Two-dimensional time dependent structure of the suspended sediment concentration over rippled seabeds

A field experiment was conducted to investigate small-scale sediment dynamics near the seabed in the nearshore region. The SandyDuck'97 experiment took place at the U.S. Army Corps of Engineers Field Research Facility, in Duck, North Carolina, U.S.A., where the seabed typically consists of fine to medium size sand. Acoustic instrumentation measured the local hydrodynamics, the suspended sediment concentration (SSC) profile, and the local bedforms.; The vertical distribution of the SSC is investigated over low amplitude wave orbital ripples in two different data sets with comparable hydrodynamic conditions but with the sediment concentration measurements located at different cross-shore positions relative to the bedforms. The bedforms have low steepness and exhibit well rounded crests compared to classical vortex ripples, so the mechanisms for sediment suspension are somewhat mysterious. The concentration profiles exhibit different temporal patterns that relate to their different locations over the bedforms. The vertical and temporal structures of the concentration profiles indicate significant horizontal advection of clouds of suspended sediment by the wave induced orbital fluid motion.; The Dune2d boundary layer numerical model is applied to investigate the structure of the turbulence and suspended sediment concentration over the observed ripples. The simulations of the flow using Dune2d indicate that a turbulent vortex is generated at the lee of the ripple crest near the time of flow reversal. This indicates that the mechanism for sediment suspension above these low amplitude, long wave ripples (LWR) is essentially similar to the vortex formation process expected over steeper vortex ripples. The turbulence kinetic energy (TKE) distributions also indicate that high TKE is concentrated at the location of the vortex. The intensity of turbulent vortex is enhanced if small wave ripples (SWR) are superimposed on the LWR. Numerical investigations of the SSC indicate that the sediment is entrained from the bed and a sediment cloud is formed by the action of the turbulent vortex. This cloud is advected horizontally over the bed by the wave orbital fluid motion.; The comparison between the data and model predictions shows an agreement with regard to the time within a wave period of the peak SSC. This indicates that the generation of the sediment cloud and its movement are well predicted over rippled seabeds by Dune2d. However, the magnitude of SSC as well as TKE was underestimated by Dune2d. This disagreement can be explained from the use of the rigid lid surface boundary condition, which results in the under estimation of sediment suspension and vertical mixing of the suspended sediments.