Abstract:
The 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.
