Morphology and evolution of submarine sand ridges and sand waves off the southwestern coast of Hainan Island, China

A substantial quantity of sedimentary sand bodies, including sand ridges, sand waves, and sand ripples, have been developed off the southwest coast of Hainan Island, China, with submarine sand ridges exhibiting the most considerable development scale. Determining the distribution, morphology, and evolutionary characteristics of the submarine sand ridges is crucial for exploiting marine sand resources and investigating engineering geological hazards. A comprehensive assessment of the study area was conducted utilizing marine survey techniques, including single-beam echo sounding, multi-beam echo sounding, side-scan sonar, single-channel seismic, and sediment sampling. The findings indicate the presence of eight substantial submarine sand ridges in the study area, delineated by tidal scour gullies and predominantly oriented in a NW-SE direction, with an average length of 21.3 km, an average width of 2.9 km, and an average height of 13.3 m. The sand ridges have a high-angle oblique bedding configuration. The grain size distribution of sediments in the study area is generally characterized by coarser near the shore and relatively finer away from the shore. The sediment type of the sand ridges is mainly sand-grade sediments (i.e., mean grain size Mz < 4Φ). A significant quantity of linear and barchan sand waves has developed within the sand ridge field, exhibiting variations in morphology, distribution, and scale. The coexistence and transformation of micro-geomorphic units, including sand waves, sand patches, and sand sheets, alongside sand ridges, reflect the complexity of the bedform-sediment-hydrodynamic system in the study area. The formation of the sand ridges may result from the late alteration of the ancient nearshore residual sand body. The study area revealed the presence of erosion-dominated, erosional-depositional, and deposition-dominated sand ridges, leading to the proposal of a development and evolution model for these sand ridges.