Particle motion in the near bed region of a turbulent open channel flow: Implications for bedload transport by saltation and sediment entrainment into suspension

The research reported herein has been aimed at improving the present level of understanding of physical processes involved in the transport of sediment particles in the near bed region of a turbulent open channel flow. Two different aspects of this phenomenon are examined, namely the mode of transport denoted as saltation, and the process of particle entrainment into suspension. The main approach followed is experimental, using visualization and particle tracking techniques to study particle motion and its interaction with near wall turbulence. A high-speed video system is used extensively with this aim. A Lagrangian approach is used to model both saltation and particle entrainment into suspension. In the former case, the interaction between saltating particles and the bed is modelled stochastically, and the effect of turbulence on saltation is investigated separately by means of a random walk model. To model particle entrainment into suspension it is assumed that particles interact with near bed coherent flow structures which are characterized using a heuristic model. Results of the study on particle saltation provide unprecedented data on the characteristics of particle trajectories and velocities, particle collision with the bed, particle rotation, particle re-entrainment, and transverse particle motion. These results are used to validate the stochastic model for saltation, which is a useful tool to simulate statistics of this process. Modelling bedload transport using results of the saltation model and a Bagnoldean formulation reveals problems with such formulation, particularly regarding the continuum hypothesis for the bedload layer. Results of the random walk model for particle saltation indicate that turbulence tends to decrease the length and height of saltation trajectories, and also to induce a variability in the process which seems to affect primarily the length of the jumps. Such effects tend to decrease as the particle size increases. Shear layers observed in the near wall region of the open channel flows interact with sediment particles lying in the channel bottom, which eventually results in the particles being entrained into suspension. Experiments in flows with a rough bed indicate that hiding effects tend to preclude the entrainment of particles with sizes finer than that of the roughness elements, and that roughness does not affect the mechanism that generates flow ejections, which would be ultimately responsible for particle entrainment. The heuristic model for particle entrainment into suspension reproduces the basic characteristics of this process relatively well, however improving the predictions of the model would require a better characterization of the coherent flow structures.