Investigating the Link Between Three-dimensional Wave Transformation Patterns and Shore Platform Morphology.

Wave propagation over shore platforms has mainly been observed along single cross-shore transects. A better understanding of multidirectional wave propagation is highly valuable near rocky shores as it will help to assess wave impact on cliff erosion, transport of cliff debris and sediment bypass. The present research aims to link three-dimensional wave transformation patterns to rocky shore platform morphological features using instrument array designs and analytical methods new to rocky shore hydrodynamic research. The focus is on the behaviour of gravity (swell: 0.06-0.125Hz) and infra-gravity waves (<0.05Hz). Directional wave analyses carried on type B shore platforms are used to compare the effects of concave, convex and flat platform edges on these two wave frequency bands. The methodology was first developed on a concave platform in Waianakarua (New Zealand). Data were collected for a month using an array of 14 pressure sensors and two directional wave recorders anchored to the platform, the largest field deployment yet accomplished on a rocky shore platform. Offshore incident wave conditions were also recorded using an ADCP in 10 m of water 850 m from the study site. The geometry of the array allowed to produced wave directional spectra which were used to investigate directional shift between incident waves and waves propagating on the platform. Non-directional spectra analysis was used to investigate the energy level of gravity waves and infra-gravity waves over the entire platform. This approach was recently applied to shore platforms presenting flat and convex edges during one of the largest storms observed in Mahia (New Zealand) over the last decade. A comparison of results from these three sites shows that gravity wave energy distribution is strongly affected by the shape of the platform edge due to refraction. In contrast, infra-gravity wave energy increases onshore and is relatively constant alongshore with strong reflection observed near the cliff toe.