Wave Driven Sand Ripple Formation on a Subaqueous Sand Mound

The MOrphological Diffusivity EXperiment (MODEX) investigated the relationship between morphodynamic change of a gaussian shaped sand mound subject to waves, currents, and combined flows. Our contribution investigates the sediment pathways for mound diffusion when subject to monochromatic waves. Two-types of bedform migration were observed – downslope ripple migration, which is in nearly the same direction as the ripple crests, and lateral ripple migration, which is in the direction perpendicular to the ripple crests. Migration rates were estimated with subsequent rectified GoPro images. Sonar and LiDAR data were used for observing the larger scale mound diffusion. Initially, ripple downslope migration drove high sediment flux until ripples reached the foot of the mound and lateral migration of ripples ensued. Lateral ripple migration included bedform splitting, where there is a flow divergence and a large downslope sediment flux generating ripples that are bigger than the wave orbital properties allow for, so the sand ripple continuously splits (or sheds ripples) laterally around the mound. Lateral ripple migration was analyzed using wavelet transforms. Beach nourishments on the shoreface, near subtidal sandbars, are known to influence sandbar dynamics, sandbars in these areas have been observed to split, as we are seeing with the ripples on top of the mound in the laboratory experiment. Insights gained here may impact the way in which we understand coastal nourishment behavior and diffusion in the field.