Laboratory experiments on the near-bed hydrodynamics over regular and irregular ripples.

Sand ripples, the smallest and most ubiquitous bedforms in coastal and seabed environments, enhance turbulence and sediment resuspension within the bottom boundary layer. Under natural wave forcing, ripples often develop three-dimensional (3D) features—such as terminations, bifurcations, and secondary crests—that reflect their complex adaptation to varying hydrodynamic conditions. To investigate the hydrodynamics over different ripple types, we conducted laboratory experiments in a U-shaped oscillatory tunnel at the Ecohydraulics and Ecomorphodynamics Laboratory, University of Illinois at Urbana-Champaign (USA). Two fixed 3D-printed ripple morphologies were studied: uniform ripples and ripples with superimposed secondary crests. Results demonstrate that the addition of secondary crests substantially modifies flow dynamics, both locally and across neighboring ripples. Compared to uniform ripples, secondary crests produce a thicker boundary layer and induce a notably higher shear velocity at the crest, indicating a greater potential for sediment transport and bedform evolution. These findings provide valuable insights into ripple morphodynamics and contribute to a better understanding of sediment processes in coastal and marine environments.