Tidal plume fronts, internal waves and sediment resuspension in a near field river plume

Internal waves are known to be an important source of mixing in the coastal ocean. Measurements from the Columbia River Plume were some of the first to demonstrate the generation of large amplitude internal waves released by a newly formed tidal plume front. Here we explore internal waves generated by multiple tidal plume fronts and their trapping in the mid-field plume of the Rhine river plume. The internal waves are released into a shallow frictional system, and their role on mixing, near shore sediment resuspension is examined. We use data collected off the Dutch coast near the Sand Engine, during the STRAINS field campaigns at a location 10 km north of the river mouth. An ADCP measured current velocity with a frequency of 1 Hz and a resolution of 0.25 m. Temperature, salinity, velocity, sediment concentration measurements, as well as turbulent stresses were measured at the 12 m site at 0.25, 0.5 and 0.75 m above the bed. The field-data and radar images show tidal plume fronts propagating towards the Dutch coast and the generation of high frequency internal waves ahead of the fronts. As the fronts propagate onshore they increase turbulence and mixing and can also increase sediment resuspension. Using an idealised non-hydrostatic model we show that the fronts can generate high frequency internal waves as they propagate towards the coast, and that these waves can break inshore. We introduce a frontal sediment pumping mechanism, and show how this is a new mechanism for sediment resuspension and offshore transport.