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

      Weppe, Simon Bastien
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      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
      Permanent Research Commons link: https://hdl.handle.net/10289/4344
      Abstract
      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.
      Date
      2010
      Type
      Thesis
      Degree Name
      Master of Science (MSc)
      Supervisors
      Healy, Terry R.
      Publisher
      The University of Waikato
      Rights
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