Robust and reliable models are needed to understand how coastlines will evolve over the coming decades, driven by both natural variability and climate change. This study evaluated how accurately five popular ‘reduced-complexity’ models replicate multi-decadal shoreline change at Narrabeen-Collaroy Beach, a sandy embayment in Sydney, Australia. Measured shoreline positions derived from approximately monthly field surveys were used for 20-year calibration and 20-year validation periods. The models performed similarly on average but with large variability between transects. The set-up of several models was modified to compensate for their sensitivity to imperfect input wave data, and further site-specific improvements were identified. Capturing interannual to decadal-scale variability in cross-shore and longshore dynamics at this site was challenging for all five models. Models appeared to aggregate key processes at this timescale into parameter values rather than representing them directly. This suggests time-varying parameters or changes to model structure may be necessary for decadal-scale simulations.

The development of dunes is characterized by the alternating effects of erosion during storm events and growth during milder conditions. The quantification of dune growth due to aeolian processes has received some attention but uncertainty remains on where the dune sand, which accommodates growing dunes, originates from in the coastal profile. In this paper we hypothesize that sediment eroded from the upper beach is the main sediment supply for aeolian sediment transport governing dune growth. To test this hypothesis we have analyzed morphological profiles collected monthly at three different field sites (Noordwijk, Vlugtenburg and Narrabeen) during several years. No significant erosive trend due to aeolian processes was found at the upper beach at this temporal resolution. We conclude that it is highly unlikely that the main supply for aeolian sediment transport governing dune growth was located at the upper beach. The intertidal zone might be a relevant alternative source of sediment in the cross shore profile.