The ice streams, which had tidewater termini, are dated by geochronometric and morphostratigraphic methods to <15.014 C kyr BP (Castleblaney ice stream) and ~13.814C kyr BP (Armagh ice stream). Bedforms along ice-stream tracks show a morphological continuum which reflects a down-ice increase in the degree of modification by ice-stream activity (i.e. resulting in unmodified →remoulded/overprinted →crosscut →streamlined bedforms). The down-ice changes in bedform types are inferred to relate to changes in subglacial drainage and sediment-transport mechanisms. Bedform and sedimentary evidence suggest that discrete subglacial meltwater channels which developed up- ice changed in a down-ice direction to unchannelized flows which deepened towards the ice margin. Meltwater release from subglacial cavities, and produced by strain heating at sheared ice-stream margins, probably helped support ice-stream flow, which ended as the volume of subglacial meltwater discharge decreased. Dated millennial-scale cycles of ice activity may be related to instability at tidewater margins, followed by complex thermal and hydraulic responses within the ice mass.

Problems of Irish drumlins and Late Devensian ice sheet reconstructions

Reconstructions of the Late Devensian ice sheet in Ireland are strongly biased towards explaining data obtained by the Geological Survey of Ireland during the last century, and observations undertaken by key workers in this century (i.e. J. K. Charlesworth). Modern field observation and analytical techniques, however, show that many of these data, such as the distribution of drumlins, are erroneous. Furthermore, models derived from these data, dealing with the significance and distribution of different bedform types, the location of ice centres and the direction of ice flow lines, can no longer be supported. New observations based on mapping from satellite imagery, and validated by detailed regional-scale field studies, show the complexity of subglacial bedform patterns which record several phases of subglacial activity during the Late Devensian. The presence of ice flow-transverse Rogen moraines and patterns of overprinted bedforms in north central Ireland suggest that subglacial environments and the location of ice centres changed dramatically during the overall glacial cycle.

Algorithm for Resistance to Flow and Transport in Sand-Bed Channels

An algorithm is developed that relates depth to discharge and determines bed- and suspended-load transport for the entire range of bed forms found in sand-bed channels; equilibrium-state geometry of lower flow regime bedforms is also predicted. Grain shear stress and form resistance are differentiated using a drag coefficient closure and a two-segment logarithmic velocity profile. A Meyer-Peter-type formulation is used to compute sand transport in the bed-load layer and for computing suspended sand transport, McLean’s procedure of 1991 and 1992 is adopted. A bed-form classification scheme is developed that uses a particle size parameter and the transport strength to predict bed-form type; it correctly identifies 73% of the 1,192 mostly flume-scale calibration data sets. For 194 sets of dune-transition bed-form calibration data, a modified version of van Rijn’s 1984 bed-form geometry predictor yields a geometric average predicted to observed height ratio of 1.00. After calibration, the algorithm produces overall geometric averages of predicted to observed depth and predicted to observed transport of 1.00. For a verification data set of 855 observations, mostly from rivers and canals, the overall geometric averages of predicted to observed depth and transport are 0.87 and 1.14.

Turbulence Measurements in Open‐Channel Flows over Artificial Bed Forms

Measurements of mean and turbulence characteristics in uniform open‐channel flows of constant mean depth over artificial fixed one‐dimensional periodic bed features were obtained using two‐component laser‐Doppler velocimetry. Two types of bed forms, based on triangular elements of the same height (≈20% of the flow depth) and wavelength (≈12.5 times the height) but of different shape, were studied: (1) 45° triangle ridges separated by a flat region; and (2) triangular elements approximating dunes with a mildly sloping upstream face instead of a flat region. The role of the total shear velocity, based on the mean energy slope and the mean flow depth, as a possible velocity scale is studied. Velocity defect and the Reynolds shear stress profiles were generally well approximated by the flat‐bed profiles for regions of flow far from the bed. Sufficiently far from the recirculation and reattachment zones, turbulence characteristics collapsed fairly well when scaled with the total shear velocity over most of the depth, but do not follow the flat‐bed behavior in the lower part of the flow. Possible implications of the clear‐water measurements for the description of suspended sediment concentration profiles and total suspended load computations are explored.

