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Predicted hydrodynamic and sediment transport impacts of breakwater construction in Tauranga Harbour, New Zealand

Abstract
The study predicted the impact of a proposed breakwater development, along the northern boundary of the Tauranga Bridge Marina, on existing hydrodynamics and sediment transport. Numerical modelling was undertaken using the DHI MIKE 21 modelling suite. A 25 m grid resolution regional hydrodynamic model of Tauranga Harbour was established to provide boundary conditions for a 4 m grid resolution local hydrodynamic model of the Stella Passage, Town Reach, and Waipu Bay region. Calibration and verification was achieved by comparing model predictions with measurements from tidal gauges and field deployed ADV instruments. A wave model was set-up to provide predictions of wave-induced sediment transport. A sediment transport model was developed to identify sediment transport pathways and areas of erosion and accretion. A pile and panel breakwater was recommended based on predictions of reduced current velocity within the marina and a limited increase along the Sulphur Point wharf. Flow diversion and channel constriction contributed to increased maximum velocities of 10% near the Stella Passage drop-off, increased peak tide velocities of up to 0.3 m.s⁻¹ west of the breakwater, and flood jet development off the western tip of the breakwater. Increased accretion north of the drop-off was predicted in response to increased annual spring transport rates in Town Reach from <50 m³/yr/m up to 100 m³/yr/m. Increased erosion through the western side of Town Reach may result in coarser surficial sediment and a westward extension of shell lag. In the western Stella Passage, peak ebb velocities increased by 0.2 – 0.5 m.s⁻¹ and annual spring transport rates increased from <5 m³/yr/m to >10 m³/yr/m. Peak tide velocities within the marina were reduced by 0.2 – 0.5 m.s⁻¹ in the north and up to 0.2 m.s⁻¹ in the south and annual spring transport rates decreased from up to 50 m³/yr/m to predominantly <5 m³/yr/m. Two dredging scenarios proposed by the Port of Tauranga were also simulated. In dredging scenario one, the Stella Passage was deepened to 16.0 m below Chart Datum. Variation in flow patterns were predicted within the Stella Passage and flood velocity increased through the western side. In dredging scenario two, the dredged area and Sulphur Point wharf were extended southward into Town Reach. In the western Stella Passage, maximum flood velocity increased by 0.2 m.s⁻¹ and peak ebb velocity increased by up to 0.3 m.s⁻¹. Peak flood velocity decreased by up to 0.3 m.s⁻¹ north of the marina. Breakwater addition to the dredging scenario one simulation produced similar results compared with the combined breakwater and existing bathymetry simulation. The breakwater and dredging extension in scenario two, both independently acted to focus flow through west side of Town Reach. Increased erosion was predicted toward the drop-off into the dredging extension. Dredging reduced the influence of the breakwater through the western Stella Passage. The breakwater increased maximum ebb velocity by 29% for the existing bathymetry, 25% for dredging scenario one, and 20% for dredging scenario two. Existing sediment transport patterns in Waipu Bay were unaltered by breakwater development or the combined breakwater development and dredging and wharf extensions. An area of high seabed elevation, in western Waipu Bay, was the preferred location for an artificial bird roost. The existing bathymetry was altered to simulate different dredge island dimensions. The recommended design was oval shaped with an east-west orientation. This design displayed consistently low annual spring transport rates of <0.1 m³/yr/m.
Type
Thesis
Type of thesis
Series
Citation
McKenzie, J. S. (2014). Predicted hydrodynamic and sediment transport impacts of breakwater construction in Tauranga Harbour, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/9292
Date
2014
Publisher
University of Waikato
Rights
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