|The Matakana Banks ebb-tidal delta is located offshore from the Tauranga Entrance to Tauranga Harbour, Bay of Plenty New Zealand. In order to access the Port of Tauranga, Matakana Banks has been dredged to provide a shipping channel with sufficient depth for navigational purposes. In 1968 the first capital-dredging programme included an Entrance Channel through the delta, which was further enlarged in 1992. A biennial maintenance dredging programme has been required to deal with subsequent infilling of the Channel.
Recently, the Port has obtained resource consent to further widen and deepen the Entrance Channel. However, while granting the consent, the Environment Court accepted that dredging could result in adverse impacts to the stability of Matakana Banks and the adjacent Panepane Point on the western side of the tidal inlet. Therefore, this study was undertaken to assess the response of the ebb tidal delta to dredging, and determine possible mitigation measures if dredging has adverse effects.
The study analysed bathymetric data from single beam echo sounder (SBES) surveys obtained from 1998 to 2011. These data indicate that the main body of the ebb tidal delta is stable and undergoes little change. However, the swash bars located on the swash platform are very mobile, and probably account for the changes reported by the earlier assessments of changes between bathymetric surveys. In order to better track swash bars and assess variations in ebb-tidal delta morphology, multibeam echo sounder (MBES) surveys have been instituted.
Comparison of morphological changes with the dredging volume and the wave climate during 1998 to 2011 indicated that the storm events may trigger erosion of the ebb-tidal delta, particularly after a maintenance dredging campaign. However, the ebb-tidal delta volume recovered quickly between dredging campaigns, suggesting that the sediment was redistibuted within the system and not permanently removed.
To provide insights into the processes affecting the ebb-tidal delta, and provide the necessary data for the calibration and verification of numerical models, and a major 27-day field programme was undertaken. Sediment traps were used to assess sedimentaion rates, and provide sediment samples for characterising the
sediment grain size and bed roughness distribution for numerical modelling. Concentric arcs of wave and current recorders were used to measure hydrodynamic processes within and around the ebb-tidal delta and tidal inlet, and along the Matakana Island shoreface.
The field and historical bathymetric data showed that the ebb-tidal delta can be divided into 3 sub-regions according to the dominant hydrodynamic regime; (1) close to the Entrance Channel and ebb-jet where tidal currents dominate; (2) the central area of the swash platform, where the influence of tidal currents is still present but waves also play important role; and (3) the margins of the swash platform where waves are dominant and tidal influence is minimal. Overall the wave influence becomes more dominant as the distance from the Entrance Channel (main ebb jet) increases, and vice versa for tidal processes.
The short- and long-term impacts of dredging on the Matakana Banks ebb-tidal delta were investigated by numerical modelling using Delft3D. The model covered dredging locations inside Tauranga Harbour and the offshore areas around the Matakana Banks ebb-tidal delta, and was calibrated by the field measurement data. A month-long time series of wave conditions were used to force a wave model coupled with a hydrodynamic model for the 2013 bathymetry to simulate the present day situation. The modelling results showed that the sediment volume of the ebb-tidal delta fluctuates with tidal range; accretion occurred during neap tides; and erosion during spring tides.
To assess the long term impact, the morphological factor (morfac) tool in Delft3D was used. A morfac of 60 was applied to 12-days simulations to predict 2-year morphological changes, corresponding to the approximate time interval between maintenance dredging campaigns. The impacts of dredging were then investigated by modelling three different conditions: (1) before dredging started in 1968, using 1967 bathymetry; (2) the present situation using 2013 bathymetry with existing dredging and dumping activities; and (3) future scenarios using the 2013 bathymetry with alternative offshore disposal locations. Conditions of average waves (no storm) and with storm waves were also simulated.
Before dredging commenced, the ebb-tidal delta had a continuous terminal lobe from the north to southeast with a minimum depth of 5 m. Sediment transport modelling indicated that bar by-passing transported sediment past the inlet via mobile bedforms on the terminal lobe, and suggested that the ebb-tidal delta was getting shallower and broader.
Simulations incorporating dredging revealed that the tidal currents in the channels of the ebb-tidal delta became more asymmetric (stronger ebb-current), and the ebb-tidal delta became bifurcated and more complex in its morphology. The system changed to inlet bypassing, although only small quantities of sediment appear to be transported past the tidal inlet (most sediment recirculates within the tidal inlet system). Overall dredging does not appear to have affected the stability of the Matakana Banks ebb-tidal delta, but there may have been an increase in the morphologic variability as bedforms circulate over the swash platform, and additional bands of sand bars have formed on the seaward margin of the swash platform.
Modelling of alternative spoil disposal sites indicated that shallow nearshore spoil disposal sites were more rapidly dispersed than the offshore spoil disposal site in 20 m depth, particularly during storm events. However, the volumetric differences between the models were small (less than 0.2%). The smallest volumetric changes compared to an initial ebb-tidal delta volume are associated with spoil disposal located northeast of the ebb-tidal delta (DaD New), and the most effective site for contributing sediment to mitigate erosion of Matakana Island is on the shoreface to the west of the ebb-tidal delta.