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Derek A. Fong and W. Rockwell Geyer

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.

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Sarah N. Giddings, Stephen G. Monismith, Derek A. Fong, and Mark T. Stacey

Abstract

Residual (subtidal) circulation profiles in estuaries with a large tidal amplitude-to-depth ratio often are quite complex and do not resemble the traditional estuarine gravitational circulation profile. This paper describes how a depth-normalized σ-coordinate system allows for a more physical interpretation of residual circulation profiles than does a fixed vertical coordinate system in an estuary with a tidal amplitude comparable to the mean depth. Depth-normalized coordinates permit the approximation of Lagrangian residuals, performance of empirical orthogonal function (EOF) analysis, estimation of terms in the along-stream momentum equations throughout depth, and computation of a tidally averaged momentum balance. The residual mass transport velocity has an enhanced two-layer exchange flow relative to an Eulerian mean because of the Stokes wave transport velocity directed upstream at all depths. While the observed σ-coordinate profiles resemble gravitational circulation, and pressure and friction are the dominant terms in the tidally varying and tidally averaged momentum equations, the two-layer shear velocity from an EOF analysis does not correlate with the along-stream density gradient. To directly compare to theoretical profiles, an extension of a pressure–friction balance in σ coordinates is solved. While the barotropic riverine residual matches theory, the mean longitudinal density gradient and mean vertical mixing cannot explain the magnitude of the observed two-layer shear residual. In addition, residual shear circulation in this system is strongly driven by asymmetries during the tidal cycle, particularly straining and advection of the salinity field, creating intratidal variation in stratification, vertical mixing, and shear.

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Ryan J. Moniz, Derek A. Fong, C. Brock Woodson, Susan K. Willis, Mark T. Stacey, and Stephen G. Monismith

Abstract

Autonomous underwater vehicle measurements are used to quantify lateral dispersion of a continuously released Rhodamine WT dye plume within the stratified interior of shelf waters in northern Monterey Bay, California. The along-shelf evolution of the plume’s cross-shelf (lateral) width provides evidence for scale-dependent dispersion following the 4/3 law, as previously observed in both surface and bottom layers. The lateral dispersion coefficient is observed to grow to 0.5 m2 s−1 at a distance of 700 m downstream of the dye source. The role of shear and associated intermittent turbulent mixing within the stratified interior is investigated as a driving mechanism for lateral dispersion. Using measurements of time-varying temperature and horizontal velocities, both an analytical shear-flow dispersion model and a particle-tracking model generate estimates of the lateral dispersion that agree with the field-measured 4/3 law of dispersion, without explicit appeal to any assumed turbulence structure.

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