Heterogeneous sediment beds under weak oscillatory flow and turbulence: Ripples' transformation and decay

Laboratory experiments were conducted on the evolution of ripples in a bimodal heterogeneous mixture under the conditions of oscillatory flow and background turbulence. First, data on sediment concentration and effective sediment size were collected during ripples' formation on an initially flat bed, which were used to clarify the characteristic grain sorting patterns (with coarse sediment on the crests and fine sediments in the troughs) reported in previous studies. Based on the differences in the mobility of sediments, a physical explanation was provided for grain sorting in ripples. Second, in a series of experiments, the ripples' decay was modeled. It was shown that when established ripples in a heterogeneous mixture are subjected to either a weak oscillatory flow (below the threshold for ripple formation) or external (shear-free) turbulence, their height decays much the same (qualitative) manner as in homogeneous sediments. The decay of ripples in bimodal heterogeneous mixture was quantified by extending a ripple “diffusion” model proposed earlier for homogeneous sediments. The results revealed that under weak oscillatory flow, the effective ripple diffusivities K for a heterogeneous mixture are of the same order of magnitude as for homogeneous sediments, while under turbulence the values of K are smaller for a heterogeneous mixture.

► Sediment ripples in heterogeneous mixture undergo a grain sorting pattern.
► A heterogeneous sediment mixture was found to be more stable.
► Weak oscillatory flow and external turbulence are two mechanisms for ripple decay.
► The ripple decay process was modeled using a Fickian diffusion equation.
► External turbulence leads to a faster decay of ripples.