The fate and dynamics of a river plume in the surf zone

Abstract

Small river outflows that directly enter the surf zone,where waves break near the shore, are a common feature of the world's coastlines. Rivers transport sediment, nutrients, and pollutants from the terrestrial to the marine environment, and the fate of this material is important for coastal morphology, ecology, and public health. Breaking waves release their energy and momentum in the surf zone, causing it to be energetic and turbulent, yet retentive, as surf zone cross-shore exchange is small on average. Thus, river water and river-borne material may become trapped in the surf zone. Trapped fresh river water is subject to energetic alongshore circulation and turbulence, and may be transported away from the river mouth, undergoing wave-driven mixing. Using observations from the Quinault River, which flows into an energetic surf zone on Quinault Indian Nation land north of Grays Harbor, WA, I investigate the interaction between river and wave forcing in the surf zone. By synthesizing data from moorings, drifters, and Unmanned Aerial System (UAS) video, I develop a conceptual model of this interaction based on river forcing, wave forcing, and the bathymetry near the river mouth. The relationships between these show how tides and bathymetry change the balance of wave and river momentum. Most frequently, wave forcing dominates over river forcing. Under these conditions the surf zone traps the outflowing river plume and the river water’s initial propagation into the surf zone is set by a plume length scale. When the river velocity is highest during low tide, and when wave forcing is low, river forcing dominates over wave forcing and river water escapes the surf zone. At high tide during low wave forcing, bathymetric effects can allow the river water to bypass wave forcing. In this case minimal wave breaking occurs in the channel and river water escapes onto the shelf. Estimates of entrainment velocity based on the drifter propagation distance indicate that mixing may be elevated above theoretical values in the near field plume by surf zone wave breaking. Based on the discharge, wave, and tidal conditions, I use the conceptual model to predict the fate of river water from the Quinault over a year, showing that approximately 70% of the river discharge is trapped in the surf zone upon exiting the river mouth. Drifter observations from the surf zone near the Quinault River mouth further indicate that the trapping of freshwater in the surf zone can result in high near-surface stratification. I investigate the rate that river water mixes with ocean water using two metrics: the rate of change of salinity in a Lagrangian reference frame, or material derivative of salinity, and the along track salinity variance. High mixing rates are observed concurrently with high near-surface stratification, as salinity gradients are collocated with wave breaking turbulence, which is unaffected by stratification. I observe a transition from low stratification and low mixing rate at low tidal stage to high stratification and high material derivative of salinity at high tidal stage as river discharge decreases. This decrease in surf zone freshwater content is driven by decreased river volume flux and increased wave-driven alongshore transport combining to export freshwater from the river mouth, and is well described by an analytical framework based on a continuously stirred reactor. In contrast with more commonly studied large river plume systems, lateral exchange and mixing could be significant, especially during periods of low stratification and at the surf zone edge. The results of this study may be applied to find the freshwater fraction of the surf zone as well as the alongshore propagation length scale of trapped river water, and to predict when such water would rapidly mix. Lastly, I synthesize these findings and offer a view of the surf zone from the perspective of trapped river water. Freshwater that is trapped in the surf zone interacts with surf zone features such as rip currents, alongshore currents, and the Stokes' drift-undertow circulation, as well as potentially altering cross-shore surf zone dynamics by introducing a baroclinic pressure gradient. As plume water propagates in the surf zone, it may undergo increased wave-driven mixing as near-surface stratification increases due to the surface intensification of wave breaking turbulence. These observations fall in line with previous work showing that wave driven mixing is greatest when the plume depth is shallow. Thus, the quantity of river water trapped in the surf zone is related to mixing, as the more river water is trapped, the less stratified the surf zone becomes, and less mixing occurs.