Measurements of sediment diffusivity profiles over rippled beds under waves

Predictions for the form of suspended sediment concentration profiles differ according to the flow, the seabed sediment and, importantly, the bed forms. Most of the formulations used are underpinned by the classical Fickian concept of gradient diffusion originating from kinetic molecular theory, where random molecular movements induce mixing. In the case of suspended sediments in field situations, it is the turbulent fluctuations in the vertical velocity component that give rise to the upward mixing process. In the simplest case the time averaged vertical turbulent diffusion of sediment, qv, is considered to be balanced by the settling of the suspended sediment under gravity