Wind and wave-influenced mixing and dynamics in the Fraser River plume

Abstract

Rivers play a large role in the transport of matter between ecosystems by connecting interior regions with lakes and oceans. River outflows, or plumes, constitute a significant delivery mechanism of pollutants, sediment, and nutrients to the coastal environment. Such plumes are common along the worlds coastlines, but are poorly understood and difficult to sample due to their complexity. The effects of external forcing (such as winds and waves) on river plumes is particularly understudied, but has been shown to play a role in the behavior of the plume. The Fraser River (the site of proposed and existing oil infrastructure) is a key system to understand, as its mouth is near both a major population center (Vancouver, BC) and unique coastal ecosystems. This study presents observations of plume mixing and stream-normal momentum balances in the Fraser river plume using Lagrangian surface drifters. The Strait of Georgia commonly experiences calm to moderate winds from the Southeast and strong storm winds from the Northwest in the winter, and there are differences in plume behavior between these conditions. Under SE winds, the plume thins, spreads, and turns to the right (North) upon exiting the river mouth, mixing intensely for a short period of time. This process is dominated by a balance between stream-normal pressure gradient, Coriolis, and rotational acceleration, as previous studies have shown. Under NW winds, the plume stays thicker, narrower, and flows directly across the Strait while forming a lateral front on its northern side, mixing at a slower rate for longer. Different momentum terms dominate under this wind condition: the stream- normal balance pits Coriolis and rotational acceleration pressure gradient against interfacial shear stress, wind stress, wave radiation stress gradient, and ambient current body force. The two configurations of the plume show that variable winds can have a substantial impact on the shape of the plume, which can lead to changes in the sea surface anomaly associated with the dome of freshwater exiting the river mouth. Additionally, while spreading causes the plume to mix intensely under SE winds, the net mixing under NW winds is larger due to the longer extent of the plume. This indicates that large spreading is not always necessary for substantial plume mixing. Understanding the conditions that change plume mixing and behavior is a critical step in describing the pathways of pollutants, sediment, and nutrients as they enter the coastal ecosystem through a river plume.