GRAIN SORTING EFFECTS ON RIPPLES

摘要

Results of an experimental research aimed at studying grain sorting effects on sand ripples in open channel flows are presented and discussed. Experimental conditions included beds formed by uniform sand and also sand mixtures of two different particle sizes. Visualizations of the evolution of the bed topography showed that patches of two-dimensional ripples emerge over the initially flat bed through the simultaneous appearance and growth of several wave fronts, and that this process is similar in both, the uniform and mixed beds studied. Grain sorting was observed over the mixed beds. Coarser grains tend to accumulate in the separation region downstream from individual ripples, and a layer of finer grains forms on the top of the bedforms. Visualizations of flow and sediment transport in the near bed region showed that quasi-periodic events, related to the existence of coherent structures of the turbulent boundary layer, are able to entrain sediment into suspension from the top of the ripples. Of the entrained grains, mostly the finer are able to by-pass the wake of the ripples, and most of the coarser particles fall into the separation region. Those grains, as well as the bedload material transported over the ripples, remain trapped in the wake of the bedforms. The by-pass mechanism together with selective transport associated to hiding and overexposure effects explain the grain sorting patterns observed. The sediment size appears to be an adequate length scale for the ripples. In the non-uniform beds, however, it was necessary to define a representative sediment size, which results to be a function of the bed shear stress. This is interpreted to be a consequence of grain sorting, which was also observed to strongly modify the behavior of the celerity of the ripples with respect to those developed on uniform sediment.