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ABSTRACT Few studies of wave processes on shore platforms have addressed the hydrodynamic thresholds that control wave transformation and energy dissipation, especially under storm conditions. We present results of a field experiment conducted during a storm on a sub-horizontal shore platform on the east coast of Auckland, New Zealand. Small (<0.5 m) locally generated waves typically occur at the field site, whereas during the experiment the offshore wave height reached 2.3 m. Our results illustrate the important control that platform morphology has on wave characteristics. At the seaward edge of the platform a scarp abruptly descends beneath low tide level. Wave height immediately seaward of the platform was controlled by the incident conditions, but near the cliff toe wave height on the platform was independent of incident conditions. Results show that a depth threshold at the seaward platform edge > 2.5 times the gravity wave height (0.05-0.33 Hz) is necessary for waves to propagate onto the platform without breaking. On the platform surface the wave height is a direct function of water depth, with limiting maximum wave height to water depth ratios of 0.55 and 0.78 at the centre of the platform and cliff toe, respectively. A relative 'platform edge submergence' (water depth/water height ratio) threshold of 1.1 is identified, below which infragravity (<0.05 Hz) wave energy dominates the platform energy spectra, and above which gravity waves are dominant. Infragravity wave height transformation across the platform is governed by the relative platform edge submergence. Finally, the paper describes the first observations of wave setup on a shore platform. During the peak of the storm, wave setup on the platform at low tide (0.21 m) is consistent with measurements from planar sandy beaches, but at higher tidal stages the ratio between incident wave height and maximum setup was lower than expected. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Waves are an important process responsible for the initiation and subsequent development of intertidal shore platforms. However, few field studies to date have described wave processes on shore platforms. A field experiment was conducted using an across-shore array of wave gauges, including a directional sensor, on a near-horizontal platform at Mahia Peninsula, North Island, New Zealand. Results show that the platform is very efficient in filtering wave energy at gravity wave frequencies (>0.05 Hz), with 40-80% reduction in wave height measured across the 140 m wide platform. By contrast, infragravity waves are present on the platform and increase in magnitude towards the cliff toe. Directional wave analysis indicates that wave reflection is mainly restricted to infragravity wave frequencies, demonstrating the differences in gravity and infragravity wave behaviour on the platform studied. Results indicate that, under fair weather conditions, the role of waves as an agent of cliff toe erosion is likely to be limited at present, although they are probably important for removing sediments accumulated at the cliff toe. The observed increase in infragravity wave energy towards the cliff toe implies that these long-period waves may be important geomorphic agents on shore platforms. [ABSTRACT FROM AUTHOR]

Abstract: Waves are generally considered to be an important control on shore platform evolution but there have been very few detailed studies of wave dynamics on rocky coasts. This study provides a detailed description of wave transformation processes across a shore platform near Gisborne, New Zealand. A field experiment was conducted on an intertidal, 250m-wide platform that is characterised by a sharp seaward edge. Mean platform elevation is 0.7m below mean sea-level (MSL) and platform gradient is 0.3°. Four nearly equally spaced pressure gauges were deployed over a 24-h period during fair-weather swell conditions characterised by a mean wave height of 0.4–0.5m and wave period of 7s. Wave energy at the seaward edge of the platform was dominated by incident swell waves with peak frequency at 0.09Hz. At all tidal stages waves broke at the seaward edge of the platform, but changing tidal stage exerted a strong control on the rate of wave energy attenuation across the platform. The relative importance of waves at wind-wave frequencies increased in the centre of the platform as the tidal stage and water level increased, whereas infragravity waves became dominant toward the cliff toe. Three different hydrodynamic zones were distinguished across the shore platform; (i) a breaker zone close to the seaward edge; (ii) a propagation/shoaling zone in the central region of the platform; and (iii) a zone of energy dissipation across the inner part of the platform. The spatial dimensions of the three zones varied with changing tidal stage, enabling different wave processes to operate across different parts of the platform at different times. The experiment showed that wave-height attenuation is strongly controlled by tidal stage, ranging between 93% attenuation of wave height during the initial flooding stage to 44% attenuation at high tide. Wave height was found to be depth-limited, with the ratios of H max and H m0 to water depth calculated as 0.7 and 0.4, respectively. The findings imply that platform geometry has an important influence on wave processes on shore platforms. The geomorphic implications of gravity and infragravity waves are discussed. [Copyright &y& Elsevier]