Impacts of Sediment-Induced Stratification on Turbulent Momentum and Sediment Fluxes In Wave- And Current-Driven Flows

Sediment entrainment and mixing processes in shallow-water environments are complicated by gravitational settling of suspended sediment because it inherently stratifies the water column, suppressing vertical transport. In this work we apply DNS to investigate the effects of sediment-induced stratification on wave and current driven estuarine flows. Through conditional statistics, we show stratification modifies the flow by suppressing the exchange of high- and low-momentum fluid by intense sweeps and ejections.This suppression manifests as a reduction in the vertical Reynolds stress, reducing shear production of TKE near the bed. However, higher in the water column shear production increases because of increased mean shear, acting to restore the vertical component of the Reynolds stress to its unstratified state. Similar dynamics occur for the vertical turbulent transport of sediment. However, no restoration mechanism exists for the sediment phase. As a result, near-bed stratification has a larger impact on the sediment dynamics than it does on the fluid dynamics.