The Influence of Wind and Waves on Spreading and Mixing in the Fraser River Plume

Abstract

This study uses drifter-based observations to investigate the role of wind and waves on spreading and mixing in the Fraser River plume. Local winter wind patterns commonly result in two distinct forcing conditions, moderate winds from the southeast (SE) and strong winds from the northwest (NW). We examine how these patterns influence the spreading and mixing dynamics of the plume. Under SE winds, the plume thins, spreads, and turns to the right (north) upon exiting the river mouth. Mixing is initially intense in the region of maximum spreading, but it is short-lived. Under NW winds, which oppose the rightward tendency of the plume, the plume remains thicker, narrower, and flows directly across the Strait with a lateral front on its northern side. Mixing is initially lower than under SE forcing but persists further across the Strait. A Lagrangian stream-normal momentum balance shows that wind and interfacial stress under NW conditions compress the sea surface height anomaly formed by the river discharge and guide the flow across the Strait. This reconfiguration changes spreading and mixing dynamics of the plume; plume spreading, which drives intense mixing under SE winds, is shut down under NW winds, and mixing rates are consequently much lower. Despite the initially lower mixing rates, the region of active mixing extends further under NW winds, resulting in higher net mixing. These results highlight that the wind, which is often a primary cause of increased plume mixing, can also significantly influence mixing by changing the geometry of the plume.Plain Language Summary Rivers transport sediment, pollutants, and nutrients from inland regions to coastal seas. Where rivers meet the ocean, freshwater flows over the ambient salty seawater, forming a river plume. The quantities that the river transports into the ocean are mixed into the seawater along with the freshwater, and so it is vital to understand this mixing process. While ocean surface waves might be the most striking visual feature of the coastal ocean, at the Fraser River mouth, south of Vancouver, Canada, we find that wind is a much more important influence on river plume mixing than waves. Wind can influence mixing by changing the geometry of the plume to either favor or discourage intense mixing occurring in the system. This is a result of the wind encouraging or discouraging plume spreading, which is the primary cause of mixing close to the river mouth. Ocean surface waves, despite being a visually striking feature of this system, do not play a large role in mixing the Fraser River plume. Thus, in order to correctly predict river plume mixing, we must take into account wind conditions near the river mouth, while waves are less important.

A Conceptual Model of a River Plume in the Surf Zone

“…As w eff is larger than w e , wave mixing is likely the relevant controlling process in the Quinault system. In larger systems it has been suggested that the ratio H S / h p determines the extent to which wave‐driven turbulence can affect plume mixing (Kastner et al, ). SWIFT drifters with three CT sensors that escape the surf zone measure strong stratification in the upper 1.2 m, indicating that H S / h p could be in a range where wave‐driven turbulence could affect plume mixing.…”

“…Stokes drift causes modest advection (hundreds of meters) of the Columbia river plume (Akan et al, ). Large plume depths relative to wave heights in the Fraser River plume shield the plume from mixing due to breaking wave‐driven turbulence (Kastner et al, ).…”

Coastal mixing in multiple-mouth deltas: A case study in the Po delta, Italy

“…Deltas, estuaries and lagoons are also characterised by the presence of buoyant fluxes, sediment transport and a relatively fast morphological evolution. Many studies focused on the classification of estuarine circulation (Hansen and Rattray, 1966;Horner-Devine et al, 2015), providing tools for quantifying their main characteristics O' Donnell, 2010). The interaction between fresh and seawater leads to a number of hydrodynamic patterns, identifying water masses mechanically confined into a coherent shape close to the estuaries, with specific buoyancy, the so-called plumes.…”

“…The interaction between fresh and seawater leads to a number of hydrodynamic patterns, identifying water masses mechanically confined into a coherent shape close to the estuaries, with specific buoyancy, the so-called plumes. Their definitions and the capability to clearly state what are their borders are still subjects of discussion O' Donnell, 2010;Falcieri et al, 2014;Horner-Devine et al, 2015 and references therein) but their interaction with coastal waters explains part of the coastal mixing processes. A certain part of river discharge is transported alongshore in downstream coastal current, while a part recirculates close to the river mouth creating an increasing area of buoyant waters, generally stabilised by tidal action (Fong and Geyer, 2002;Isobe, 2005, Chant et al, 2008.…”

“…Mixing processes occurring in coastal areas are of several types and respond to a number of additional factors: coastal currents are affected by the general hydrodynamics of the main basin and are modulated by local inputs, like rivers, lagoons and wetlands, modifying their thermohaline characteristics and dynamics (Androulidakis et al, 2015). Moreover, on approaching the coast, from deep to shallow waters, mixing is also intensified by wave actions, which also have consequences on sediment resuspension (Karstner et al, 2018). The morphology of the coast can contribute, with its complexity, in modifying the mixing dynamics.…”