Longshore Sediment Transport and Morphological Changes Under Climate Change—A Comparison Between CMIP5- and CMIP6-Derived Forcings and the Use of Wave Climate Bias Correction

Abstract

Changes in wave climate can impact coastal zones by altering the sediment supplied to coastal compartments via longshore sediment transport (LST). Estimating these changes is challenging, and biases and uncertainty in wave climate projections contribute to uncertainty in LST and morphological change projections. This paper compares wave climate, LST projections, and morphological changes derived from two iterations of the Coupled Model Intercomparison Project (CMIP), and the implications of applying wave climate bias correction in these projections for the late 21st century under high emission scenarios. LST and morphological changes were simulated in a process-based model calibrated with data from a sand bypassing system. Bias correction improved representation of wave climate, including extremes, and reduced variance between climate models. Although bias correction did not change projected mean LST, it reduced the spread of model ensembles by 20% and 10% for CMIP5 and CMIP6, respectively. Both CMIP5 and CMIP6 suggest a future reduction of LST in the study area. However, CMIP6-derived projections show: (a) 50% less variance in wave forcing; (b) greater consistency between ensemble members; and (c) double the reduction in LST. This reduction is attributed to changes in the frequency, intensity and direction of modal and extreme waves. Morphological changes suggest steepening of the beach profiles in line with the historical record. This contribution highlights the value of a bias-corrected model ensemble and improvements in CMIP iterations in providing coherent projections of future wave climate change and its impacts on regional coastal processes.