MODIFICATIONS IN NEAR BED FLOW OVER SEDIMENT BEDS AND THE IMPLICATIONS FOR GRAIN ENTRAINMENT

摘要

There is a growing interest in understanding how the flow over a mobile sediment bed and bed structure develop under non equilibrium conditions. In situations of limited upstream sediment supply, armouring is well known to occur during erosional phases. Recent work has shown that the bed structure develops specific grain arrangement patterns during the armouring process. However, very little experimental evidence has been collected so far on the evolution of the flow field associated with changes of particle organisation on the bed surface. An experimental technique has been developed to measure the near-bed flow field and the bed structure in mobile bed experiments simultaneously. Results are at a preliminary stage, but they are already capable of providing strong evidence that armouring processes are associated with a significant variation of the near-bed flow velocity pattern. The data also indicates that different flow velocities are responsible for particle entrainment at various stages of the armouring process.