A steady state model for buoyant surface plume hydrodynamics in coastal waters

Abstract

Shallow, surface plumes of buoyant water are frequently discharged into coastal waters by rivers, bays, power plants, and other sources. The most readily observed portion of these plumes is the near field where the initial expansion of plume water occurs, and where there is large contrast between plume and ambient water. In contrast to existing turbulent jet models of such plumes, the present model divides the plume near field into two domains, a frontal domain where turbulent exchange between plume and ambient water is intense, and the remainder of the flow where the dynamics of inviscid, nonlinear gravitational spreading of a shallow buoyant layer dominates. Using this approach, the steady state flow produced by the supercritical outflow of buoyant water from a channel at a coast into ambient water with a uniform alongshore current is modelled. Earth rotation is neglected. Frontal jump conditions developed in an earlier paper are applied to match the flow at the frontal boundary with the remainder of the plume. Numerical solutions for the entire flow are found by the method of characteristics, but much of the flow can be determined by simpler analytic computation. The computed flow fields show that a frontal boundary forms on the upstream or offshore side of the plume where the oncoming ambient current contains the gravitational spreading of the buoyant water. No front forms on the corresponding downstream, inshore side. In the body of the plume the flow simultaneously expands and turns downstream toward alignment with the ambient current. Relatively little change in plume water speed occurs, but the pressure field required for turning the flow downstream results in deepening of the plume interface offshore near the front. This, in turn, concentrates plume volume flux along its offshore side near the front. The maximum angle through which the flow may be turned initially is less than 66°. Outlet channel angles greater than this produce plumes which are separated from the shoreline by ambient water. The model results explain many of the observed features of the Connecticut River plume in Long Island Sound. DOI: 10.1111/j.2153-3490.1982.tb01818.x