Flow separation and resuspension beneath shoaling nonlinear internal waves

Laboratory observations are presented showing the structure and dynamics of theturbulent bottom boundary layer beneath nonlinear internal waves (NLIWs) of depressionshoaling upon sloping topography. The adverse pressure gradient beneath the shoalingwaves causes the rear face to steepen, flow separation to occur, and wave-inducednear-bottom vortices to suspend bed material. The resuspension is directly attributed to thenear-bed viscous stress and to near-bed patches of elevated positive Reynolds stressgenerated by the vortical structures. These results are consistent with published fieldobservations of resuspension events beneath shoaling NLIWs. Elevated near-bed viscousstresses are found throughout the domain at locations that are not correlated to theresuspension events. Near-bed viscous stress is thus required for incipient sedimentmotion but is not necessarily a precursor for resuspension. Resuspension is dependent onthe vertical velocity field associated with positive Reynolds stress and is also found tooccur where the mean (wave-averaged) vertical velocity is directed away from the bed.The results are interpreted by analogy to the eddy-stress and turbulent burstingresuspension models developed for turbulent channel flows.