Wave groups and sediment resuspension processes over evolving sandy bedforms

High resolution measurements of the water velocity, bedforms and suspended sediment concentration were made using an Acoustic Doppler Velocimeter, acoustic bedform scanners and an Acoustic Backscatter System, under irregular free-surface waves. The waves were generated in a large scale flume facility above a number of bedform types. These data were analysed in (i) the frequency domain in order to examine the frequency at which sediment suspensions occurred in the oscillatory bottom boundary layer and the free stream; and (ii) the time domain in order to examine the instantaneous entrainment and vertical transport of sediment at intra-wave, wave average and wave group time scales. During the course of the experiments the significant wave height was systematically incremented enabling the character of sediment suspensions to be studied under a number of flow and bedform regimes. Wave groups were identified as an important control over sediment suspensions in both the wave boundary layer and free stream, with fluctuations in the suspended sediment concentration occurring at low, wave group, frequencies. However, the initial entrainment process, within the wave boundary layer, occurred at intra-wave frequencies. In contrast, in the free stream, sediment suspensions were dominated by the vertical transport of sediment at wave group time scales. During wave groups the sediment suspension field was characterised by the upward transport of sediment due to the continual injection of turbulence under a series of waves which generated a wave pumping effect. The character of a wave group is considered to be an important control over sediment suspensions in the free stream. Four distinct types of wave group were identified and the instantaneous sediment suspension field below each type examined. Such comparisons were possible using the high resolution Acoustic Backscatter System which enabled both intra-wave and wave group processes to be resolved up to 0.8. m above the bed.