Sediment transport on the eastern Canadian continental shelf

A synthesis is presented of the sediment transport pathways resulting from hydrodynamic processes over the eastern Canadian continental shelf. This is based on interpretation of bedform type and orientation, grain size trends, sediment stratigraphy, visual observation and direct measurements. These pathways are then compared with results of numerical simulations of the transport of 0.35 mm (medium) sand for the Grand Banks of Newfoundland, the Scotian Shelf and the Gulf of Maine. The abundance of hydrodynamically reworked sediment increases from north to south. There is no evidence for sediment transport on the Baffin Shelf or Arctic Island Channels due largely to ice cover and deep (200 m) water. Sediment transport on the Labrador Shelf, by contrast, is apparent though restricted to bank tops in depths less than 120 m. This transport is predominantly to the southeast under the influence of strong wind-driven and geostrophic flows. The Grand Banks of Newfoundland is subject to storm-induced sand transport in depths less than 120 m. Transport here is to the south and southeast over much of Grand Bank and Avalon Channel. Above approximately 60 m the seabed is devoid of sand, suggesting scouring; between 60 and 120 m sand transport takes place principally as bedload. The shelf immediately south of Newfoundland is subject to strong easterly sediment transport that has scoured the banktops of all but gravel-sized sediment. The Scotian Shelf and Georges Bank also exhibit storm-induced sediment transport that is tidally-modulated in depths less than 100 m. The predicted and observed sand transport pathways are remarkably consistent. Predicted zones of sand convergence and divergence correspond to known regions of sand accumulation and depletion, respectively. Quantitative predictions are, by contrast, only within an order of magnitude of observations. Percentage exceedence predictions of sand transport for the Hibernia (Grand Banks of Newfoundland) and Venture (Sable Island Bank) discovery sites show strongly linear trends on a Gumbel distribution. These plots show that intense storms (greater than 20 year return interval) dominate net transport at Hibernia, whereas short-term storms (annual or shorter return interval) dominate at Venture.

Reconstruction of Eolian Sequences--A Function of Scale, Jurassic Page Sandstone, Arizona: ABSTRACT

Interpretation of sequences of ergs and bed forms that comprise an eolian unit is strongly dependent on the scale of mapping. A pentagon-shaped, three-dimensional outcrop of eolian Middle Jurassic Page sandstone near Page, Arizona, measuring 36 m high and nearly 1 km in perimeter, was mapped with the aid of an electronic tacheometer. The extent and orientation of bounding surfaces and sets of cross-strata were mapped in detail, foreset orientation were measured, and a general depiction was made of the lateral and vertical distribution of stratification types within sets and of features associated with the surfaces. The level of mapping is at a middle scale with respect to architectural elements and proved adequate for an interpretation of most bed forms in terms of shape (two or three dimensions, sinuosity, symmetry, and crestline-phase relationships), size (height and width), aerodynamics (overall primary and secondary airflow patterns), and migration behavior (degree of climb and recognition of cyclicity). These interpretations allow the determination of bed-form hierarchy (simple dunes or draas) and of the bed-form type according to the descriptive (crescentic, linear) and dynamic (transverse, oblique, longitudinal) classifications. In conjunction with this level of mapping, large-scale or regional mapping was used in order to identifymore » distinct, stacked, dune-field, and erg deposits that comprise the Page sandstone. Local mapping of the out-crop at a fine scale was necessary for refined interpretation of cyclicity, bed-form size, migration rate, and airflow pattern.« less

Ripple generation under the combined influences of waves and currents on the Canadian continental shelf

A multi-parameter instrument package ( Ralph) was deployed for 15 days in 22 m of water on Sable Island Bank, Scotian Shelf. The instrument successfully recorded mean current velocity, statistics on near-bed wave motion, wave height, period and propagation direction, and time-lapse photographs of the seabed. The seabed at the deployment site was composed of well-sorted, fine sand (0.23 mm mean diameter) which moved during peaks in tidal flow and during periods of high wave activity. Eight distinct bed types were seen in time-lapse imagery: (1) wave ripples; (2) straight-crested current ripples; (3) linguoid current ripples; (4) wave and current ripples; (5) transitional wave ripples; (6) transitional current ripples; (7) poorly developed (biodegraded) ripples; and (8) flat bed. Each bed type occurred at well-defined combinations of near-bed wave motion, mean current speed and biodegradation. Wave-current ripples intermediate in form and orientation, were not seen. Bedform types were found to be well separated in plots of wave Reynolds number (U mb·d 0/ν) and dimensionless mean flow (U 100/W s). A better separation was found if the wave Shield's parameter (after Grant and Madsen, 1979 , Journal of Geophysical Research, 84, 1797–1808) and the current Shield's parameter (after Sternberg, 1972 , Shelf sediment transport, process and pattern, pp. 61–83) were used. Bedform stability plots showed that wave ripple threshold is influenced by currents, and current ripple threshold is influenced by waves. Despite this, ripple type is defined by the partitioned, wave and current components of stress and not the total stress as no intermediate (wave-current) ripple type was observed. Thresholds for sand transport and the generation of well-developed bedforms were influenced by the combined, near-bed motions of waves and currents even when the bed exhibited wave ripples or current ripples only. The threshold for sand transport under combined flows is vague, but is adequately represented by a line drawn between the threshold for transport under unidirectional flow based on the modified Shield's curve of Miller et al. (1977 , Sedimentology, 24, 507–527) (θ c= 0.04) and the threshold specified for pure wave motion of Komar and Miller (1973 , Journal of Sedimentary etrology, 43, 1101–1110) (θ w = 0.04). A first approximation to a threshold criterion for fine sand is proposed as [θ w + θ c]crit = 0.04 .

