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The Alongshore Transport of Freshwater in a Surface-Trapped River Plume

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  • 1 Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts
  • | 2 Department of Applied Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
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Abstract

The alongshore transport of a surface-trapped river plume is studied using a three-dimensional model. Model simulations exhibit the previously observed rightward veering (in the Northern Hemisphere) of the freshwater and establishment of a downstream geostrophically balanced coastal current. In the absence of any ambient current, the plume does not reach a steady state. The downstream coastal current only carries a fraction of the discharged freshwater; the remaining fraction recirculates in a continually growing “bulge” of freshwater in the vicinity of the river mouth.

The river mouth conditions influence the amount of freshwater transported in the coastal current relative to the growing bulge. For high Rossby number [O(1)] discharge conditions, the bulge shape is circular and the coastal current transport is smaller than for the model runs of low Rossby number discharges. For all model runs conducted without an ambient current, the freshwater transport in the coastal current is less than the freshwater discharged at the river mouth.

The presence of an ambient current (in the same direction as the geostrophic coastal current) augments the transport in the plume such that its downstream freshwater transport matches the freshwater source, and the plume evolves to a steady-state width. The steady-state transport accounted for by the ambient current is independent of the strength of the ambient current. The amplitude of the ambient current only determines the time required to reach a steady-state plume width. A key result of this study is that an external forcing agent (e.g., wind or ambient current) is required in order for the entire freshwater volume discharged by a river to be transported downstream.

Corresponding author address: Dr. Derek A. Fong, Environmental Fluid Mechanics Laboratory, Dept. of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020. Email: dfong@stanford.edu

Abstract

The alongshore transport of a surface-trapped river plume is studied using a three-dimensional model. Model simulations exhibit the previously observed rightward veering (in the Northern Hemisphere) of the freshwater and establishment of a downstream geostrophically balanced coastal current. In the absence of any ambient current, the plume does not reach a steady state. The downstream coastal current only carries a fraction of the discharged freshwater; the remaining fraction recirculates in a continually growing “bulge” of freshwater in the vicinity of the river mouth.

The river mouth conditions influence the amount of freshwater transported in the coastal current relative to the growing bulge. For high Rossby number [O(1)] discharge conditions, the bulge shape is circular and the coastal current transport is smaller than for the model runs of low Rossby number discharges. For all model runs conducted without an ambient current, the freshwater transport in the coastal current is less than the freshwater discharged at the river mouth.

The presence of an ambient current (in the same direction as the geostrophic coastal current) augments the transport in the plume such that its downstream freshwater transport matches the freshwater source, and the plume evolves to a steady-state width. The steady-state transport accounted for by the ambient current is independent of the strength of the ambient current. The amplitude of the ambient current only determines the time required to reach a steady-state plume width. A key result of this study is that an external forcing agent (e.g., wind or ambient current) is required in order for the entire freshwater volume discharged by a river to be transported downstream.

Corresponding author address: Dr. Derek A. Fong, Environmental Fluid Mechanics Laboratory, Dept. of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305-4020. Email: dfong@stanford.edu

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