Abstract:
Previous studies have shown that episodic suspension events which develop under 'groupy' waves have vertical extents and involve suspended sediment concentrations (SSC) that are not predicted by current theory or models. The aim of this work is to improve the understanding of the mechanisms associated with suspension of sand induced by wave groups over rippled beds. This is accomplished through detailed analysis of high-resolution measurements of sand suspension produced by wave groups over a mobile sand bed under controlled laboratory conditions at full scale. The observations presented are based on data collected from an array of instruments including a multi-transducer acoustic backscatter sensor in wave basin and wave flume experiments run in the National Hydraulics Laboratory in Ottawa, Canada. Analysis techniques are introduced that provide detailed and statistically robust measurements of group induced suspension patterns over bedforms that allow the driving mechanisms behind these patterns to be revealed. Visualisation of the development and convection of sediment-laden vortices through measurements collected from a multi-transducer acoustic backscatter sensor showed that the persistence of vortices over several wave cycles and subsequent vortex pairing produced events reaching vertical extents much larger than the height of the bedforms. Ensemble averaging suspension events from repeated wave groups reveal that the convection of vortices and turbulence produces vertical temporal lags in the group induced peak in SSC and a pattern of boundary layer development at the temporal scale of the passage of the wave group. A redefined 'pick-up' function, based on the Nielsen reference SSC model, is developed which provides better prediction of wave-by-wave reference SSC under the developing stages of group induced suspension events. The rate of vertical propagation of SSC away from the nearbed region at the time scale of the wave group is also examined by the application of cross-correlation techniques to the ensemble-averaged SSC measurements. From the statistical analysis convection rates are estimated. A simple heuristic convection-diffusion-settling model is produced that incorporates the redefined 'pick-up' function and the convection rates. The model was shown to reproduce the essential elements of a group induced suspension event.