Potential Impacts of Wharf Extensions on the Hydrodynamics of Stella Passage and Upstream Regions of Tauranga Harbour, New Zealand

摘要

The Port of Tauranga have proposed extending the berthage of both the Sulphur Point and Maunganui wharves southward to provide a combined length of 1,303 m. The dredged channel (Stella Passage) will also be extended south into Town Reach, with the dredged sediment used to reclaim 4.7 ha behind the wharf extensions and along the Sulphur Point shoreline. In this thesis a hydrodynamic model of the southern basin was developed and used to predict the potential impacts of the wharf extensions on the hydrodynamics in Stella Passage and the upper harbour. Numerical modelling of the southern basin was undertaken with Deltares Delft3D FLOW modelling software, using a 2D model with a 20 x 20 m rectangular grid. Sensitivity analysis identified bottom roughness and bathymetry as having the largest influence on model outcomes. Successful calibration and verification of the southern basin model was carried out using field data collected from instruments deployed in Stella Passage and the upper harbour. Statistical analysis of the modelled water levels showed ‘excellent’ agreement with the field data. The modelled current velocities did not match quite as well, but the results were sufficiently good to provide confidence in the model predictions.The modelled hydrodynamics in Stella Passage were similar to those predicted by previous studies. Compression of the tidal volume and acceleration over the steep boundary between the two areas meant current speeds within the shallower Town Reach were significantly higher than those in the dredged Stella Passage. A clockwise eddy of residual velocities indicated increased sediment transport on the ebb dominant western side of Town Reach. No previous models of the southern basin have modelled the hydrodynamics in the upper harbour beyond the Railway Bridge. Within the upper harbour the largest effects on the existing hydrodynamics were caused by the bridge causeways and the size and shape of the basins. Residual velocity eddies were created around the causeways from velocity gradients caused by shadow zones on the lee sides of the causeways. As the residual velocities and net sediment transport rates were low, the upper harbour was deemed to be in dynamic equilibrium.Modelled existing hydrodynamics within Stella Passage, Town Reach and the upper harbour were compared to three modelling scenarios simulating the 2015 2016 capital dredging, and proposed wharf extensions, dredging and reclamation. The modelled harbour developments had no significant impact on the hydrodynamics in the upper harbour; changes to water levels and current speeds were less than 0.025 m and 0.05 m.s-1, which were smaller than model errors and the impacts of weather events. The largest impacts were localised within Stella Passage and Town Reach close to the proposed developments. The 2015-2016 dredging reinforced the existing patterns in residual velocity and potential sediment transport pathways. Differences in current speeds between models indicated that the largest impacts on the hydrodynamics within Stella Passage and Town Reach were from the extension of the dredged channel into Town Reach rather than the wharf extensions and reclamation. Current speeds decreased significantly within the newly dredged channel, but this effect was compensated for to a degree by the restrictions of the channel width when the wharves were constructed. In western Town Reach, current speeds increased due to the drop-off moving south and the asymmetrical shape of the dredging extension channelling the tidal flow. The potential for sediment transport and erosion increased in western Town Reach, however the actual sediment transport may be reduced following the formation of a shell lag facies which are common areas of high flow within the harbour.