Laboratory experiments of rotating stratified exchange flows over a sediment bed

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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 δ < 192 ) and Burger numbers ( 0 . 4 < B u < 2 . 1 ). For low Burger numbers ( B u < 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 ∗ , 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.