Mechanisms and variability of salt transport in partially-stratified estuaries

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2000The variability of salt transport determines the variation of the length of the salinity intrusion and the large-scale density gradient in an estuary. This thesis contains three studies that address salt transport and the salt balance. The variation of salt transport with the depth, the along-channel salinity gradient, and the amplitude of the tidal velocity is investigated with analytic and numerical models. The results indicate that salt transport increases dramatically during stratified periods when vertical mixing is weak. Analysis of salt transport from observations in the Hudson Estuary show that stratified periods with elevated estuarine salt transport occur in five-day intervals once a month during apogean neap tides. Oscillatory salt transport, which is hypothesized to be primarily caused by lateral exchange and mixing of salt, appears to play a more minor role in the salt balance of the estuary. The salt balance of the estuary adjusts very little to the spring-neap modulation of salt transport but adjusts rapidly to pulses of freshwater flow. A simple model is used to investigate the process and time scales of adjustment of the salt balance by connecting variations of salt transport to the variations of freshwater flow and vertical mixing. The results show the length of the salinity intrusion adjust via advection to rapid and large increases in freshwater flow. The salinity intrusion adjusts more rapidly to the spring-neap cycle of tidal mixing the higher the freshwater flow.The National Science Foundation provided support through a National Science Foundation Graduate Fellowship and NSF Grant OCE94-15617. Grants from the Hudson River Foundation (HRF Grant 006j96A) and the Office of Naval Research (Grant Number N00014-97-1-0134) have also contributed towards the work in this thesis. This work is also partially the result of research sponsored by NOAA National Sea Grant College Program Office, Department of Commerce, under Grant No. NA46RG0470, Woods Hole Oceanographic Institution (WHOI) Sea Grant project no. R/O-30