Hydrodynamics of Manukau Harbour, New Zealand

A calibrated two-dimensional finite-element model, which handles flooding/drying of intertidal areas, complemented by field data, has produced information on tidal propagation in Manukau Harbour, New Zealand with reasonably good accuracy. Excluding the upper-harbour inlets, c. 50% of both the amplification and phase lag in the dominant M-2 tidal constituent occurs through the deep 10-km-long entrance channel to Puponga Point. The calculation of tidal residuals has isolated flood-directed and ebb-directed residual circulation respectively on either side of Puponga Point, which is a typical pattern for alternating flows around headlands. The model has established the dominance of tidal-driven over wind-driven circulation in most of the harbour except in the upper intertidal areas. Wind-driven circulation in the inner Harbour is characterised by downwind flows over intertidal sand banks with pressure-driven return flows (vertically averaged) in the deeper main channels. Tidal dissipation rates are sufficiently high to inhibit the onset of any summer stratification.