Sediment suspension under water waves
Data collected in a large scale laboratory wave flume by a research team using the autonomous bottom boundary layer rig, (STABLE) was the subject of this study. The near bed suspension processes were examined relating them to the hydrodynamics. The deployment of a number of sensors allowed the assessment of their individual performance including the analysis of the pump-sampling and acoustic concentration data. Wavelet analysis was applied to identify the influence of STABLE on the vortex ripples in the vicinity of the rig. It revealed that the modification of the ripple dimensions around STABLE's feet was very localised. Sediment suspension was found to be strongly correlated to wave groups. The measured concentrations and empirical models based on convective and diffusive entrainment mechanisms were compared. A model based on the jet like ejection of particles between a vortex pair was developed and showed that lifting of sediments up to ten ripple heights above the bed was possible. A second model, capable of simulating the pumping effect, included this entrainment process to simulate the suspension under wave groups taking the suspension history into account. The behaviour of neutrally buoyant particles in a laboratory wave flume was videoed and revealed jet like ejections and horizontal movement over two or more ripple wavelengths.