On the Transport of Buoyant Coastal Plumes

Abstract

The role of discharge conditions and shelf geometry on the transport of coastal plumes is studied with a fully nonlinear, primitive equation hydrodynamic model. The physical setting is an estuarine channel with a small discharge Rossby number. By simulating different discharge magnitudes, buoyant plumes are shown to be succinctly described by a simple coastal front model. Three results emerge from the model analysis. First, the plume transport is given by T = ?0(g?ph2/2f?), where ?0 is a parameter dependent on the ratio of the front and the plume widths, g?p is the plume reduced gravity, h is the plume maximum depth, and f is the Coriolis parameter. Second, this model links the plume transport directly to upstream river conditions with T = ?Qr, where Qr is the river outflow and ? is a parameter that relates to entrainment, the geometry of the plume front and shelf slope, and the fraction of freshwater carried downshelf. Third, these equations reduce to analytic results previously established for special cases, providing useful formulas to estimate the plume transport from hydrographic and river discharge observations.