The effects of planform morphology on two-dimensional wave transformation over near-horizontal shore platforms

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Shore platforms fronting cliffs have highly irregular planform configurations that could modify the alongshore distribution of wave energy, thereby influencing cliff erosion. However, research aiming to understand the geomorphological control of platform morphology on two-dimensional wave transformation remains scarce. We conducted detailed wave measurements within the intertidal zone of three commonly observed mesoscale planform geometries (straight, concave and convex seaward edges). Waves were measured using a phased array of pressure transducers near the seaward edge of three near-horizontal shore platforms in New Zealand. Waves were analysed at infragravity (IG: 0.002–0.05 Hz) and swell frequencies (SW: 0.05–0.125 Hz), with wave energy derived from the zeroth moment wave height, directional wave spectra derived from the Extension of the Maximum Entropy Method and phase lag derived from the cross-spectrum characteristics. Results showed that wave translation occurred on the straight edge platform, divergence on the concave platform, and convergence on the convex platform. These patterns generated areas of wave ray convergence and divergence over the platforms. Wave energy convergence coincided with increased IG energy and reduced SW energy decay, whereas divergence had the opposite effects. Wave reflection generated cross-shore standing IG waves, for which nodal states varied alongshore with platform width. These phenomena resulted in alongshore variations in normalised wave energy Ê over the three platforms, which were more pronounced for IG than for SW. Compared to the straight platform, alongshore variations of IG were twice as large on the concave platform and three times larger on the convex platform. This study shows that planform morphology impacts along-platform wave energy distribution. These effects likely influence planform rock coast evolution and should be considered in future numerical modelling studies.