Water Mass Modification in Near-field River Plumes
Robert Hetland
Texas A&M University

This paper describes numerical solutions to a simple layer model of a near-field river plume. The near-field is defined as the region where buoyant outflow from an estuary is supercritical. The layer model is intended to reproduce the scales of the near-field plume, in particular the density anomaly of the water leaving the near field, by balancing the competing processes of entrainment and plume spreading. The structure of the solutions is qualitatively similar to observations and three-dimensional simulations of near-field plumes, and agrees reasonably well with three-dimensional simulations of near-field plumes. The properties of water exiting the near-field plume depend functionally on the normalized entrainment velocity, Wn = We * 2 * Wo / Qo, where We is either a representative or minimum background entrainment velocity, Wo is the width of the estuary mouth, and Qo is the volume flux of water leaving the estuary. Water leaving this region will determine to a large extent the water properties of the larger scale river plume structure over the continental shelf. Thus, this simple model may be useful in estimating the characteristic density of buoyant water in larger scale analytic models of buoyancy-driven flow on the continental shelf, or in estimating unresolved mixing near buoyancy sources in numerical models of coastal ocean flow.