Surf-Swash Interactions on a Low-Tide Terraced Beach

Through an integrated approach, this paper investigates the role of coupled surf-swash dynamics on outgoing waves using data collected at a low-tide terraced beach, Grand Popo, Benin (Gulf of Guinea, West Africa). Observed reflection is 8 %. Analyses are conducted from deep water directional wave spectra measurements, daily beach surveys and remote video measurements. Our results show that the swash can be a non-negligible component of the nearshore energy balance (14% of total dissipation) and is closely tied to reflection. Reflection thus depends on waves at swash inception (offshore waves and surf zone saturation), and shoreface slope varying with tide and morphological evolution. An outgoing cut-off frequency (shortest reflected waves) can be linked to swash saturation with a strong dependence on shoreface slope. A phase-resolving Boussinesq model was validated and used to investigate the influence of terrace width, upper shoreface slope and tidal elevation over the terrace. This papers puts forward the role of the coupled system surf-swash and underlines potential key interactions between a rapid shoreface evolution and surf zone hydro-morphodynamics.