Citation

Black K, Mead S. J. Coast. Res. 2001; (SI 29): 115-130.

Copyright

(Copyright © 2001, The Coastal Education and Research Foundation)

DOI

10.2307/25736210

PMID

unavailable

Abstract

The first offshore reef developed to amalgamate world-class surfing, public amenity and coastal protection is described. The reef is at Narrowneck Beach on the Gold Coast, Australia, a long, sandy coast experiencing large cyclone waves and high net longshore transport. The reef design was based on bathymetric and wave breaking studies of world-class surfing breaks and numerical model predictions. A broad range of different shapes was numerically tested using refraction, Boussinesq refraction/diffraction and sediment transport models. The shape that met the design criteria, for both the surfing and shoreline protection characteristics, was found to be a submerged double-sided headland. The designed reef is 400 m long and extends from the natural 2 m isobath to a toe in 10.4 m depth, with the crest at 0.4 m below the Lowest Astronomical Tide. The shape allowed both 'left' and 'right' surfing waves to be included. Peel angles are rarely less than 40° (in predominant small waves) and seabed gradients of up to 1:12 produce steep plunging wave faces, making the surfing wave 'fast' and 'hollow'. In small swell, a large, focussing underwater segment will more than double the deep water wave height at the breakpoint. A slower 100 m long segment at the shoreward end is designed for surfing begineers and casual tourists. Because the adjacent beaches experience close-out conditions in big swells, a padding channel has been placed at the offshore tip of the reef to give surfers access during large swell. A lagoon shoreward of the reef provides sheltered padding and, at low tide, sheltered swimming for beach-goers. Wave interference patterns inshore of the reef are expected to provide challenging 'wedgey' waves for body boarders, while the offshore placement of the reef provides ideal wave jumping conditions for the sailboarder.