Observations of shoreline-sandbar coupling on an embayed beach

- Bar-shore coupling is related to the reduced separation between the bar and shore.
- The degree of rotation is a strong driver of this coupling on embayed beaches.
- The coupling length scale is controlled by this rotation.

We analyse a seven-year dataset (1999-2005) of shoreline and sandbar variations derived from video observations at the embayed Tairua Beach, New Zealand, to explore sandbar-shoreline coupling and to determine how this coupling is related to alongshore-averaged sandbar-shoreline separation and beach rotation. Our results quantify the coupling between the sandbar and the shoreline which is directly related to the reduced separation between the sandbar and the shoreline. Using a simple predictive shoreline model, we show that this behaviour is related to the energy of the incoming waves. Our observations are obtained from a headland-enclosed beach and show aspects of the morphological state that relate to the existence of headlands that are not considered in the Wright and Short beach state classification. Both the sandbar and shoreline at Tairua Beach rotate relatively quickly (< 1 month) during the winter storm events, when the storms are accompanied by strong alongshore components, gradually returning during the summer months. The sandbar-shoreline coupling length scale is controlled by this rotation, with large wavelengths occurring when the sandbar is strongly rotated.