Experiments on bar formation in a straight flume: 2. Graded sediment


  • Stefano Lanzoni


Laboratory experiments have been carried out in a large laboratory flume using a strongly bimodal sediment mixture in a range of flow conditions such that the initially flat bed of the flume became unstable enhancing the development of alternate bars. In order to elucidate the effect of grain sorting, the present experiments are compared with a previous set of experiments performed in the same flume under similar hydraulic conditions but using a nearly uniform sand with a mean geometric grain diameter equal to that characterizing present experiments. The comparison suggests that sediment heterogeneity may appreciably affect bed form characteristics. The development of smallscale (ripples) and mesoscale (dunes) sediment waves tended to be inhibited thus allowing a decrease in flow resistance. Owing to the bimodal character of the adopted mixture at low values of the bed shear stress a condition of partial transport was attained for which only the finer-grained portion of the mixture was observed to move, while the coarsergrained fractions remained essentially immobile throughout the experimental run. However, complete mobilization of all size fractions was observed to occur in runs carried out at higher slopes (i.e., at higher bed shear stress) in order to generate an alternate bar pattern. Selective transport of individual grain size fractions, coupled with the characteristic bar topography pattern led to an intense longitudinal sorting which accreted the coarser particles on bar crests. Furthermore, bar migration caused, through scour and fill, a significant vertical sorting. As a consequence of these sorting processes and in accordance with previous experimental observations, bar height turns out to be invariably damped with respect to uniform sediment experiments. The trend exhibited by the wavelengths is less clear and suggests that in the present experiments sorting effects do not enhance the clear shortening of bar wavelengths typically observed in other series of flume experiments carried out with weakly bimodal mixtures.