Formation Of Antidunes Research Articles

The influence of turbidity currents and contour currents on the distribution of deep‐water sediment waves offshore eastern Canada

AbstractSediment waves are commonly observed on the sea floor and often vary in morphology and geometry according to factors such as seabed slope, density and discharge of turbidity currents, and the presence of persistent contour currents. This paper documents the morphology, internal geometry and distribution of deep‐water (4000 to 5000 m) bedforms observed on the sea floor offshore eastern Canada using high‐resolution multibeam bathymetry data and seismic stratigraphy. The bedforms have wavelengths of >1 km but fundamentally vary in terms of morphology and internal stratigraphy, and are distinguished into three main types. The first type, characterized by their long‐wavelength crescentic shape, is interpreted as net‐erosional cyclic steps. These cyclic steps were formed by turbidity currents flowing through canyons and overtopping and breaching levées. The second type, characterized by their linear shape and presence on levées, is interpreted as net‐depositional cyclic steps. These upslope migrating bedforms are strongly aggradational, indicating high sediment deposition from turbidity currents. The third type, characterized by their obliqueness to canyons, is observed on an open slope and is interpreted as antidunes. These antidunes were formed by the deflection of the upper dilute, low‐density parts of turbidity currents by contour currents. The modelling of the behaviour of these different types of turbidity currents reveals that fast‐flowing flows form cyclic steps while their upper parts overspill and are entrained westward by contour currents. The interaction between turbidity currents and contour currents results in flow thickening and reduced sediment concentration, which leads to lower flow velocities. Lower velocities, in turn, allow the formation of antidunes instead of cyclic steps because the densiometric Froude number (Fr′) decreases. Therefore, this study shows that both net‐erosional and net‐depositional cyclic steps are distributed along channels where turbidity currents prevail whereas antidunes form on open slopes, in a mixed turbidite/contourite system. This study provides insights into the influence of turbidity currents versus contour currents on the morphology, geometry and distribution of bedforms in a mixed turbidite–contourite system.

Finite-amplitude river dunes

The linear and weakly nonlinear stability of a uniform flow in an infinitely wide open channel with erodible bottom is studied. Under suitable conditions the flow is found to be unstable, leading to the formation of dunes and antidunes. At a linear level, the corresponding regions of existence are presented and compared with experimental data. A weakly nonlinear analysis is then performed in a neighbourhood of the critical conditions for dune and antidune formation. The analysis shows that, for values of the ratio of the shear velocity to the depth-averaged velocity of practical interest, dune bifurcation is supercritical, whereas antidune bifurcation is subcritical. The latter result suggests a possible interpretation of the plane–antidune transition, where plane bed and antidune configurations are observed to coexist for the same values of the flow and the sediment parameters. The supercritical behaviour of the dune bifurcation allows for the prediction of an equilibrium amplitude that successfully compares with the amplitudes measured in laboratory experiments.

Bed morphology and generation of step–pool channels

AbstractFlume experiments have been carried out to study the formation processes and the bed morphology of step–pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step–pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand‐bed rivers. This observation points out that step–pool field‐data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand‐ and gravel‐bed rivers or step–pool units in boulder‐bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self‐stabilization by modifying the step–pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley & Sons, Ltd.

Sedimentology and geochemistry of extensive very coarse deepwater submarine fan sediments in the Middle Jurassic of Oman, emplaced by giant tsunami triggered by submarine mass flows

Unusual fining upwards coarse conglomerates overlain by sandstones, thin cherts and green shales occur at the top of the deep-water submarine fan deposits of the Oolitic Limestone Member of the Jurassic Guwayza Formation of Oman. They separate the dominantly submarine fan deposits of the Guwayza Formation from the pelagic shales, fine-grained limestones and cherts of the overlying Sidr Formation. The cross-bedded and graded framework conglomerates occur in extensive, tabular units and are dominated by earlier Mesozoic carbonate clasts with sandy oolitic and peloidal grains derived from fault escarpments and shelf sediments far to the southwest. Subordinate inverse grading, very thick beds, very large floating clasts (up to 100 m long in places) indicate deposition from catastrophic debris flows. Though most palaeocurrents indicate flow from off the platform to the southwest, hummocky cross-bedding shows divergent palaeocurrents suggesting movement in part by deep-water waves. The beds are too coarse for antidune formation and the conglomerate to sand hummocks indicate decelerating flow. There are no nearby large objects to deflect turbidity currents to form divergent flows. We consider that the hummocky cross-stratification, like that in shallow water, was formed by interfering waves. That such coarse, tabular conglomerates affected by wave action occur over extensive areas across deep submarine fan environments, suggests deposition by high-velocity seaward-moving debris and grain flows followed by reworking by waves large enough to redistribute coarse sediment in deep water. The only waves large enough are those of giant tsunami. Petrology and geochemistry show no impact or explosive volcanic constituents in the finer units and the waves involved are too large for generation directly by submarine fault displacements. We suggest that the top Guwayza conglomerates were deposited by very large submarine slides which were then reworked by the tsunami generated by them. Such contemporary massive slope failure deposits are present on the adjacent slope and shelf margin.

