The role of seaward morphology on wave transformation onto and across a microtidal shore platform

Waves are a primary erosive agent on intertidal shore platforms. In microtidal environments, as waves cross a shore platform the energy transforms from gravity to infragravity frequencies. The morphology of the intertidal zone is a key boundary condition for this energy transformation. Waves approaching the shore however often interact with the seabed at some distance from the intertidal platform. In this study we explore this wave interaction over a gently sloping subtidal ramp (1 km wide) down to 11 m water depth fronting a semi-horizontal intertidal platform (180 m wide). Five non-directional pressure sensors were installed on the sea floor from 5 to 11 m depth for one week with a further four placed on the intertidal platform for two tidal cycles during the offshore deployment. It is found that a shift in wave frequency starts to occur at the base of the seaward ramp at 7 m water depth, with the primary development of infragravity waves occurring at less than 5 m depth. The offshore energy signal was also tidally modulated with infragravity frequency waves hypothesised to be able to escape seaward across the platform edge at high tide. This new exploration of nearshore wave energy shows that wave dynamics over the entire platform surface should be considered rather than just focussing on the intertidal zone. •Pressure sensors were deployed in a transect across a microtidal shore platform from the beach toe to wave base at > 10 m depth.•Infragravity waves are found across the intertidal and subtidal portions of the platform.•There is a strong tidal modulation of the wave energy across the entire platform.•Offshore IGW waves are influenced by energy reflection off the low tide cliff and by seaward propagating leaky-mode waves.