Estuary Plumes and Fronts in Shelf Waters: A Layer Model

Abstract

A layer model that treats fronts as discontinuities is developed to study the steady state behavior of shallow estuary plumes on the continental shelf. The complete range of earth rotation effect is evaluated from small-scale or nonrotating plumes (Kelvin number equal zero) to large-scale, rotating plumes (Kelvin number equal order one). Supercritical flow is assumed in the outlet channel and the method of characteristics is used to compute the flow downstream. Nonrotating plumes have strong boundary fronts and concentrate their greatest layer depth and mass transport offshore near the front, but form no coastal current. Rotating plumes have boundary fronts that weaken soon after discharge, form a turning region where Coriolis action deflects the flow toward shore, and subsequently set up a coastal current. Soon after its formation this coastal current is bounded offshore by a strong front called the coastal front, across which the momentum balance changes from nearly inertial in the turning region upstream to nearly geostrophic in the coastal current itself. In traversing this front the flow loses total energy, but gains potential vorticity. Farther downstream the coastal front weakens, and meanders of the coastal current begin. Their wavelengths are short, about two Rossby radii, and their amplitudes grow, doubling after about 20 Rossby radii. The presence of supercritical speeds and fronts generates a plume dynamics that is remote from any linear description but shows analogous behavior to supersonic, compressible gas flow with shock waves.