An insight into headland sand bypassing and wave climate variability from shoreface bathymetric change at Byron Bay, New South Wales, Australia

The headland sand bypassing mechanisms in the Eastern Australian longshore sand transport system are investigated at Cape Byron, in response to wave climate variability. The mechanisms are interpreted from shoreface bathymetric change between surveys in 1883, 2002 and 2011 CE. They involve a split in the sand transport to follow a nearshore path along the inner bar and a cross-embayment path connecting the up-coast and down-coast outer bars. The relative magnitude of the net sand transported via the two pathways is controlled by a rotation in directional wave conditions. Two bypassing mechanisms were interpreted: (i) a predominantly cross embayment transport during unimodal east-southeast wave climate such as those interpreted for the period prior to 1883; and, (ii) a split transport between the inner nearshore and cross-embayment paths during a bimodal dominant south-south-easterly and sub-dominant east-north-easterly wave climate such as in the 2000s. The net sand transport bypassing Cape Byron was dominated by a connected outer bar system prior to 1883 and conversely, a stronger inner bar system during the 1960s to 2000s. This is manifest in the 10° rotation in seabed morphology and shoreline planforms. These changes are in accordance with decadal climate variability described by the Interdecadal Pacific Oscillation (IPO). The switching between headland sand bypassing mechanisms on interannual to decadal timescales determines the geometry of the bypass strand with the downcoast littoral zone and has important implications for understanding the shoreline rotation and the application of the headland-bay beach concept to predicting planform curvature in open compartments.