Observations of river plume mixing in the surf zone

Abstract:

This work investigates the rate that river discharge is mixed with ocean water in the surf zone. Turbulence is high due to wave breaking in the surf zone, which is expected to lead to high mixing. At the same time, however, exchange between the surf zone and coastal waters is limited; thus, mixing in the surf zone may be reduced if the surf zone isolates fresh from salt water. We use mooring and drifter observations to quantify the mixing of river discharge and understand how the processes above determine the mixing in the surf zone near the Quinault River mouth, on Quinault Indian Nation land north of Grays Harbor, WA. We quantify mixing using two metrics in the drifter’s Lagrangian reference frame: the rate of change of salinity and the along track salinity variance. Over the course of a tidal cycle, surf zone turbulence is observed to be constant and unaffected by changes in stratification. We observe a transition from low stratification and low mixing rate at low tidal stage to high stratification and high mixing rate at high tidal stage as the freshwater content of the surf zone decreases. Changes in freshwater content are interpreted using a surf zone control volume near the river mouth. Increased wave-driven alongshore transport out of the control volume and decreased river volume flux into it explain the observed reduction in surf zone freshwater content. We estimate lateral exchange processes using lateral eddy diffusivity values from previous studies and estimates of the cross-shore salinity gradient from our measurements; we find that lateral mixing is of a similar magnitude to our vertical mixing estimates, especially during periods of low stratification. In summary, we find that while wave breaking provides high turbulence in the surf zone, the rate at which river water is mixed with ocean water is ultimately set by the stratification, which is determined by variability in the tidal freshwater flux, alongshore current and surf zone volume.