Wave Group Effects on Breaker Height on a Uniform Slope

SHAND, T.D., PEIRSON, W.L. AND COX, R.J., 2007. Wave group effects on breaker height on a uniform slope. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 767 – 772. Gold Coast, Australia, ISSN 0749.0208 The maximum wave height that can be achieved during shoaling is of key importance in the design of coastal structures and the assessment of near shore currents and water levels. Previous investigations, upon which current design guidelines are based, have tended to focus on regular, monochromatic waves thereby neglecting group effects. Studies incorporating random wave testing include these group effects but may require prolonged testing periods to achieve the design condition. The shoaling transformations of wave groups as they progress into shallow water and break has been investigated in laboratory wave flume experiments. The water surface elevations were recorded by capacitance type wave probes. The type and position of breaking was recorded and video imaging used to observe wave kinematics during shoaling and breaking. Results show that the interaction of horizontal (intra-group) and vertical (depth-induced) energy fluxes plays a key role in the shoaling and breaking processes of the wave group. Notably, the evolutional stage of the wave group as it approaches a critical threshold depth facilitates the emergence of two distinct shoaling cases – an early shoaling case and a delayed shoaling case. These distinctive shoaling cases appear to cause significant variation in the wave breaking properties (breaker type and breaking position) and in the maximum depth limited wave heights (Hb/d) observed. In particular, the delayed shoaling case appeared to stabilise the wave groups, delaying breaking and yielding Hb/d ratios in excess of current design guidelines. These preliminary results indicate that current design guidelines based on monochromatic waves may underestimate the ‘worst case’ scenarios associated with wave groupiness with consequent implications for engineering design, coastal modelling and hazard mapping and planning. ADDITIONAL INDEX WORDS: Wave group, depth-limited wave breaking, shoaling, wave kinematics