Dynamic variation of sand spit in the Dafeng River estuary, Beibu Gulf

The rapid change process of sand spit is directly related to the estuarine geomorphological migration with potential impacts on water and sediment exchange between land and sea. The movement and the driving mechanism of sand spit landform have received especial concerns. However, limited information is related to sand spit morphodynamics due to difficulties in obtaining monitoring data. Based on multi-satellite remote sensing images from 1990 to 2023, surface sediments collected in November 2022, and regional hydrological data, the evolution of the sand spit in Dafeng River estuary in Beibu Gulf, Guangxi, SW China over a medium-strong tidal environment were analyzed. Results show that ① the sand spit in the estuary exhibited continuous extension to the northwest, and the width and area of the spit featured seasonal variation with increased width in flood season and decreases in dry season; ② The main sediment source of the sand spit was from riverine material in flood season. The average grain size of sediments on top the spit seaside was the coarsest and then gradually became finer from sea to land, and to the middle and tail parts of the spit. The sediments were transported mainly from the top of the sediments seaside towards the middle and tail of the spit, so that the spit continued to extend northwestward; ③ Runoff from sand is the source of sediment in the spit, waves are the main factor controlling the long-term morphological evolution of the sand spit in the estuary. Tidal actions in mild-strong tidal environment are the driver to the sediment transport shoreward and the continuous northwestward extension of the sand spit. The significant difference in seasonal runoff was the main factor on the variations in width and area of the sand spit: greater in flood season and smaller in dry season.