Grain Size Characteristics and Transport Trends of Nearshore Sediments off Eastern Hainan Island, China

The area from Tonggu Cape to Qinglan Bay off eastern Hainan Island, China, is a bedrock headland coast with a river-mouth lagoon and hosts typical habitats such as coral reefs. Qinglan Bay is located on the path of sediment transportation along the north continent of the South China Sea, the crucial area of clastic sediment diffusion in the continent and Hainan Island; therefore, investigating the sedimentary dynamic environment in the nearshore area off Qinglan Bay is particularly important. Grain size analysis was performed on 321 seafloor sediment samples collected from the waters off Qinglan Bay. The seafloor sediments in the study area can be classified into eight types, dominated by muddy sand, silty sand, and sand. In this study, the Gao–Collins method was applied to the grain size data to perform grain size transport analysis. Combined with submarine topography, the headland effect of Tonggu Cape, the sheltering effect of coral reefs, and the combined transport and shaping effects of tidal currents, waves, the coastal current, and human activities, we infer the possible sediment source compositions of the northern and central-southern sedimentary environment zones. In the northern zone, eroded material from Tonggu Cape and open-sea sediments are mainly controlled by seabed characteristics composed of coral reefs and rocks during deposition. In the central-southern zone, coastal erosion material and open-sea sediments are mainly controlled by the mixing of terrestrial and open-sea sediments, driven primarily by currents, waves, and tidal currents. These findings improve the understanding of modern sedimentary processes in the study area and provide basic scientific data for coral reef conservation, coastal protection, and port and navigation support.