Revisiting the linear theory of sand dune formation

A linear stability theory for dune and antidune formation is presented that implements a rotational two-dimensional flow model. As in previous linear theories, the phase-lag between sediment transport and bed elevation remains the main mechanism driving instability. However, it is shown that this phase-lag varies significantly in a neighbourhood of the bed. Moreover, since the layer in which sediments are moving has a finite (though small) thickness, it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself. It is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process. This also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes. Following the above ideas, antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia. The present analysis ultimately allows a successful unification of the theories of dune and antidune formation.

Local scour around bridge piers

Local scour around cylindrical bridge piers in uniform, cohesionless sediment was investigated experimentally. Three empirical functions which relate equilibrium scour depth with approach velocity, sediment size and How depth were obtained. The variations of equilibrium scour depth with approach velocity show that the depth of scour decreases at velocities just above the threshold until it reaches a minimum at about twice the threshold velocity. Thereafter, it increases again until a maximum is reached at the transition flat bed condition. At still higher velocity, the equilibrium scour depth decreases due to the formation of antidunes. Both the effects of sediment size and flow depth were found to be independent of the stage of bed particle motion and the approach velocity. Adjustment factors, for the effects of sediment size and flow depth were defined and evaluated.

Antidunes on Modern and Ancient Washover Fans

ABSTRACT Antidune deposits on a washover fan on Seabrook Island, South Carolina display a variety of bedding types. The predominant stratification form comprises nested, very thin lenses which are internally cross-laminated and which become thinner and more closely spaced downflow. In addition, cross laminae within the lenses tend toward backset forms in the bermward portions of the flow, and become more foreset-oriented in a downfan direction. These characteristics, together with the antidune field's relationship to other bed features on the fan, suggest that antidune formation is related to the occurrence of an undular hydraulic jump just downfan from the berm crest. Ancient deposits interpreted as washover sequences, including Ordovician and Pleistocene sandstones in South Africa and a Carboniferous sandstone in Kentucky, display sedimentary structures similar to those observed in the Seabrook antidune field. Because washed-out antidunes and their associated cross laminae are very subtle features, antidunes are possibly responsible for many of the gentle irregularities in otherwise flat beds and are therefore possibly far more common in the geologic record than previously recognized. Their sedimentary structures are distinct from degraded forms of ripple cross-lamination, and in many cases are identical to features usually described in the literature as low-angle truncation surfaces.

Origin of Step-Pool Systems in Mountain Streams

Mountain streams very often present a step-pool aspect. Antidune formation alone; dispersion and sorting theory or velocity reversal as advocated for the origin of riffle-pool sequences do not explain how steps and pools form. Experiments have been conducted to clarify their origin. It was found that the step-pool aspect appears after high flows had deformed the bed. During the presented tests a self armoring process occurred which resulted in a stabilization of the bed. When coarsening of the top layer was little classic antidunes resulted. An important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance. The substantial increase in resistance was an essential factor in bed-stabilization.

Effects of Shock Waves, Channel Width, and Walls on the Formation of Antidunes Moving Downstream and Alternating Bars

Effects of Shock Waves, Channel Width, and Walls on the Formation of Antidunes Moving Downstream and Alternating Bars

Backset Bedding in Turbidites: A Further Example from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec

ABSTRACT Backset bedding was found in a thick (144 cm) turbidite bed in the Cloridorme Formation, Gaspe Peninsula, Quebec. The backsets occur in coarse-medium sand below a train of three trochoidal profiles (wavelength 30 cm, amplitude 4 cm) and above a basal division (9-13 cm thick) of parallel lamination. Flute casts on the sole of the bed, and ripple-drift cross-lamination--forming the preservation blanket covering the trochoidal profiles and backsets--allow paleocurrent direction measurements. The backset bedding is interpreted to result from the upstream migration of antidunes. A suitable analysis for the formation of antidunes by density currents uses an equation appropriate for internal waves at a density interface between two superimposed fluids which are moving parallel and in the same direction.

Paleohydraulic Interpretation of Antidune Bedforms with Applications to Antidunes in Gravel

ABSTRACT Antidune-like bedforms in the alluvium of the gravel-bedded North Saskatchewan River, Alberta, form regular trains of gravel mounds. Crest lengths, transverse to the flow, are about 2 meters, and crest spacing, along the flow, is between 2 and 3 meters. Flume experiments with gravel beds show that published equations describing flow conditions for the formation of antidunes on sand beds are applicable to antidune formation on gravel beds. Paleohydraulic reconstructions based on antidunes give rough estimates for depth and velocity, but cannot be used to estimate bedslope. Comparison of slope estimates based on antidune dimensions and observed bedslopes show the inadequacies of such estimates.

Sediment Inertia as Cause of River Antidunes

The formation of antidunes on riverbeds is explained in terms of sediment inertial effects. Due to inertia, bed sediment in transport cannot respond immediately to flow changes. To quantify the effect, a simple equation for sediment momentum balance is derived. A stability analysis indicates that riverbeds are unstable for a specific range of Froude numbers and that the instability leads exclusively to antidune formation. The instability, which is scaled by a dimensionless number representing the ratio of sediment inertia to friction, is a result of a spatial lag of bed load behind bed stress. An analytical relation for spatial lag is obtained, and the lag is estimated for some experimental data. The limits of antidune formation are compared with data.