Longshore sediment transport variability changes by the end of the 21st century.

Projections of wave climate for the Gold Coast, Queensland suggest a reduction in wave height combined with an anticlockwise shift in wave direction which may lead to an overall local reduction in longshore sediment transport (LST). Understanding changes in LST variability into the future is crucial to better manage the coastline and prepare for future adaptation pathways. The aim of this paper is to analyse the variability of LST at the end of the 21st century and the implications it may have on coastal management. To do so, a benchmark model was established to assess the LST along the entire coast by using a Delft3D model, where boundary conditions are derived from a global wave hindcast forced with the Climate Forecast System Reanalysis (CFSR). The Delft3D model was calibrated using the pumping rates of an artificial sand bypassing system at the northern Gold Coast for three representative years encompassing: low (2004), average (2001) and high (2003) LST rates. Then a hybrid wave downscaling technique was applied to obtain the wave data at the breaking point at three locations along the coast where the Coastal Engineering Research Center (CERC) equation was calibrated based on the benchmark model. Finally, future wave climate scenarios (Contributions to the Coordinated Ocean Wave Climate Project, COWCliP) derived from CMIP5 (seven GCMs) and CMIP6 (two GCMs) were bias-corrected and used to generate ensembles to analyse future projected changes in the variability of LST along the Gold Coast by comparing historical and future projection model runs. The changes in interannual variability projections are inconclusive with the CMIP5 ensemble indicating a decrease in LST interannual variability while the CMIP6-derived models (ensemble with two members) indicate an increase. To allow for a fair comparison, more models to compose the CMIP6 ensemble are required. As for the seasonal variability, CMIP5-derived models suggest a decrease in LST between February and May and virtually no changes for the rest of the year, decreasing the seasonal variability in LST. Changes in LST and its variability may have flow-on effects for coastal management with changes to the creek dredging programs, potentially reduced input to the by- and back-passing systems on the coast and the dispersion of beach nourishment campaigns. [ABSTRACT FROM AUTHOR]