On the Role of Ripple Secondary Crest on Nearbed Hydrodynamics

Ripples are ubiquitous in sandy beds and their geometry plays an important role in determining seabed roughness and intensifying near-bed turbulence. Ripple geometry in natural settings often deviates from equilibrium configurations. To understand how such nonequilibrium geometry and structures impact near-bottom hydrodynamics and hydraulic roughness, we performed laboratory experiments examining the effect of two different types of ripple configurations. We employed two distinct fixed 3D-printed ripple morphologies, uniform ripples and ripples with superimposed secondary crests, and replicated natural conditions by adhering sand grains, matching in size to the ripple scale, onto their surfaces. Our results show that the introduction of secondary crest disrupts the flow over not only the modified ripple but also over its neighbors. Secondary crests induce a thicker boundary layer than regular ripples. Velocities over the upstream side of secondary crest show substantial deviation from the regular ripple baseline case, while the downstream side experiences a lower effect. The shear velocity at the crest of ripples with a secondary feature is significantly higher, indicating an increased capacity for sediment transport and bedform evolution. The turbulent kinetic energy over ripples with secondary crests is twice as high as that over regular ripples. Our results further affirm that the hydraulic roughness is a function of not only the height and wavelength of the ripples, but also of specific structures and ripple geometry.