Sea level oscillations, hydrology and sedimentology of Mercury Bay

This study was undertaken to improve the understanding of the hydrodynamics and sediment transport patterns in Mercury Bay, Coromandel Peninsula. Abstract from Thesis: 1. Mercury Bay is an example of a shallow coastal embayment with a complex and variable hydrodynamic regime resulting from the interaction of its configuration with the tidal wave, oceanic influences, estuarine discharges, barotropic waves, storm surges, swell and locally generated seas. 2. Sea level oscillations are investigated by analysis of specific frequency bands of the tidal record using harmonic, spectral and numerical band pass filtering techniques. 3. Bay circulation is determined using mapped, three dimensional salinity, temperature and seston structures, while the magnitudes of tidal currents and mean circulation are determined from current recordings. 4. Sediment textural characteristics and distributions are used to demonstrate depositional environments and sediment transport directions in Mercury Bay. 5. FORTRAN IV computer programs are presented for: a) the harmonic analysis and prediction of tidal elevations (for 60 tidal constituents) together with the extraction and plotting of the residuals; and b) the calculation of the mean and daily mean sea levels, low frequency sea level variations (20-365 day periods) and higher frequency sea level fluctuations (1-20 day periods), using a range of band pass filters. 6. The central band of sea level oscillations (0.5 - 6 cycles per day) contains the semi-diurnal tidal wave that is dominated by the M2 tide of 0.72 m amplitude. 7. The low frequency band (0-0 c.p.d.) contains at its low frequency end (20-365 day periods): a) a non-secular rise in mean sea level between late 1972 and late 1974 of 0.6 m; and b) a secular annual oscillation coupled with oceanic changes of 0.07 m amplitude; c) barotropic global surges of 20-50 days duration and amplitudes of 0.10 m. The high frequency end (1-20 day periods) has: d) regional barotropic shelf waves of 0.07 m amplitude; and e) storm surges typically below 0.30 m amplitude and a maximum recorded height of 0.80 m. 8. The high frequency band (>6 c.p.d.) is dominated by free oscillations. The main oscillations are the first harmonics along and across the Bay with periods of about 35 and 20 minutes and the longitudinal first harmonic for Mercury Bay and the outer Bay with a period of 50 minutes. 9. A clockwise mean circulation pattern is present in Mercury Bay. This is driven by: a) a weak northward moving coastal current passing across the entrance of the Bay; b) tidal flows; and c) the reentrant ebb tidal momentum jet discharged with semi-diurnal frequency from the Whitianga Estuary. 10. Sediments of Mercury Bay are typically very well sorted, very fine sands. Textural parameters show that beach and Bay sediments are distinctive. Mud deposition occurs in areas shown by wave refraction analysis to wave shadow zones. 11. Evaluation of velocity data on factors contributing to water movement in Mercury Bay has revealed that for sediment movement to take place, energies of at least two major water movements have to reinforce one another. Optimum conditions for sediment transport and coastal erosion occur during storm periods when elevated water levels and increased wave activity result in greater sediment suspension which can then be moved by tides and storm-driven circulation.

Sea level data collected and analysed - harmonic, spectral and numerical filtering techniques. Subtidal sediment samples taken from throughout Mercury Bay and analysed for texture. Thesis includes FORTRAN IV computer programs for sea level analysis. Salinity, temperature and seston measurements. Tidal current measurements. Discussion of natural processes in Mercury Bay.

Mercury Bay

1972-1980

See above abstract and data content.

Thesis available in University of Waikato library No confidentiality.

Study completed.

Related information: Later studies in Mercury Bay by University recorded in this database (Cooper, Steeghs)