Large-scale bedforms as indicators of mutually evasive sand transport and the sequential infilling of wide-mouthed estuaries

Large-scale bedforms (2–10 m in vertical and 10 2–10 3 m in horizontal dimensions) found in wide-mouthed estuaries are described. Different bedform types occur depending upon the local availability of sand. With an increasing sand supply, sand ribbons grade into elongate trains of sand waves and then form sandwave fields. Inshore, headland-associated sand banks are formed which multiply into en-echelon sand banks. Based upon a review of data on directions of sand transport from the Bristol Channel and Thames Estuary, U.K., and from Moreton Bay, Australia, charts of ebb- and flood-dominant transport zones are constructed for lower estuarine environments which have undergone different degrees of infilling. Linear sand banks are seen to delimit partially the boundaries between opposing sand transport zones. Transport paths demonstrate how sediments derived from outside of the estuary are dispersed through ebb and flood transport zones, to supply other areas of net deposition. A comparison between different estuaries reveals that variations in the compexity of ebb- and flood-dominant transport zones and the morphologies of large-scale bedforms are coupled with apparent changes in the relative amounts of sand available to each system. A model for the sequential infilling of estuaries and the evolution of large-scale bedforms is presented and applied to the interpretation of present day examples. Vertical sequences predicted to be generated by such bedform evolution are described and discussed, in terms of their preservation in the geological record.

Spatial variability and variations in bedload transport direction in a subtidal channel as indicated by sonographs

AbstractThree sonographs of a part of a subtidal channel, located in the Oosterschelde, are described and interpreted. The sonographs are used to determine the asymmetry of the bedforms. Current profiles and directions are used to explain differences in dimensions and characteristics of the bedforms. The relations between current velocity parameters and bedform characteristics in intertidal zones do not apply to subtidal forms. The division in the depth‐velocity diagrams between the types of 2d and 3d bedforms of Dalrymple et al. (1978) is not valid.

Objective Identification of Pools and Riffles

This paper presents a new technique for objectively identifying pools and riffles. The technique is termed the bed form differencing technique and as its data base has bed elevations surveyed at a fixed interval along the channel. These elevations are differenced, and the difference or elevation change values are used to identify pools and riffles. Where the cumulative elevation change since the last bed form exceeds a certain tolerance value, a new bed form is considered to exist, the type of bed form being determined by the direction of the elevation change. Analysis of a small stream in western New York suggests that the most realistic pool‐riffle sequence is obtained where the tolerance value is 0.75 times the standard deviation of the difference values. The bed form differencing technique can be used to identify bed forms other than pools and riffles, such as ripples and dunes, by simply modifying the surveying interval to reflect the scale of the bed forms.

