Physical controls in the empirical relationship of current ripple wavelength

Ripples are ubiquitous on Earth and other planets and can act as valuable (paleo) environmental indicators of terrestrial and extraterrestrial sedimentary landscapes. Recently, Lapotre et al. established an empirical relationship that describes current ripple wavelengths and indicates a dependence on bed shear stresses. However, the fluid mechanics controlling ripple wavelengths are still poorly understood, and empirical relationships cannot be easily applied to extraterrestrial landscapes. We used numerical modeling to investigate the physics linking flow conditions to current ripple morphology and spacing. Our analysis used large eddy simulations based on a Lattice-Boltzmann numerical flow model. Results showed that shear velocities varied spatially depending on ripple separation, with largest stresses found between separation bubbles and downstream crests. We further considered differences in the sum of non-dimensional bed-load fluxes caused by positive (downstream directed) and negative (upstream directed) shear stresses at ripple crest. Preliminary analyses indicate that modeling results were closer to the empirical relationship as differences in bed-load fluxes at ripple crest decreased. This observation might suggest that the empirical relationship linking ripple wavelengths and bed shear stresses represents sediment transport conditions at ripple crests, possibly a minimum in net bed-load fluxes.