Incorporating high-quality surfing breaks into multi-purpose reefs
Mead, S. T. (2000). Incorporating high-quality surfing breaks into multi-purpose reefs (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13967
Permanent Research Commons link: https://hdl.handle.net/10289/13967
The studies presented in this thesis have focussed on surfing. Bathymetries, aerial photographs and wave vortex information at natural surfing breaks were measured and collected. These data were applied to understand the morphology that creates world-class surfing reefs, to predict wave-breaking intensity and to develop a dataset of surfing breaks (seabed bathymetries, peel angles and wave vortex shapes). In addition, the results were applied to the production of a coastal resource consent to construct a multipurpose reef at Mount Maunganui Beach in north-eastern New Zealand. Bathymetry surveys were carried out using a custom-built, portable, surveying system that was developed to enable surfing reef surveys at remote sites worldwide. Along with peel angle and wave vortex information, the bathymetric surveys of 28 surfing locations around the Pacific Rim and Indonesia were recorded and constitute the first international dataset of world-class surfing breaks. A full geomorphic range was measured including coral reefs, rocky reefs, headlands, rock ledges, river and estuarine deltas and sand beaches. Analysis of these bathymetries revealed meso-scale surfing reef geomorphic components that were classified as ramp, platform, wedge, ledge, focus, ridge and pinnacle, which constitute and account for the quality of world-class surfing reefs. The function of these components in relation to wave refraction and wave breaking is considered. There were repeating combinations of meso-scale surfing reef components and, with numerical modelling, it was found that the combinations of reef components occur in configurations that explain why these breaks are consistently high-quality. Components are sub-categorised by function into two basic groups, components that pre-condition waves and components that break waves. Components are arranged in functional order, with larger offshore components (ramp, focus and platform) aligning and shoaling waves prior to breaking on the smaller inshore components. Small wave breaking components (ridge and pinnacle) rest on the larger wave breaking components (wedge and ledge) and modify small sections of the wave. The study indicated that relative sizes and placement of the components determines the overall quality and length of the surfing break and that changes to either will reduce surfing wave quality. A surf break that very effectively combines the components is Bingin Reef in Bali, Indonesia. Refraction modelling of the waves at Bingin was in close agreement with field measurements and highlighted how the reef components behave as a unit to produce consistent, high-quality waves. One feature of Bingin was that it maintains a fast surfable peel angle (~35 °) over a range of wave heights and directions. A defined take-off zone was also persistent, resulting from wave-focusing over a large-scale reef component. Also very obvious in the model simulations, was the fast-breaking, steeper faced, part of the wave at Bingin, that is produced by a smaller scale reef component positioned on top of a larger feature. When the reef components that comprise Bingin were manipulated, and sometimes omitted, in most cases, the consequent changes to wave refraction produced waves that broke with less than world-class characteristics. The most common result was that waves broke too fast for surfing, or 'closed-out. In some cases this could be overcome by re-orientating components at angles greater than those that exist at Bingin. However, this resulted in greater changes to peel angles with changing wave height and directions than normally experienced at Bingin. Re-positioning or omitting smaller reef components had less effect on wave breaking, but these changes still down-graded the quality of the wave for surfing. Because the components combine and interact in a holistic way through wave refraction and pre-conditioning of the wave orientations, designs of artificial surfing reefs must apply these holistic principles in order to produce high-quality surfing facilities that optimise the characteristics of specific sites. The dataset of world-class surfing break bathymetries and the accompanying wave vortex profiles were used to develop a, method for predicting and describing the breaking intensity of plunging surfing waves. This method uses the orthogonal seabed gradient to predict the wave vortex height to width ratio, which was found to be the best indicator of wave breaking intensity. The subtle differences in the vortex shape of plunging waves on different seabed gradients can now be described much better than with simplistic indicators, such as the Irribarren number. Description of the shape of plunging waves, or the tube-shape, is critical for defining quality surfing waves. These quantitative predictions of tube shape will be incorporated into artificial surfing reef design. A multi-purpose, artificial, offshore reef was designed for construction at Mount Maunganui Beach, New Zealand. The proposed reef will form the basis for research into coastal protection, amenity enhancement (particularly surfing, but also diving, fishing and beach recreation), biological response and social and economic impacts. In order to proceed with reef construction, a 5-year resource permit is being sought from the regulatory authority, and this application required an assessment of the likely environmental impacts of the proposed reef. The studies undertaken for the Assessment of Environmental Assessment for resource consent included physical, biological, reef design and socio-economic impacts. A comprehensive design process was undertaken to incorporate the amenity of surfing into a submerged reef shape. Programs to monitor physical and biological responses, as well as social and economic impacts, were also established. These studies support the use of multi-purpose, artificial, offshore reefs as an environmentally-friendly solution to coastal protection. The reefs also cater to the growing demand for more coastal-amenity development.
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