High-resolution field measurements and numerical modelling of intra-wave sediment suspension on plane beds under shoaling waves

Intra-wave sediment suspension is examined using high-resolution field meas urements and numerical hydrodynamic and sediment models within 120 mm of a plane seabed under natural asymmetric waves. The detailed measurements of s uspended sediment concentration (at 5 min vertical resolution and at 4 Hz) showed two or three entrainment bursts around peak now under the wave crest and another at now reversal during the decelerating phase. At flow reversa l, the mixing length was found to be approximately double the value attaine d at peak flow under the crest. To examine the cause of multiple suspension peaks and increased diffusion at flow reversal, a numerical "side-view" hy drodynamic model was developed to reproduce near-bed wave-induced orbital c urrents. Predicted currents at the bed and above the wave boundary layer we re oppositely directed around flow reversal and this effect became more pro nounced with increasing wave asymmetry. When the predicted orbital currents and an enhanced eddy diffusivity during periods of oppositely directed flo ws were applied in a Lagrangian numerical sediment transport model, unprece dented and extremely close predictions of the measured instantaneous concen trations were obtained. The numerical models were simplified to incorporate only the essential parameters and, by simulating at short time scales, emp irical time-averaged parameterisations were not required, Key factors in th e sediment model were fall velocities of the full grain size distribution, diffusion, separation of entrainment from settlement, and non-constant, but vertically uniform, eddy diffusivity. Over the plane bed, sediment convect ion by wave orbital vertical currents was found to have no significant infl uence on the results. (C) 2001 Elsevier Science B.V. All rights reserved.