Hydrodynamics and sedimentary structures of antidunes in gravel and sand mixtures

Abstract

This thesis firstly reviews the current literature available on antidune bedforms and their hydrodynamic environment, alongside recent studies of the turbulence environments associated with bedforms in unidirectional flow. Based on this understanding, three suites of experiments were designed and conducted to elucidate turbulent flow structure within the standing waves above antidunes and to record the sedimentary response of a loose mobile bed that constituted the antidunes. The first suite of experiments used Acoustic Doppler Velocimetry (ADV) to quantify and characterise the flow structure above fixed bedforms and this was supported by a second suite of experiments that used high-speed video to visualise flow structure. Finally, in the third suite of experiments a loose bed of sediment was allowed to deform into antidunes beneath standing waves and the resultant sedimentary structures were recorded and related to the growth and decay of both standing waves and antidune form. Taken together these data have been interpreted in order to identify and elucidate the bulk-flow, turbulent environment of the flow field above antidunes and the sedimentary structures that characterise the preserved antidune bedding.
The ADV experiments have shown that a coherent and organised spatial pattern of turbulence exists above antidune bedforms. Initially, when antidune amplitude is small, turbulent stresses are relatively equally distributed along the entire bed boundary layer, however as antidune amplitude increases there is a progressive concentration of turbulent stresses. Turbulence becomes increasingly concentrated in the near-bed region within the trough between upstream and downstream contiguous antidunes and on the upstream flank of the antidune immediately downstream. Velocities in the trough region drop significantly below the mean velocity elsewhere over antidune bedforms. A clear distinction can be drawn between sand and gravel antidunes, with gravel antidunes having comparatively much lower velocities in the trough region, and turbulence stresses (ejections, sweeps, turbulence Intensity, TKE and Reynolds Stress) an order of magnitude higher than for sand bedforms. Further, experiments over a porous gravel bed indicate levels of near bed turbulence higher than over a gravel-surfaced concrete bedform without interstitial flow. High-speed photography and interpretation of streak images further supports this ADV data.
It is proposed that antidunes break when turbulence reaches an ‘intensity’ that constitutes a threshold above which rapid erosion occurs in the trough causing a pronounced increase in turbulent ejections laden with sediment and consequent rapid deposition on the downstream antidune flank. Flow then stalls over the downstream antidune; the standing wave collapses and erodes much of the bed. In terms of distinctive sedimentary structure, three types of bedding were observed in sediment sections taken after mobile bed runs where antidunes had been active. Type I bedding is formed by the erosion of the bed and marks the lowest surface formed by antidune downcutting during active migration or collapse. Type II bedding is formed by turbulent sweeps during antidune growth and migration. However the contrasts in sediment size and type that mark bedding are dependent on the heterogeneity of bed sediment. A third type of downstream dipping, bipartite planar bedding was observed to form under an upstream migrating standing wave. The preservation of a suite of sedimentologic features produced by a period of antidune activity is however dependent on the degree of downcutting and erosion during standing wave collapse.

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