Physical processes in a time-dependent salt wedge estuary with a region of repeated sharp curvature

Abstract

This thesis presents a comprehensive study of physical processes in the Taieri River of the South Island, New Zealand, a poorly studied time-dependent salt wedge estuary with a region of repeated sharp curvature. A series of Conductivity Temperature Depth and Acoustic Doppler Current Profiler observations of the landwards advance, break-down and seawards retreat of a salinity intrusion were obtained through the use of surface and bed moorings, as well as through vessel-mounted operations. Intratidal variation of physical properties is examined from the perspective of internal hydraulics, lateral circulation and turbulent mixing, with an emphasis on the influence of system stratification. A theme is that physical characteristics observed in the Taieri River broaden the range of observed conditions for a time-dependent salt wedge estuary.

During flood tide, a salt wedge and tidal intrusion front region advances and a laterally asymmetric surface front typically arrests in Bend B. A novel description of the internal hydraulic state of the entire, advancing two-layer structure details an advancing, typically strongly supercritical, tidal intrusion front region that seemingly undergoes an internal hydraulic jump before advancing as a typically supercritical salt wedge region. The oblique arrest of the surface front then provides a novel example of control through the Froude angle. During slack and ebb tides, there is an adjustment, break-down and retreat of the salinity intrusion. Internal hydraulics reflect an estuary-wide tendency to supercritical flow, with conditions that appear to differ with system stratification.

Lateral circulation in and around Bend B is driven by the temporal and spatial variation of centrifugal acceleration and the lateral baroclinic pressure gradient. The interplay of the salinity and velocity fields is complex and at times intense, and conditions appear to differ with system stratification during slack and ebb tides. Apparent oscillations (i.e. lateral rebounds) of the salinity field, associated with the adjustment, break-down and retreat of the salinity intrusion, are attributed to a rebounding lateral baroclinic pressure gradient. Partial overturning of the salinity field, which can occur during slack and ebb tides, is driven by centrifugal acceleration and helical flow. Both features are novel for a time-dependent salt wedge estuary. Partial overturning only follows lateral rebounding under weakly stratified conditions.

Turbulent mixing in Bend B gives some context to the internal structure of the advance and arrest of the salinity intrusion during flood tide. Although internal shear instabilities are driven by vertical shear across the pycnocline, there is a general internal stability of the pycnocline and the salt wedge and tidal intrusion front region persist. Turbulent mixing in Bend B also gives some context to the adjustment, break-down and retreat of the salinity intrusion during slack and ebb tides as it differs with system stratification. Mixing mechanisms - direct generation of mixing through the rapid lateral motion of the salinity field, internal shear instabilities and advectively induced instabilities - are considered with reference to lateral rebounding and partial overturning of the salinity field and reveal internal mixing of the pycnocline that is novel for a time-dependent salt wedge estuary.