Controls of local geology and cross-shore/longshore processes on embayed beach shoreline variability

Shoreline variability along the 3.6-km long Narrabeen Beach embayment in SE Australia is investigated over a 5-year period. We apply the one-line shoreline change model LX-Shore, which couples longshore and cross-shore processes and can handle complex shoreline planforms, non-erodible emerged headlands and submerged rocky features. The model skilfully reproduces the three dominant modes of shoreline variability, which are by decreasing order of variance: cross-shore migration, rotation, and a third mode possibly related to breathing. Model results confirm previous observations that longshore processes primarily contribute to the rotation and third modes on the timescales of months to seasons, while cross-shore processes control the shoreline migration on shorter timescales from hours (storms) to months. Additional simulations carried out in simplifying progressively the bathymetry show how the inherent geology strongly modulates the spatial modes of shoreline variability. The offshore central rocky outcrop is found to limit the rotation. In contrast, the submerged rocky platforms that extend from the headlands enhance the shoreline rotation mode and increase alongshore variability of the cross-shore migration mode, owing to increased alongshore variability in wave exposure. Offshore wave transformation across large-scale submerged rocky features and headland shape are therefore critical to contemporary shoreline dynamics.