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.
