Mixing in the Tidal Environment

Abstract

A review is given of mathematical and computer modeling of many of the physical processes that effect the dilution of tracers released in tidal flows: tidal trapping, differences across the flow in tidal speeds, density currents, flow separation. The modeling approaches given most emphasis are tidal trapping, shear dispersion, chaotic stirring, and particle tracking. The central theme of this review is that during the first few tidal cycles, stretching, twisting, and splitting of the tracer patch gives rapidly increasing rates of dilution. There is a water depth (about 30 m) or estuary width (about 200 m) at which the tidal contribution to the eventual dilution rate is largest (about 3 km² d⁻¹ or 35 m² s⁻¹). Careful selection of the position, timing, and release rate of a discharge can allow a given amount of effluent to be diluted much more rapidly and to have a much reduced environmental impact.