Morphodynamics of single-barred embayed beaches : Shoreline and barline morphodynamics at the scale of the embayment

Seven years of shore and sandbar positions extracted from hourly time-averaged images collected at Tairua Beach (New Zealand) are used to study the morphodynamics of the shoreline and the nearshore parallel sandbar of single-barred embayed beaches. First, a semi-empirical model is proposed, that validates the concept of sandbar rotation and relates it to the wave energy gradient along the embayment. Then, a statistical study confirms the role of the alongshore wave energy flux in the rotation of embayed beach shorelines. Finally, principal component analyses and semi-empirical models are used together to characterize the dominant dynamic patterns of the shoreline and sandbar at the scale of the embayment. Dominant, simultaneous cross-shore migrations relate to beach state transitions and to variations in beach planform curvature (breathing). Rotations are largely asynchronous, confirming that different drivers are likely to be involved in shoreline and sandbar rotations.