Intense bedload transport in non-uniform flow
Creators: name: Paulos, Yonas Kinfu
Publisher: name: University of British Columbia
Year: 2009

description: A theoretical and experimental investigation of intense bedload transport in non-uniform flow has been carried out. The study includes the process of shear stress transfer from fluid to carpet; the effect of convective acceleration induced pressure field on the carpet flow; efficiency factor of intense bedload transport; the profiles of sediment and fluid velocities; and frictional behavior of carpet flow. The theoretical study considers two aspects of carpet flow: The first deals with the dynamics of flat-bed carpet flow and the second deals with the effect of the convective acceleration of the fluid produced by flow over a bedwave. Employing Bagnold's (1954) semi-theoretical relationships, based on his rotating-drum experiments, the theoretical analysis gave two results: 1) velocity profiles for the grains and fluid; and 2) the trend of shear stress transfer from the fluid to the grains. These flat-bed relationships were then extended to the bedwave situation which deals with the effect of the pressure gradient produced by convective acceleration on a carpet flow. Control volume analysis shows that an additional shear stress, other than fluid induced turbulent shear stress, acts to mobilize the sediments. This is confirmed in two ways from the experimental measurements: 1) from the sediment transport kinematics; and 2) from the weight of sediment supported within the carpet layer. Experiments were carried out to verify the theory and to establish the relative importance of the various stress mechanism. The experiments were conducted using a re-circulating closed conduit, which avoided the effect of surface waves and also produced the necessary high shear stress and other flow parameters. The sediment concentration and velocity profiles were measured by using a high-speed digital camera that was able to record 1000 frames per second. The velocity profile of the overlaying fluid was measured by using an acoustic doppler velocimeter. Pressure measurements at various points were made by using pressure transducers that logged data to a computer. A relationship for estimating fiictional behavior of intense bedload on flat bed is given. It was found that the amount of resistance depends on the Shields parameter, the critical value being close to 0.8. This theory indicates that for flows with intense bedload and Shields parameter above this critical value, the fiictional resistance is higher than an equivalent clear-water and rough-surface flow, and lower for values below the critical. Identically shaped dunes were observed for a range of discharges which indicated that the fluid induced shear stress, both from friction and convective acceleration, scale almost similarly for a given range of flow. Moreover, the geometry of the dunes are possibly influenced by the scale of the experiments. A reasonably good matching of the grain velocity profile was obtained between the theoretical and experimental results. And it appears that carpet sediment transport under the influence of convective acceleration is more efficient than the flat bed case, because the additional force on sediments from the pressure gradient is transmitted directly with less energy loss.