Observations of Enhanced Sediment Transport by Nonlinear Internal Waves

The mechanisms responsible for sediment resuspension and transport by nonlinear internal waves (NLIWs) remain poorly understood largely due to a dearth of detailed field measurements. We present novel observations of the turbulent benthic boundary-layer (BBL) beneath trains of NLIWs of depression in the ocean. At the 250 m deep, low-gradient (<0.2%) continental shelf site the BBL was near well mixed to an average height of about 10 m above the bottom. Above this bottom mixing-layer, stratification constrained the extent of vertical sediment transport. NLIWs drove sediment transport by a combination of bed-stress intensification, turbulent transport, and a vertical pumping mechanism associated with the compression and subsequent expansion of the mixing-layer. There was no evidence that the observed dynamics were associated with a global instability, as proposed in previous studies. The results have implications for cross-shelf mass transport and highlight future challenges for measuring and modeling boundary-layer processes within shelf seas.