engleski

Thermohaline structures, turbulence and mixing processes within shallow Maketu Estuary

After the 1996 partial re-diversion of the Kaituna River into the Maketu Estuary, the salinity-temperature structures of the estuarine waters have been changed. Conditions are now dependent on the specific operating regime of the Ford’ s Cut floodgates which control freshwater input. Synoptic plots of the salinity-temperature contours along the estuary main channel and analyses of salinity-temperature measurements at fixed sites within the estuary revealed a large variability in salinity over the course of the tidal cycle at the surface and bottom (24-30 psu). A Principal Component Analysis (PCA) showed that the major influence on the salinity stratification is river inflow (~33%) and wind stress (~21%) associated with prevailing south-westerly winds blowing across the estuary during the observation period, August 1997. The residence time of the estuary is estimated to be ~ 13.3 hours, approximately a half-tidal cycle shorter than it was prior to the partial re-diversion of the Kaituna River. The turbulence and mixing processes which characterised water mass behaviour is a combination of small-scale turbulent diffusion and a larger-scale variation of the field of advective mean velocities. The components of Reynolds stress parallel to the boundary in the direction of mean flow were found to be variable in the range of 0-12 cm2s-2 depending on tidal phase, while the energy spectra of turbulent velocity fluctuations were found to be similar (both during the flood and ebb phase) indicating the weak turbulence associated with a weak mean current and stratified water column. The Brunt-Väisälä frequency and the local gradient Richardson number indicate dynamically stable ebb flow (Ri >12), but during the flood phase the vertical distribution of the Richardson number (Ri Ł 1) indicates instability of the flow, likely associated with the river inflow (a fully turbulent mid-flood flow). The Ozmidov scale varies from 0.7-1.8 m and is comparable to the largest scale of the flow at the sites examined, while the smallest scale, the Kolmogorov scale was estimated to be ~ 3 x 10-2 cm. An axial convergence front was formed only during the late stage of the flood tide extending continuously up the estuary main channel, while occurrence of internal waves was identified in the upper reaches of the estuary. Derived typical values of Nz ranged from 8.3-23.8 x 10-4 m2s-1, Kz ranged from 4.4-9.2 x 10-4 m2s-1 and Kx ranged from 1.24-8.3 m2s-1 Overall, vertical density gradients caused by the presence of the newly-formed salinity-temperature regime due to the re-diverted river had a stabilising effect on the vertical mixing processes within the water column.

Temperature; turbulence; mixing; shallow estuary

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