Coherent flow structures over mixed grain sized surfaces and their role in sediment transport
The prediction of grain entrainment rates in river flows is of great importance to an engineer, as the movement of sediment can cause permanent changes to the form and character of a river. One of the key elements in achieving accurate prediction of grain entrainment rates is the understanding of the near bed flow regime, as research has revealed the existence of a number of organised flow structures which are thought to control the magnitude of shear stress applied to the bed surface. However, most of the knowledge gained on these turbulent flow structures has been from experiments conducted over smooth walls often at low Reynolds numbers. Many geophysical flows differ greatly from this scenario, having flows with high Reynolds numbers and boundaries which are rough and deformable. A series of flume experiments have therefore been conducted to investigate whether the flow structures identified over smooth walls are also present over mixed grain sized sediment beds. The first experiment used a new flow measurement technique known as particle image velocimetry to record flow field measurements over a fixed bed. Both horizontal and vertical planes of the flow were investigated (at different times) in order to obtain an understanding of the three dimensional nature of the flow. A second set of experiments was conducted over the same fixed bed, but also involved the introduction of sediment grains into the flow upstream of the measurement area. The same techniques were used to examine the flow in this case, thus enabling significant differences between the circumstances to be identified. A third set of experiments was conducted over a natural sediment and in this case flow field measurements were taken at periods of high and low transport rates, a difference in transport rate being associated with bed armouring. A short time series of near instantaneous velocity field measurements were recorded for each experimental condition to aid the identification of the flow structures. Visual inspection of the time series revealed that sweeps and ejections occur at different scales, the smallest scaling with grain size or bed features and the largest scaling with flow depth.