Multiple Tidal Plume Fronts, Internal Waves and Near Shore Sediment Resuspension

Internal waves are an important source of mixing in the coastal oceans. Studies in the Columbia river plume demonstrated the presence of large amplitude internal waves that were released by the propagating tidal plume front. Here we explore internal waves generated by tidal plume fronts released into a shallow frictional system, the Rhine river plume, and their role on mixing, flocs and near shore sediment resuspension. To do this we use a detailed data set collected off the Dutch coast, 10 km north of the river mouth, near the Sand Engine, during the STRAINS field campaigns. Current velocity was measured with an ADCP with 0.25 m resolution and a frequency of 1 Hz. The data consists of temperature, salinity, and velocity measurements, as well as SPM and LISST data. In addition turbulent stresses were measured at the 12 m site using high frequency ADV velocity data collected 0.25, 0.5 and 0.75 m above the bed. We show that tidal plume fronts form on each ebb tide, as they propagate they generate high frequency internal waves ahead of the fronts. As the fronts and internal waves move onshore they increase turbulence and mixing. Notably they can also increase sediment resuspension near shore. The field-data and radar images show tidal plume fronts propagating past the 12 m mooring towards the Dutch coast. Using an idealised non-hydrostatic model we show that the fronts generate high frequency internal waves as they propagate towards the coast, that break inshore. We introduce a frontal sediment pumping mechanism, due to breaking internal waves nearshore, and discuss how this is a potential mechanism for sediment resuspension and offshore transport.