The fate of river discharge on the continental shelf: 1. Modeling the river plume and the inner shelf coastal current

Abstract

We study the development and evolution of buoyant river plumes on the continental shelf. Our calculations are based on three-dimensional numerical simulations, where the river runoff is introduced as a volume of zero salinity water in the continuity equation and mixing is provided by the model's turbulence closure scheme and wind forcing.In the absence of wind forcing the modeled fiver plumes typically consist of an offshore bulge and a coastal current in the direction of Kelvin wave propagation. We propose a plume classification scheme based on a bulk Richardson number, which expresses the relative magnitude of the buoyancy-induced stratification versus the available mixing. When the ratio of the discharge and shear velocities is greater (less) than 1, the plume is categorized as supercritical (subcritical); that is, the width of the bulge is greater (less) than the width of the coastal current. Supercritical plumes are often characterized by a meandering pattern along the coastal current, caused by a baroclinic instability process. For a given discharge, subcritical plumes are produced by large mixing and/Or shallow water depths. In the presence of wind forcing the favorable conditions for offshore removal of coastal low-salinity waters include high river runoff and strong upwelling-favorable wind stress. When the rivers are treated as individual sources of freshwater ("point source" behavior), the wind-driven flow may exhibit substantial spatial variability. Under the above removal conditions, strong offshore transport takes place in "jetlike" flow regions within the river plume, in contrast to the downwind acceleration of adjacent waters. When the rivers are treated as a long "line source" of freshwater, the plume region resembles a coastal low-salinity band and the above removal conditions trigger offshore transport that is most pronounced at the "head" of the source. control the fate of fivefine waters and related materials afterPaper number 95JC03024. 0148-0227/96/95JC-0302455.00 their release in the coastal ocean. Our main objective is to describe the generation and evolution of a fiver plume on the continental shelf and determine the important factors that govern its offshore expansion and, consequently, the removal mechanism of fiver-borne materials.The frontal structure of a fiver plume has been discussed by several investigators. Earlier studies, such as those by Kao et al. [1977], Kao [1981], Ikeda [1984], and Csanady [1984], recognized the importance of nonlinearity, Coriolis, and friction in the development of the buoyant plume. McClimans [1986] identifi•ed the following three major processes that characterize the dynamics of the seaward expansion of the river flow: (1) acceleration, resulting from the balance between inertia and gravity (buoyancy) forces; (2) mixing, governed by turbulence due to bottom and interfacial friction; and (3) geostrophy, where the balance between Coriolis and the developed cross-shore pressure gradient (due to "freshening" of coastal waters) generates an alongshore coas...

The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields

“…Weak southward flow inshore of plumes has been observed in model results [e.g., Kourafalou et al, 1996a] and is usually attributed to frictional effects; i.e., along-shelf flow responds first on the inner shelf to shifts in wind direction. Although this is also the case for the Columbia region (not shown), the southward flow commonly observed inshore of the Columbia plume is more consistent with the existence of geostrophic flow around the plume.…”

“…Models predict enhancement of surface Ekman flow within the plume [Kourafalou et al, 1996a]. Cross-isobath currents in the upper 5 m of the Columbia plume frequently exceed those outside the plume by up to a factor of 2.…”

The influence of the Plata River discharge on the western South Atlantic shelf

“…[7] The meridional penetration of a river plume is largely controlled by the magnitude of Q and the direction and amplitude of the wind stress forcing [Kourafalou et al, 1996]. To illustrate these relations we computed t y and Q for the period 1949 -2001 (Figure 4).…”

Buoyant gravity currents along a sloping bottom in a rotating fluid

“…However, the relevance of these studies to the typical geophysical applications where the buoyant gravity current propagates along a sloping bottom is unclear. Previous laboratory (Whitehead & Chapman 1986) and numerical modelling studies (Chao 1988;Chapman & Lentz 1994;Kourafalou et al 1996;Yankovsky & Chapman 1997;Xing & Davies 1999;Garvine 1999) suggest a sloping bottom has a profound influence on the characteristics and dynamics of buoyant gravity currents (hereafter referred to as simply gravity currents).…”