A Conceptual Model of a River Plume in the Surf Zone

• Published in: Journal of geophysical research. Oceans 2019-11, Vol.124 (11), p.8060-8078
• Main Authors: Kastner, S. E., Horner‐Devine, A. R., Thomson, J. M.
• Format: Article
• Language: English
• Subjects: Beach erosion; Beaches; Breaking waves; buoyancy; Coastal erosion; Coastal zone; Coasts; Drifters; Ecology; estuary; Geophysics; Length; Marine pollution; Momentum; Mooring; Nutrients; Offshore; Pollutants; Public health; River channels; River discharge; River flow; River mouth; River mouths; river plume; River plumes; River water; Rivers; Sediment; Surf; Surf zone; surface gravity waves; Tides; Unmanned aerial vehicles; Water depth; Water discharge; Wave breaking
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2019JC015510

We 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. Plain Language Summary Small rivers are an important source of sediment, nutrients, and pollutants to the coastal ocean, but they are less well studied than their larger siblings. The coastal discharge from small rivers often meets large breaking waves in the surf zone. Our work investigates the effect of the large waves present at the mouth of one such river, the Quinault River in Washington State. We find that the fate of Quinault River water is determined by the relative importance of river, wave, and depth effects, all of which are modulated by the tide. When wave forcing dominates, river water is trapped in the surf zone. When river forcing dominates, river water escapes the surf zone. The difference in depth between the offshore channel and the rest of the beach can also allow river water to escape the surf zone by reducing the effect of the wave forcing. When we apply our conceptual model to 1 year of wave, river, and tide data, we predict that 70% of Quinault River discharge is trapped in the surf zone. The sediment and pollutants carried by this trapped river water can thus have important impacts on beach erosion, public health, and local