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Oceanographic and morphodynamic monitoring of a multi-purpose reef at Mount Maunganui, New Zealand

The innovative concept of an offshore submerged multi-purpose reef combining a coastal protection function with recreational benefits including improved surfing, diving, or fishing conditions provides an appealing solution for the management of beaches. However, since the technology is relatively recent, there is still a lack of empirical knowledge about its performance in the field. The present research monitored the effect of the prototype research reef constructed at Mount Maunganui in New Zealand on the local beach morphodynamics and oceanographic conditions, being primarily concerned with implications on the coastal protection function of the technology. The beach morphodynamic response to the reef was investigated from a set of already available high resolution bathymetric surveys imaging the foreshore and surfzone morphology prior to, and throughout reef construction, and a new postconstruction survey collected as a part of this research. The reef implementation was found to disturb the pre-existing beach state functioning including the onshore/offshore migration of the underlying long shore bar, rather than cause a persistent salient response. A possible additional beach width of ~ 20 m, extending ~ 150 m alongshore was identified in the lee of the reef from shoreline analysis, but the pattern was transient throughout the monitoring period since it was superimposed on comparatively large pre-existing fluctuations. In addition, the reef structure provided a control point on the offshore morphology able to trap sediment updrift and erode sediment downdrift. Both field measurements and numerical modelling of waves and currents were used to monitor the effect of the reef on the oceanographic conditions. Wave propagation over the reef without breaking (H less than 0.5 m) resulted in transmitted heights larger than incident by up to a factor of 2. When the reef triggered breaking, transmitted heights were reduced by up to ~40 %. A concurrent process was the shift of the wave energy spectra towards higher frequencies landward of the reef that resulted in reduced transmitted wave periods. Wave modelling showed that the wave shadowing was associated with significant wave rotation around the reef that induced two zones of divergent wave angles near the shoreline in the lee of the reef. The hydrodynamic response to the wave energy dissipation was the development of stronger onshore directed currents landward of the reef. Hydrodynamic modelling indicated that the reef-induced flow forced a cellular circulation in the lee side under shore normal waves, and an onshore deviation of the ambient (unidirectional) long-shore currents under oblique waves. The research provided a rare empirical test to the theoretical design concepts, and potential supplements or refinements. To obtain the required beach protection, the reef impact on the beach morphodynamic coupling including wave transformation, nearshore hydrodynamics, and small/large scale beach state response needs to be carefully assessed.
Type of thesis
Weppe, S. B. (2010). Oceanographic and morphodynamic monitoring of a multi-purpose reef at Mount Maunganui, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/4344
The University of Waikato
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