GRAIN SORTING EFFECTS ON RIPPLES

摘要

Results of an experimental research aimed at studying grain sorting effects on sandripples in open channel flows are presented and discussed. Experimentalconditions included beds formed by uniform sand and also sand mixtures of twodifferent particle sizes. Visualizations of the evolution of the bed topographyshowed that patches of two-dimensional ripples emerge over the initially flat bedthrough the simultaneous appearance and growth of several wave fronts, and thatthis process is similar in both, the uniform and mixed beds studied. Grain sortingwas observed over the mixed beds. Coarser grains tend to accumulate in theseparation region downstream from individual ripples, and a layer of finer grainsforms on the top of the bedforms. Visualizations of flow and sediment transport inthe near bed region showed that quasi-periodic events, related to the existence ofcoherent structures of the turbulent boundary layer, are able to entrain sediment intosuspension from the top of the ripples. Of the entrained grains, mostly the finer areable to by-pass the wake of the ripples, and most of the coarser particles fall into theseparation region. Those grains, as well as the bedload material transported overthe ripples, remain trapped in the wake of the bedforms. The by-pass mechanismtogether with selective transport associated to hiding and overexposure effectsexplain the grain sorting patterns observed. The sediment size appears to be anadequate length scale for the ripples. In the non-uniform beds, however, it wasnecessary to define a representative sediment size, which results to be a function ofthe 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 rippleswith respect to those developed on uniform sediment.