Dynamics of Sediment Resuspension and Transport by Fission of Nonlinear Internal Waves Over a Mild Slope
Internal wave , Stratified flow , Sediment transport , Resuspension , Bedform , Coastal Engineering
Breaking nonlinear internal waves (NLIWs) of depression on boundary slopes drive mixing and mass transport in the coastal ocean. Of the different breaker types, fission is most commonly observed on the mild slopes of continental margins. However, fission on mild slopes has rarely been investigated in the laboratory owing to limitations in flume length. In the present work, a train of NLIWs of depression was generated in an 18.2 m wave flume and shoaled upon a mild uniform slope (S = 0.04). During fission, each NLIW of depression scattered into one or two NLIWs of elevation at the turning point and transformed into a bolus at the bolus birth point (BBP), where the flow separates from the bed and shear instability occurs through the pycnocline. The bolus propagated upslope, decreasing in size until it degenerated by shear and lobe-cleft instability, while losing volume to a return flow in the lower layer. The location of the BBP, bolus propagation length scale, initial size and the number of boluses from each incident wave were parameterized from the wave half-width and the wave Froude number associated with the incident NLIW. The same experimental facilities were used to investigate sediment transport, as both bedload and resuspension, driven by NLIW fission on the mild uniform slope, which was covered with acrylic sediments. Incipient sediment movement started as bedload under NLIWs of elevation after the turning point. Further upslope, sediments resuspended under the front face of the NLIW of elevation, where bottom currents converged, causing upward flow. The intensity of sediment movement peaked at the BBP. The sediment movement mechanisms were the same for boluses as for NLIWs of elevation, except boluses carried sediments upslope in a quasi-trapped core. Different methods for estimating the bed shear velocity were evaluated and compared with the critical thresholds for bedload and resuspension. The logarithmic law-of-the-wall was best able to predict sediment movement as bedload through the inclusion of the mean horizontal velocity component, and the turbulent kinetic energy method was best able to predict sediment resuspension by considering the ability of fluctuating vertical velocity components to lift sediment.