A Conceptual Model of a River Plume in the Surf Zone

Abstract

AbstractWe use observations from the Quinault River, a small river that flows into an energetic surf zone on the West Coast of Washington state, to investigate the interaction between river and wave forcing. By synthesizing data from moorings, drifters, and Unmanned Aerial System video, we develop a conceptual model of this interaction based on three length scales: the surf zone width, LSZ; the near‐field plume length, LNF; and the cross‐shore extent of the channel, LC. The relationships between these length scales show how tidal variability and bathymetric effects change the balance of wave and river momentum. The most frequently observed state is LSZ>LNF. 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 LNF. When the river velocity is highest during low water, and when wave forcing is low, LNF>LSZ and river water escapes the surf zone. At high water during low wave forcing, LC>LSZ, such that minimal wave breaking occurs in the channel and river water escapes onto the shelf. Based on the discharge, wave, and tidal conditions, the conceptual model is used 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.