River plume mixing and transport in the surf zone

Many small rivers empty into energetic surf zones. Recent work has shown that river water is often trapped in the surf zone by strong wave forcing, suggesting that river-borne sediment, nutrients, and pollutants can influence nearshore morphodynamics and productivity as well as public health in coastal communities. This work synthesizes observations from the mouth of the Quinault River, WA, focusing in particular on the drivers of plume transport and mixing in the surf zone. We present a scaling analysis that predicts when river water is trapped in the surf zone and investigate the relative importance of velocity shear and wave breaking to river plume mixing. The study period spanned most of one spring-neap cycle, during which river discharge ranged from 100 to 200 m^3/s and offshore wave height varied from 1.0 to 2.5 m.

The fate of drifters deployed at the river mouth indicates that river water was mostly trapped in the surf zone, as wave forcing often dominated over river forcing. Nearshore mooring and drifter salinity measurements show a cross-shore salinity gradient such that salinity on the inner shelf decreases shoreward, with near-zero salinity observed near the shoreline. This cross-shore salinity gradient indicates the importance of plume mixing in the surf zone, which we estimate with the reduction of salinity variance over a drift track. We relate this estimated mixing to the turbulent kinetic energy dissipation rates calculated from Doppler profiles of turbulence collected onboard the drifters. We then scale these estimates of mixing rate and dissipation with measured and inferred kinetic and potential energy inputs from the incident waves and river flow to determine the relative importance of these processes in mixing a river plume in the surf zone.