Hydrodynamic Model: Whitianga Estuary and Buffalo Bay

Extracted from thesis Abstract (Reeve, 2008): Whitianga Estuary is a bayhead barrier type estuary located on the east of the Coromandel Peninsula, North Island, New Zealand. The catchment has undergone many land-use changes since pre-European settlement. In some areas of the catchment land-use has changed from native forest to grasslands to forestry and back to pasture. These changes in catchment land-use all contribute to increased sedimentation into the estuary. Development of the estuary itself has also occurred in recent times. Much growth has been focussed around the estuary due to Whitianga town-ship having a large boating community, and includes a 170 berth marina and wharf situated at the tidal inlet entrance to the estuary. These, and the extensive canal development engineering works can have substantial impacts on the sedimentation regime, and may modify tidal circulation, flushing, and sediment deposition within the estuary. LiDAR, singlebeam data, and recent rectified aerial photographs were interpreted for the creation of a bathymetric grid file to be used for hydrodynamic modelling of the estuary. The 3DD numerical model was used to determine tidal flows and current velocities. From the hydrodynamic model a particle-tracking model was created to determine sediment transport pathways within the estuary. From the initial 20 m model it was then possible to create a number of nested model grids for the purpose of determining the best practice scenario for the creation of a proposed boat ramp and associated approach channel near the harbour entrance. Hydrodynamic results suggest that residual circulation in Whitianga Estuary is nearly in balance, with a low ebb tidal domination present. Particle tracking results suggest that sediment entrained and carried into and within the estuary will accumulate on the intertidal flats. Sediment transport modelling indicates that the impact of a proposed boat-ramp will result in sedimentation of the dredged approach channel due to reductions in residual and tidal velocities. Purpose: A bathymetric grid and hydrodynamic model (3DD) was created to investigate a range of physical processes, including tidal circulation, sediment transport and coastal hazards. Grid was created as part of MSc research, and has been used for further research since.

Dataset includes a 20 m model grid and hydrodynamic modelling over the Whitianga estuary and Buffalo Bay. Particle (sediment) modelling also undertaken using POL3DD. Nested models at smaller grid size created in the area of the boat ramp and marina at 10 m scale.

Whitianga Estuary and Buffalo Bay

2007-2008

Bathymetry for model grid collected with Multibeam, single beam depth sounder, LiDAR data and RTK GPS Survey. Boundary conditions for model came from (coastal) S4 current meter, river boundary conditions from EW records.

Gaps in collection: n/a Data quality: Measured S4 and FSI current meter water velocities and elevations were used in the model calibration and to determine appropriate bed roughness values. Calibration of eddy viscosity was undertaken by changing the 3DD model viscosity value until the ebb tidal jet predicted the eddy formations that occur as the tidal jet exits the entrance into Buffalo Bay. Results from the model showed good results for both spring and neap elevations and velocities within the estuary. A full discussion is given in Chapter 6 of Reeve (2008). Attribute accuracy: Discussed in Reeve (2008).

Some data available in printed and digital form with thesis.

Master of Science thesis and subsequent research.

Data available through University Sensitivity/Confidentiality: N/a

Research using and improving model is ongoing.