Modelling and evaluation of beach morphodynamic behavior: A case study of Dongsha Beach in eastern China

Beaches provide natural protection for coast and ecosystems. Storm surges and waves make significant impacts on the topographic changes of beaches, subsequently causing crises of coastal erosion and management. Beach protection and nourishment projects is growing considerably along the world. Five profiles of Dongsha Beach located in the southeast of Zhujiajian Island, eastern China, were measured and used to analyze the cross-shore profile changes, longshore sediment transport, beach rotation and equilibrium shoreline and profile. Delft3D Flow/Wave and XBeach models were established and calibrated to investigate the morphological responses of Dongsha Beach. It was found that: (1) the longshore sediment transport dominated the changes of erosion and accretion sediment volume compared to the cross-shore sediment transport; (2) the shoreline of Dongsha Beach was not in equilibrium condition compared to the profile equilibrium conditions; (3) the longshore sediment transportation was the main reason of the erosion because the shoreline changes caused by longshore sediment transport during the storm condition could not be restored during calm sea periods; (4) the correlation between wave induced currents and Hs2·T was good and the dominating wave direction was 90°. Finally, sediment nourishment projects named N1∼N3 were assumed and simulated to identify the optimum for the dynamic balance of Dongsha beach. Sediment sources with D50 of 0.5 mm should be chosen in sediment nourishment projects, and the nourished area should be selected in N1 or N3 of the beach. The numerical model established in this study could be a useful tool to predict the future erosion and accretion of beaches. This study can provide a useful understanding and reference especially for the embayed beach nourishment, making the beach management decisions more scientific and reasonable.