The Role of Non-Uniform Roughness in the Formation of Sand Ribbons*

Abstract Sand ribbons typically are shallow-water sedimentary features characterized as thin, elongate bands of sand overlying a coarser lag sediment which is exposed between them. The ribbons are oriented along the direction of the dominant current, and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons. A perturbation expansion, coupled with flume experiments, indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips. The non-uniform turbulence field arising from this roughness distribution drives a secondary helical flow. The expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth (∼4:1) for unstratified channel flow conditions. The secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress. This serves to retard the movement of sand through such a system.