LONGITUDINAL DYE DISPERSION AND SALT FLUX IN ESTUARIES

Abstract

Estuarine dispersion plays an important role in determining the fate of waterborne materials. It is a long-standing question in estuarine dynamics that is still not well understood. This dissertation revisits this problem by utilizing two tracers: dye and salt.

Dye-release experiments and numerical modeling are conducted to investigate horizontal dispersion in a partially mixed estuary. Longitudinal dispersion of a dye patch shows strong flood-ebb asymmetry at early times after a dye release, with most of the dispersion occurring during ebb tides. Tidal straining enhances vertical current shear on ebb tides and promotes longitudinal dispersion. There are also large differences in the dispersion rate between spring and neap tides. Due to strong spring mixing, a dye patch quickly extends from the bottom to the surface, exposing to the full vertical shear in the water column and leading to strong longitudinal dispersion. In contrast most of the dye patch is limited to bottom few meters during neap tides. Although weak vertical mixing facilitates longitudinal dispersion, the vertical shear across the thin dye patch is much weaker, leading to weak longitudinal dispersion during neap tides. In first four tidal cycles, the second moment of the dye patch in the along-channel direction increases with time at a power of between 2 and 3. The longitudinal dispersion rate varies as the four-third power of the dye patch size, indicating scale-dependent diffusion.

Salt dispersion and transport are examined in a comparative numerical modeling study between the partially-mixed Chesapeake Bay and the well-mixed Delaware Bay. To investigate how different physical mechanisms drive the salt transport into the estuaries, the longitudinal salt fluxes are decomposed using the Eulerian and quasi-Lagrangian methods. Under the Eulerian framework, the salt flux is decomposed into three parts: an advective term associated with the barotropic forcing, a steady shear dispersion term associated with the estuarine exchange flow, and a tidal oscillatory salt flux. In both estuaries, the advective term is dominant over steady shear dispersion and tidal oscillatory salt flux in the temporal variation of total salt flux. In Chesapeake Bay, the steady shear dispersion is the dominant mechanism and the tidal oscillatory salt fluxis small. In Delaware Bay, the steady shear dispersion and tidal dispersion are comparable. The along-channel variation of tidal oscillatory salt flux is mainly due to changes of the phase difference between the tidal current and salinity. Isohaline analysis using the quasi-Lagrangian methodology yields a new interpretation of the estuarine exchange flows and describes the evolution path of salinity classes.