| Accurate modeling of the nearshore morphology and coastal littoral process depends on reasonable estimates of the sediment transport rate or the pick-up function. At present, due to our limited understanding of sediment suspension and deposition mechanisms in complex conditions, appropriate sediment transport rates or pick-up functions for the nearshore environment are not yet available. Therefore, this study focuses on a small-scale sediment transport process, called sheet flow.; Sheet flows occur when the shear stress is so high that a significant thickness of highly concentrated sediment becomes mobile and ripples in the bed disappear. Because large amounts of sediment can be transported within a sheet flow, they are responsible for the major sediment repositions and changes of morphology in the nearshore region.; Averaged two-phase continuum equations are presented for concentrated flows of sediment that are driven by strong, fully developed, steady or oscillatory turbulent shear flows over a porous, mobile bed. A two-equation model, based on two-phase theory, is implemented for the closure of the fluid turbulence. An improved closure upon Bagnold's relation for the sediment stresses using the kinetic theory of granular flow is adopted. A failure criterion is introduced to determine the location of the stationary bed. The proposed model is solved numerically with a finite difference algorithm and is able to describe the sediment transport under both steady and unsteady conditions. The predictions of sediment concentration and velocity are tested against experimental measurements. The sheet flow model is further employed to study unsteady effects on sediment dynamics. It is found that the sediment transport depends not only on the external flow but also upon its acceleration.; A dilute model for sediment transport is further studied by neglecting the sediment stresses of the proposed sheet flow model. Adopting boundary layer approximations, a semi-analytical solution for dilute flow is developed that improves upon the single-phase Rouse formula. The improvement is due to the incorporation of dissipation of the flow turbulent energy due to the presence of sediment.; Future work on both the improved closures for sediment stresses and more comprehensive measurements are suggested. |
