Tidal and spring‐neap variations in horizontal dispersion in a partially mixed estuary

A sequence of dye releases in the Hudson River estuary provide a quantitative assessment of horizontal dispersion in a partially mixed estuary. Dye was released in the bottom boundary layer on 4 separate occasions, with varying tidal phase and spring‐neap conditions. The three‐dimensional distribution of dye was monitored by two vessels with in situ, profiling fluorometers. The three‐dimensional spreading of the dye was estimated by calculating the time derivative of the second moment of the dye in the along‐estuary, cross‐estuary and vertical directions. The average along‐estuary dispersion rate was about 100 m 2 /s, but maximum rates up to 700 m 2 /s occurred during ebb tides, and minimum rates occurred during flood. Vertical shear dispersion was the principal mechanism during neap tides, but transverse shear dispersion became more important during springs. Suppression of mixing across the pycnocline limited the vertical extent of the patch in all but the maximum spring‐tide conditions, with vertical diffusivities in the pycnocline estimated at 4 × 10 −5 m 2 /s during neaps. The limited vertical extent of the dye patch limited the dispersion of the dye relative to the overall estuarine dispersion rate, which was an order of magnitude greater than that of the dye. This study indicates that the effective dispersion of waterborne material in an estuary depends sensitively on its vertical distribution as well as the phase of the spring‐neap cycle.