Bedforms of the Canadian eastern seaboard: A comparison with global occurrences

This paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern Canadian continental shelf. In order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found, and thereafter to relate the subsequent terminology to any diagnostic, generic characteristic identifiable from the records. Sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples. These latter features are identifiable adopting the classification of Dalrymple et al. (1978) which proves adequate for the acoustic differentiation of 2-D and 3-D types; though we observed a greater coherence in bedform spacing of the 2-D types than reported hitherto. Sand ridges were adequately defined and recognised using the description of Swift et al. (1978) with respect to storm-generated ridges and that of Off (1963) and others with respect to tidal-current ridges. Such ridges appear not to be uniquely described by ratios of their metrics. The bedforms on the southeastern Canadian continental shelf are abundant and wide-spread. Virtually all types of bedforms, recognisable on the U.S. shelf are present to the north. The majority of these bedforms appear to be storm generated and are considered intermittently active. Many of the sand waves (s.s.) have no measureable relief though retaining their distinctive spatial geometry. Both active and moribund sand ridges were found. Of these ridges both tidal- and storm-generated types were identified. The storm-generated ridges were further broken down into shoreface-connected ridges and offshore ridges. A metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height—length ratios provided the bedforms are active and fully developed. There is some overlap between megaripples and small sand waves and indeed between 2-D and 3-D megaripples, yet the distinction is good between sand waves and sand ridges (even moribund types). Sand ribbons in general can be confused with moribund sand waves and megaripples. A detailed morphological examination is required to distinguish each feature. Data from sand ridges of the eastern Canadian shelf, combined with data from similar features found globally, shows that ridge heights tend to cluster into three distinctive modes. These modes are at heights of 7, 13 and 27 m. The first mode is correlated with storm-generated ridges, the second with tidal-current ridges and the third possibly with submerged barrier islands.

Stability of bedforms in a tidal environment

Large-scale bedforms developed in a tide-dominated salt estuary near Sapelo Island, Georgia, are morphologically separated into two distinct groups. Megaripples include bedforms 4 to 15 m in wavelength and 10 to 75 cm in height. Sand waves are 15 to 45 m in wavelength and 85 to 130 cm in height and can be further subdivided into megarippled forms and smooth-crested forms. Ebb-oriented geometry of large-scale bedforms is maintained through both ebb and flood portions of the tidal cycle. Bedforms are in equilibrium with peak tidal flow, and ebb orientation is the direct result of large pressure gradients and related bed-shear values that develop during the ebb. Bedform heights can be statistically related to flow power, shear velocity, water depth and sediment grain size. Mean flow velocity values used in many previous studies of bedform stability did not produce significant F-statistic values in a multiple linear regression procedure used to discern relationships among variables. Stability fields of bedform types can be empirically defined using parameters statistically significant in the regression analysis. Flow power, shear velocity, water depth, and sediment grain size were used in various combinations to construct bivariate plots showing separate stability fields of sand waves and megaripples.

Neap–spring tide sequences of intertidal shoal deposits in a mesotidal estuary

ABSTRACTIn the mesotidal (tidal range 3·5–4·9 m) Westerschelde estuary (The Netherlands) the intertidal part of a sandbank was the subject of systematic observations of: (1) hydrographic properties, (2) the distribution and response pattern of various types of bedforms, and (3) the sedimentary structures produced. Ebb and flood usually differ considerably in strength, giving rise to a clear dominance of one over the other, which may change over the neap‐spring tide period. Parallel, long‐crested sand‐waves and irregular, short‐crested dunes have a different response to the neap‐spring variation in current velocity.Because one tide (usually the flood in our area) predominated over the other, the internal structure largely consists of unidirectional cross‐bedding, separated into a succession of tidal bundles, each formed during one tide. The tidal bundles are arranged in a lateral sequence reflecting neap‐spring tide periods and differing in character with location.Within the tidal bundle, reactivation, full vortex and slackening structures reflect acceleration, full stage and deceleration of flow respectively in the dominant tide. The full vortex structures tend to be well developed around spring tide but disappear towards neap tide. The subordinate tide carves ‘pause planes’ which enclose the tidal bundles. These pause planes are either erosional or depositional (mud).

Sediment transport in a microtidal estuary: Matanzas River, Florida (U.S.A.)

Tidal channels and shoals at the mouth of the Matanzas River are the site of considerable sediment transport. Rates and patterns of transport were determined using fluorescent dyed sediment tracers which were introduced on three intertidal locations. Temporal sampling after introduction was at the surface and by coring. Transport rates and patterns were examined for both quartz and shell hash fragments in the distinctly bimodal sediment. Wave activity and time-velocity asymmetry of currents produced a net landward sediment transport over the channel-margin bar and flood delta. The combination of time-velocity asymmetry and deflection of currents by the ebb shield causes landward transport over the flood delta. Ebb transport of sediment dominates in the south flood channel. Minimum sediment transport rates vary with morphology, currents, and grain-size parameters, but these rates average 15–20 m over one tidal cycle. Wave activity enhances transport of shell hash. Vertical variation in tracer concentration consistently decreases with depth. Depth of disturbance values are typically 10–15 cm on the channel margin bar, but are variable elsewhere, being controlled by type and height of bedforms.