Coupled flow-wave numerical model in assessing the impact of dredging on the morphology of Matakana Banks

The short- and long-term impacts of dredging on the Matakana Banks ebb-tidal delta was 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 an extensive field programme to measure hydrodynamic processes and sediment characteristics, and high resolution bathymetric surveys. The impacts of dredging was 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; and (3) future scenarios using 2013 bathymetry with alternative offshore disposal locations. The field data and hydrodynamic model results showed that the ebb-tidal delta can be separated into two main regions that are dominated by tidal processes or wave processes. Overall, the wave influence becomes more dominant as the distance from the entrance channel (main ebb jet) increases, and vice versa for tidal processes. Sediment transport pathways were inferred using spatial patterns of erosion and accretion from bathymetric surveys and cross-sectional profiles along the shoreline. The pathways reflected the interactions between the identified wave and tide processes. A month-long time series of wave conditions were used to force a wave model coupled with the hydrodynamic model for the 2013 bathymetry to simulate the present day situation. The results showed that the sediment volume of the ebb-tidal delta fluctuated 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. Morfac of 60 was applied for 12 days simulation time and the results will be presented in this pape