Observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves

Acoustic measurements of near-bed sediment diffusivity profiles are reported. The observations were made over two sandy rippled beds, classified as ‘medium’ and ‘fine’ in terms of sand grain size, under slightly asymmetric regular waves. For the medium sand, the ripples that formed had relatively steep slopes, while for the fine sand, the slopes were roughly half that of the medium sand. In the medium sand case, the form of the sediment diffusivity profiles was found to be constant with height above the bed, to a height equal approximately to the equivalent roughness of the bed, ks, while above this the sediment diffusivity increased linearly with height. For the case of the fine sand there was no constant region; the sediment diffusivity simply increased linearly with height from the bed. To understand the difference between the respective diffusivity profiles, advantage has been taken of the high temporal-spatial resolution available with acoustic systems. Using intra-wave ensemble averaging, detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed. These intra-wave observations, combined with measurements of the bed forms and concepts of convective and diffusive entrainment, have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds. These mechanisms centre on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness.