Coastal change along Noosa, Australia during a triple back-to-back La Nina (2020-2023).

The capacity of sandy beaches to provide coastal protection varies across time and space. Recent research suggests climate change could result in substantial erosion of most sandy beaches by the end of the century, with significant social and economic impacts. Furthermore, storm clusters and cyclone events generate extreme erosion of sandy beaches which are not fully recovered. Three back-to-back La Nina events occurred between 2020-2023. La Niña is associated with warmer waters in the western Pacific Ocean, which increase storminess off Australia's east coast. Chances of a higher number of tropical cyclones increase, as do the chances of cyclones travelling further south and more frequent passages of east coast lows. The aim of this study is to present preliminary coastal change observations along the Noosa coast, Queensland, Australia during this rare, but not unprecedented, period. Detailed (3 cm spatial resolution) and frequent (monthly) dronederived subaerial beach volumetric surveys were undertaken along six wave-dominated sandy beaches. The frequency of coastal storms considerably increased between an average 7.2 storms/yr during 2015-2020 and 12.6 storms/yr during the 2020-2023 triple La Nina period. Even though storms were generally less energetic during La Nina period, average duration was longer. Changes in subaerial sediment volume derived from precise drone surveys across the several studied sites showed a general signal of erosion, particularly after impactful storms such as the one in February 2022, where between 30-100m3/m of sediment were lost from the dune/beach system across our sites. A general trend of accretion is observed more recently but large variability across space and time is present. Most of this variability can be explained by the presence of dynamic coastal creeks, but other factors such as the presence of dune vegetation, beachrock or nearshore megacusps warrant future investigation. Frequent, high-quality 4D spatial information (3D plus time) across the coastal zone, particularly including the upper shoreface, is required to monitor, manage and predict coastal changes and associated hazards. Building an understanding of coastal response to both gradual and extreme events is especially critical in an era of progressively rising sea levels which are likely to exacerbate already existing trends. [ABSTRACT FROM AUTHOR]