Wave climate and sediment transport within Tauranga Harbour, in the vicinity of Pilot Bay

Abstract

Previous studies of hydrodynamics and circulation in Tauranga Harbour, a meso-tidal estuarine lagoon (168km² total area), have concentrated on tidally-induced processes. However, non-tidal processes, such as wind generated waves, are also responsible for sediment transport in estuaries. This study was initiated to characterise the wave climate and investigate the effect waves have on sediment transport and deposition within the southern basin of the harbour (112km²), particularly adjacent to the Port of Tauranga in the vicinity of Pilot Bay. A review of previous investigations of the hydrodynamics and sedimentology of Tauranga Harbour was undertaken, and predictions concerning tidal circulation patterns and sediment transport pathways were examined in light of more recent data. Two model-based studies were considered in detail: the physical model run by the Wallingford Hydraulic Research Station; and the numerical models used by the Tauranga Harbour Study (THS). Both methods simulated large-scale circulation patterns reasonably well, the numerical model results being more exact than those of the earlier physical model. However, discrepancies were evident when considering more local circulation patterns, such as in Pilot Bay. Tidal circulation patterns and sediment facies distributions for Pilot Bay were compared with detailed model predictions from the two previous studies. Both failed to correctly define the circulation present, although the THS numerical sediment transport model did agree with the facies distribution. However, the facies distribution is relict, resulting from flow conditions prior to port development. Wind and pressure parameters were measured to assess the role local weather conditions play in determining the wave climate within the harbour. The harbour weather patterns are dominated by a diurnal sea breeze cycle which results in a relatively high average speed of 4ms-¹ compared to inland locations. To define the wave climate, two main groups of waves were considered: high-frequency (short-period) waves, including wind-generated waves; and low-frequency (long-period) non- tidal waves, including seiches, storm surges and tsunamis. The wave climate within Tauranga Harbour is dominated by wave energy transmitted through the harbour entrance. This source accounts for ~70% of the energy in the average harbour spectrum; local wind-generated waves account for ~9%; and the rest represents low frequency waves, including tides. A relationship, based on the JONSWAP spectral form, was derived to predict the local wind-generated component of the wave field. The equation is more peaked than the normal JONSWAP form, reflecting the extreme limiting conditions found inside the harbour. Seiching occurs frequently, usually in association with local winds exceeding 9.5ms⁻¹, but also in response to external forcing by tsunamis and large swell waves. Analysis of the seiche frequencies indicates that seiches are not controlled by harbour channels and dredged shipping basins. The largest oscillations within the harbour are caused by tsunamis and storm surges. Normally high-frequency waves inside the harbour are too small to move significant quantities of sediment, except when breaking or in very shallow water. However, wave-induced suspension of sediment combined with tidal currents can produce large sediment fluxes, particularly over the extensive intertidal regions of the southern basin (64km²). This process is most significant for regions where tidal velocities are below sediment threshold (<0.35ms⁻¹). The annual sediment fluxes for shallow regions with low tidal velocities are comparable to those occurring in major tidal channels within the harbour. A renourished beach in Pilot Bay was monitored between 1983 and 1987. Based on measured volumetric changes the renourished beach is expected to last for between 7 and 16 years. The net sediment flux measured within the nearshore zone is in close agreement with the Tauranga Harbour Study numerical model predictions. Textural analyses of monthly sediment samples were used to predict sediment transport directions. The method employed produced directions consistent with the observed patterns of erosion and accretion.