Analysis of dynamic geomorphology at the east entrance of Qiongzhou Strait based on tidal current numerical simulation

The tidal pattern of the east entrance of Qiongzhou Strait that separating Hainan Island from Chinese mainland is irregular diurnal tide, and characterized by east-west reciprocating flow, in which strong tidal current is the main dynamic factor of shaping the seabed morohology. The study of sediment transport characteristics under tidal current is crucial for better understanding the fluxes of water and sediment, sedimentary processes and geomorphological evolution in Qiongzhou Strait. Survey data of surface sediments collected in 2008 and 2015 from 249 stations show that the sediments at the east entrance of the strait were mainly sandy and gravelly, forming the bed load that transported on seabed. To study the sediment transport and seabed morphology, the tidal current process, sediment transport rate and erosion and sedimentation intensity of the east entrance of Qiongzhou Strait were dynamically modeled. The results show that the maximum eastward flow is between 0.9 and 2.0 m/s, and the maximum westward flow is between 0.8 and 1.7 m/s, and the tidal flow in the middle and neap tides is roughly 50% to 60% of that in the spring tides. In general, the eastward flow is stronger than the westward flow, but in the north and the middle part of Qiongzhou Strait, the westward flow is always stronger than the eastward. During spring tides, the frequency of sediments movement in the study area is generally high, with an average of 60%, and the average transport rate is 10⁻² kg/(m·s). During the middle and neap tides, the average frequency is only about 18%, and the average transport rate is 1-2 orders of magnitude smaller than that during the spring tides. The sediment transport direction is mainly eastward. We found that there is a higher accumulation rate near shoals and a higher erosion rate, reaching 10⁻¹ m/a.