Distribution characteristics and migration trend of surface sediments in Riyue Bay, Hainan, South China

The Riyue Bay in Hainan, South China, is a typical headland arc-shaped bay with various types of coasts and the grain size of the substrate is relatively coarse. By analyzing the sea current and the particle size of the surface sediments obtained in 2011, the surface sediments distribution in the Riyue Bay was clarified, the incipient motion and transport fluxes of the surface sediments calculated, and the net sediment transport trend discussed. Results show that the surface sediments in the study area were relatively coarse, and were mainly composed of gravel-bearing sand, followed by gravel sand and sand, and sandy gravel and silty sand were sporadically distributed. The sediment grain sizes ranged from the clay the finest to the gravel the coarsest; fine particles distributed mainly in the east and west coastal waters of the study area, while coarse particles distributed in NS-direction stripes in the central and southern areas. In addition, the sorting was poor in the central area and good in the surroundings. The incipient velocity of sediment increases from northeast to southwest in the nearshore area, while in the offshore, it firstly decreases and then increases from northeast to southwest. The influence of particle size on the incipient velocity of sediment is greater than that of water depth. The simple tidal current action in the sea area near the Fenjiezhou Island can only cause some particles to resuspension. The incipient motion was between 22 and 39 cm/s. The average daily net transport flux of surface sediments was between 0.04 and 3.7 kg m⁻¹ d⁻¹. The direction of net sediment transport was generally along the direction of SW. The average daily net transport flux in the northeastern sea area was significantly higher than that in the southwestern sea area. The single width sediment transport rate during the rising tide period is slightly higher than that during the falling tide period (excluding station R4), and the sediment transport direction during the rising and falling tide periods is basically the same.