The Influence of Sand Bar Morphology on Surfing Amenity at New Zealand Beach Breaks

• Published in: Journal of coastal research 2019-12, Vol.87 (sp1), p.44-54
• Main Authors: Bryan, Karin R., Davies-Campbell, Jai, Hume, Terry M., Gallop, Shari L.
• Format: Article
• Language: English
• Subjects: beach geomorphology; climate patterns; rip current channels; Sand banks
• ISSN: 0749-0208; 1551-5036
• DOI: 10.2112/SI87-005.1

Bryan, K.R..; Davies-Campbell, J.; Hume, T.M., and Gallop, S.L., 2019. The influence of sand bar morphology on surfing amenity at New Zealand beach breaks. In: Bryan, K.R. and Atkin, E.A. (eds.), Surf Break Management in Aotearoa New Zealand. Journal of Coastal Research, Special Issue No. 87, pp. 44-54. Coconut Creek (Florida), ISSN 0749-0208. Wave breaking patterns on many of New Zealand's prominent surfing beaches are controlled by surf zone sand bar morphology. In turn, sand bars change in response to wave breaking and surf zone current patterns, with well-known theories predicting more linear bars during winter, and more three dimensional patterns during lower energy conditions. Here four databases of sand bar morphology from New Zealand, collected at Aramoana, Piha, Lyall Bay, and Tairua, are analysed to detect changes in key parameters that control wave surfability: the length and orientation of the bar. Longer bars provide the potential for longer surfing rides, whereas the orientation determines the peel angle or the difficulty of the ride. Computer algorithms were used to detect sandbars from light intensity maxima patterns obtained from averaged, geo-rectified video imagery collected at each beach. The length of bar was inversely correlated with orientation, with longer beaches also having longer bars. Sand bar length and orientation was highly variable, both spatially (with location along the beach) and temporally (seasonal and interannual variability), making it difficult to detect any significant changes between location or with time. However, sand bars were generally shorter and more obliquely-oriented near the ends of these headland-enclosed beaches. Although no significant seasonal variations were detected, there were detectable interannual variations on the two beaches with the longest datasets, which were correlated with the Southern Oscillation Index. Implications of the study are that, unless changes are substantial such as caused by groins and seawalls, long monitoring datasets are needed to detect anthropogenic impacts on surf breaks.