Time and Patterns of Development of Dunes Subjected to Sudden Changes in Flow Depth

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Wiebe, Joshua Daniel
Bed Forms , Dunes , Unsteady Flow , Bed Resistence , Floods
In unsteady flows, dune dimensions may vary considerably from fully-developed dimensions produced from a flat bed under a steady and uniform flow. Specifically, dune height and length are observed to lag discharge when the flow is non-steady, resulting in dimensions that are out of phase with the prevailing flow. This research attempts to provide some insight into the behaviour of dunes when the flow is suddenly changed, as well as the time-scale of the related dune changes. Nineteen experimental runs were carried out in the 21-m long, 0.76-m wide Sediment Transport Flume at Queen’s University. In ten of these runs the flow depth, h, was suddenly increased (h2/h1 > 1) and in nine runs the flow depth was suddenly decreased (h2/h1 < 1). In all runs, the slope of free surface was kept equal to the slope of the bed such that the change in flow rate is represented by the change in the flow depth. Seven ratios of the two flow depths, h2/h1 (varying between 0.49 and 2.29), were performed for three slopes (S = 1/792, 1/534, 1/341). The bed material was a coarse, poorly-graded silica sand (D50 = 1.0 mm). Longitudinal bed profiles were taken along the centreline of the flume approximately every 6–8 minutes to determine the transient dune dimensions and the time at which the dunes acquired their new equilibrium dimensions. This time is termed the duration of dune development, Td. Through dimensional and physical considerations, Bielenberg (2006) established that the dimensionless counterpart of Td should be a function of the material number, relative flow depth, relative flow intensity, and ratio of the flow depths h2/h1. The present experiments were carried out to investigate the influence of the relative flow intensity and h2/h1 on the duration of dune development. Results indicate that equilibrium dune dimensions do not depend on the initial shape of the bed. After the period Td, the dunes tend to be similar to those produced from a flat bed. It is found that Td is strongly dependent on h2/h1, and weakly dependent on the flow intensity. All other determining variables remaining the same, Td decreases with increasing values of flow intensity. Semi-empirical relations for the dimensionless duration of dune development are presented.
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