Laboratory experiments of rotating stratified exchange flows over a sediment bed

We present a pioneering experimental study of stratified, rotating exchange flows interacting with a bottom, mobile sediment bed that simulates large estuaries. Two-dimensional velocity fields are coupled with bed scan that allows to reconstruct the bed morphology. The experiments span a large parameter range, notably laminar to turbulent Ekman layer regimes ($33<R{e}_{\delta }<192$) and Burger numbers ($0.4<Bu<2.1$). For low Burger numbers ($Bu<1$), meandering of the lower salty layer occurs due to baroclinic instability, leading to the formation of columnar vortices, whose size and phase speed are in agreement with theory (Pedlosky, 2013). The Ekman layer thickness is well identified in the experiments over a smooth bed giving access to an eddy viscosity, hence the friction velocity $u_{\ast }$, which is used as the characteristic velocity of sediment erosion. We show that sediment transport is driven mainly by Ekman dynamics with a net transport across the channel cross-section in direction of the geostrophic slope. For low Burger numbers, meandering induces further variability in cross- and along-channel velocities affecting sediment transport. The estimated non-dimensional bed-form wavelengths align closely with previous values of ripples in gravity currents reported in the literature. Notably, these wavelengths do not indicate a transition from ripples to dunes with increasing Yalin numbers, unlike in open channel flows. Finally, the sediment suspension model of Maggi et al. (2024) is extended by including across-channel sediment transport, that explains the observed temporal change in bed-forms and the effect of Ekman pumping on the suspended sediment layer thickness.