Grain Sorting on Sand Ripples in Heterogeneous Sediments

摘要

Sand ripples are commonly observed coastal benthic features that form under the action of waves/currents. In most previous studies of ripple dynamics homogeneous sand was used (see, e.g., Testik et al., 2005 and references herein). In nature, sands are mostly heterogeneous. In the figures above we give an example of typical grain sorting as observed in a bimodal sand mixture under steady oscillatory flow in a water channel. The mixture consists of 34 % of fine sand (d = 0.3 mm, blue) and 66 % of course sand (d = 0.6 mm, brown). Initially flat (Fig. 1), sand mixture is subject to oscillatory flow (from left to right). With time, established ripples are formed (Figs. 2 and 3) with typical sediment segregation pattern: fine grains (blue) are mostly seen on the ripple troughs with very narrow strips on the tops of the crests; coarse grains (brown) are seen mostly on the ripple crests.