Investigations of Offshore Wave Preconditioning

Abstract

Atkin, E.A.; Mead, S.T., and Phillips, D., 2019. Investigations of offshore wave preconditioning. 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. 78–90. Coconut Creek (Florida), ISSN 0749-0208.The way in which a wave breaks is largely a function of the path wave orthogonals and the bathymetry that wave orbitals encounter as the wave propagates shoreward. This effect is commonly referred to in surf science literature as preconditioning and occurs at a range of scales. This work considers seabed features that do not cause wave breaking but influence the way in which they break. An assessment first quantifies the dimensions of a number of known wave preconditioning features, and then involves iterative numerical model simulations of heuristic seabed features. The impact of an offshore feature is descried in terms of the wave propagation in the lee of the feature and breaking patterns, or surf zone plan-shape, near the shoreline. It is proposed that offshore features be considered as either disruptive or focus preconditioners. The former results in complex and chaotic interference patterns resulting in multiple peaks at the shoreline, with the latter creating a more singular or defined peak with less widespread disruption. A key component for surfing wave quality to be influenced by offshore preconditioning is the establishment of longshore wave height gradients, which are often associated with the bifurcation, or “snapping” of wave crests. The degree to which a feature sets up longshore wave height gradients and bifurcates wave crests is shown to be influenced by the size of the preconditioning feature relative to the incident wave conditions.