Wave transformation on a sub-horizontal shore platform, Tatapouri, North Island, New Zealand

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, 250 m-wide platform that is characterised by a sharp seaward edge. Mean platform elevation is 0.7 m 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 0.4–0.5 m and wave period of 7 s. Wave energy at the seaward edge of the platform was dominated by incident swell waves with peak frequency at 0.09 Hz. 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 Hmax and Hm0 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.

► The first detailed measurements of wave processes on a rock platform are presented.
► Three different hydrodynamic zones are distinguished across the shore platform.
► Wave height on the study platform is depth-limited.
► The ratio of significant wave height to water depth observed on the platform is 0.4.
► The inner platform surface is dominated by energy at infragravity wave frequencies.