On the Generation and Evolution of Morphological Features on Idealised Macro-Tidal Embayed Sandy Beaches

The morphodynamics of macro-tidal embayed sandy beaches were studied by numerical experiments. The focus was placed on the generation and evolution of morphological features on schematised plane-sloping embayed beaches subjected to both large semi-diurnal tides and energetic wave conditions through a spring–neap tidal cycle. We relied on the advanced process-based XBeach model to simulate coupled hydrodynamics, sediment transport and morphological changes. Two differently sized rectangular beaches were considered. The model predicted on both beaches an extensive longshore morphological system made of a subtidal bar and an intertidal runnel. Both features were found to be controlled by the ratio of wave height to tidal range and spring–neap tidal variation. Boundary effects were revealed by the occurrence of small transverse sand deposits along headlands and the accretion and curvation of the longshore bar ends. The impact of bordering headland features on predicted morphodynamic evolutions appeared mainly associated with overall and specific embayment characteristics. Topographic headland rips were revealed to be of primary importance in regulating these morphodynamics. A suite of involved processes and underlying mechanisms was furthermore proposed. The wave obliquity was thus found to asymmetrise the entire bar–runnel system in both alongshore and cross-shore directions. The level of asymmetrisation appeared positively correlated to the level of embaymentisation.