Observations of River Plume Mixing in the Surf Zone

Abstract

Abstract 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 Kz = (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 × 10−2 m2 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. Significance Statement River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.