A simplified energy and mixing budget for a small river plume discharge

Abstract

The formation and evolution of a tidally modulated river plume that spreads and mixes into English Channel waters is investigated by combining results from two observational studies. Remote sensing imagery showed a near‐semicircular plume expansion as its diameter increased linearly during the ebb tide. Subsequent ship surveys made repeated transects across the plume front and showed that regions of mixing and instability (Froude numbers > 1) were confined to horizontal length scales <10 m in the region of the bounding plume front. These observations were used to compute the rate of frontal mixing energy, and results were compared to the rate of buoyancy energy input and the mixing rate over the plume area due to wind and tidal flows. Analysis showed mixing at the front, although spatially confined and subject to considerable variability, was the dominant dispersion process. On the basis of these results, a simple continuity model was developed where the ratio of frontal mixing rate to buoyancy input rate, fb, was estimated for the early stages of plume formation only on the basis of given values of freshwater input and the estuarine tidal prism. During the field experiments, fb ≈ 0.44 and suggested that the initial frontal mixing rate was much smaller than the buoyancy input rate, consistent with the formation of a well‐defined plume. However, on the basis of river flow statistics, for 65% of the time during spring tidal conditions, the run‐off rates are small enough for fb > 1, suggesting that a plume cannot form.