Observations of River Plume Mixing in the Surf Zone


We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone trapped river plume. We quantify mixing based on the rate of change of salinity, DS/Dt, in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kɀ = 2.2±0.6×10−3 m2 s-1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1×10−2m2 s-1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage or offshore wave height; as a result we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.