Morphological processes of two artificial submerged shore-parallel sandbars for beach nourishment in a nearshore zone

Artificial submerged sandbar is regarded as an assistant structure for beach nourishment with two beneficial functions of protecting beach and supplying sand source. Due to local representative physiognomy of subaqueous sandbars and the aim of longer resident time of beach nourishment, two artificial submerged sandbars parallel to the shore, are implemented at Tiger-Rock Beach, Qinhuangdao, China. It is essential to investigate and identify the systematic morphological implications of artificial submerged shore-parallel sandbars to beach nourishment practice. In this study, a three-level nested flexible triangular unstructured mesh system is set up for a three-dimensional sediment transport and morphological model. The sediment transport and morphological model, driven by coupled tidal currents and waves, is well validated by field measurements. The validated model is used to evaluate the responses of sediment transport and morphological evolution to beach nourishment with two submerged shore-parallel sandbars. The most significant effect of the artificial submerged sandbars is inducing wave breaking (even secondary wave breaking) and forming shadow zones. The artificial submerged sandbars increase westward sediment transport at Public Bathing Place and Specialist Bathing Place, and eastward sediment transport at Langtaoyu Bathing Place. Sediment from sandbars can be transported in three pathways: i) onshore by a wave-induced current due to breaking wave at the crest; ii) longshore due to a longshore current; and iii) seaward by a reversed asymmetrical current. Overall, the sandbars protect beach through forming wave shadow area in their rears with lower wave height and the sediment transport capacity, and supply sediment toward the shore by sacrificing themselves, i.e., sandbars are eroded and have a low-crest asymmetrical shape. The findings in this study contribute to the understanding of morphodynamics of artificial submerged sandbars and can provide a reference for beach nourishment to preserve their eco-friendly nature.