Abstract
This study presents detailed observations of two-dimensional wave transformation processes over near-horizontal shore platforms. This research was undertaken to answer a fundamental question of shore platform hydrodynamics: How does planform morphology affect two-dimensional wave transformation processes over near-horizontal shore platforms? The impact of planform morphology on directional wave patterns and their subsequent influence on wave energy distribution was first assessed by deriving wave energy and directional wave patterns from a two-dimensional phased array of pressure transducers on a convex platform. Results show that swell and infragravity waves (SW, IG) are subject to refraction on the platform surface and reflection at the cliff toe. A dominance of oblique and reflected wave trains was observed over the inner platform, while shore-normal onshore waves represent only 36% of the SW energy and 22% of the IG energy. These observations underline the impact of the directional wave patterns controlled by planform morphology on wave energy variations over platform surfaces, which was investigated in further detail by comparing directional and energy patterns over three platforms of straight, concave and convex edges. The results show that wave refraction patterns over concave and convex platforms generate localised wave convergence and divergence over the platforms. Wave convergence results in a local increase in IG energy and a reduction in SW decay rate, while wave divergence exerts opposite effects on these frequency bands. Additionally, in the presence of reflection, patterns of crossshore standing IG patterns can vary alongshore depending on the platform geometry. Combined, wave refraction and reflection patterns affected the alongshore distribution of wave energy near platform edges, notably for IG for which alongshore energy variations were twice as large on the concave platform and three times larger on the convex platform than on the straight platform.
A numerical Boussinesq wave model was used to identify the effects of platform edge geometry on wave height distribution over near-horizontal platforms. A harmonic analysis permitted to identify platform edge curvature as a factor controlling the energy balance of wind waves (WW), SW, and IG across platforms and the subsequent cross-shore decay of significant wave height (𝐻𝑠), notably over convex platforms. For non-breaking waves, increasing convex curvature promotes the amplification of SW and WW (corresponding to 29% and 11% of the incident wave height, 𝐻0), particularly over the outer platform. For broken waves, high degrees of convex edge curvatures promote the amplification of SW and WW over the outer platform and inhibit the amplification of IG over the inner platform, while low degrees of curvature have the opposite effect. The contrasting behaviours between high and low convex curvatures depend on the balance between focusing from refraction and defocusing from wave breaking, which in the present case relates to a convex edge curvature threshold of 1.8. Employing a high-order spectral decomposition analysis, the present study identifies coherent wave amplification as a key mechanism at the origin of SW and IG stationary patterns over the inner platform. These stationary patterns resulted in alongshore variations in 𝐻𝑠 at the shoreline, marked by a decrease behind concave edges (up to 6% for non-breaking waves and 4% for broken waves) and an increase behind convex edges (up to 15% for non-breaking waves and 8% for broken waves).
The present study has shown that the exponential functions used to describe cross-shore wave attenuation in profile evolution models could over-predict (up to 50%) and under-predict (up to 40%) wave decay near concave and convex edges, respectively. Such misrepresentation can lead to errors in profile evolution predictions. Thus, an alternative expression of the cross-shore wave decay accounting for two-dimensional wave transformation effects is proposed