Perspectives on wave transformation over a near-horizontal shore platform: Comparison of a two-dimensional and transect approach

Understanding wave transformations over intertidal shore platforms is necessary for managing rock coasts. Previous studies have investigated the effects of intertidal shore platform morphologies on wave transformation using instruments aligned in cross-shore and longshore arrays. This approach has provided a fundamental understanding of the influence of platform profile characteristics on wave transformation properties. However, it precludes the consideration of shore platform planform morphologies on two-dimensional wave transformation, which has been overlooked or only of secondary concern. We developed a two-dimensional approach to measure the wave direction of both swell waves (SW: 0.05 Hz < frequency < 0.125 Hz) and infragravity waves (IG: frequency < 0.05 Hz) during transformation across a near-horizontal shore platform (NHSP). Waves were measured using a grid of 14 pressure transducers (PT) recording at 2 Hz. Directional wave spectra were derived using the Extension of the Maximum Entropy Principle (EMEP) method. Both SW and IG were subject to strong directional changes over the NHSP induced by refraction on the lateral edges of the platform and reflection at the cliff toe. As a result, a dominance of oblique and reflected wave trains was observed over the inner platform while onshore waves propagating along the transect represented only 36% of the SW energy and 22% of the IG energy. These directional changes affected the wave energy distribution over the shore platform surface, generating complex energy patterns. A 77% decrease of refracted SW energy was recorded from the intertidal platform edges to the platform centre. Two dominant IG energy patterns were identified. The first was a long-shore increase of energy toward the central axis of the platform and a cross-shore increase of energy from the platform centre to the seaward and landward extremities. This pattern was seen for 81% of the submersion period. The second pattern also reported in previous research was an increase in IG from intertidal platform edges to the shoreline, but this was only observed for 15% of the submersion period. The two-dimensional approach developed in this paper allowed to provide field-based observations of the combined effects of wave reflection and refraction on wave energy distribution over a NHSP.