Why does Tauranga Harbour exist?

Abstract

Within the Bay of Plenty, Tauranga and Ohiwa Harbours are unusual in that their hypsometry indicates that they are less mature estuaries than the rest of the estuaries in the region, which is not consistent with the assumption that the Tauranga Harbour is being impacted by increased sedimentation due to changing land use over the last 200 years. The evolution of estuary hypsometry is a function of the estuarine hydrodynamics (currents and waves), sediment supply, and vertical land movements. Focussing on Tauranga Harbour, recent studies have shown most of the harbour, particularly the lower intertidal and sub-tidal areas, has low to very low sedimentation rates (<1 mm/y). Observations and previous studies also indicate that waves combined with tidal currents are effective at flushing fine sediment from unconfined areas of the harbour. The predominant supply of coarser sediment to the harbour, beyond the flood tidal deltas at the harbour entrances, is erosion of volcaniclastic materials around the shoreline, and there appears to be minimal transport of sand derived within the harbour to the open coast. There is no unequivocal evidence of an increase in sedimentation in response to modern land use changes. Shallow seismic reflection survey data and limited exposures in coastal cliffs indicate that potentially active faults are present within the harbour. Differential rates of sea level rise within the harbour, and archaeological evidence, suggest that vertical land movements are on going and contribute to increasing accommodation space within the Harbour. Therefore, Tauranga Harbour exists as a mesotidal lagoon due to a restricted sediment supply, a hydrodynamic regime that exports fine sediments from the harbour, and subsidence that increases the sediment storage capacity of the main basins. These factors have implications for coastal hazards and the management of Tauranga Harbour.