STEEP DUNES NEAR DUNE ASYMMETRY TRANSITIONAL AREAS IN THE BEIBU GULF, NORTHWEST SOUTH CHINA SEA: MORPHOLOGY AND MAINTENANCE

Symmetrical dunes can be found in areas where dune asymmetry changes sense, but in areas where dunes are not expected to be moving the development and maintenance of symmetrical dunes are poorly understood. In the southeastern Beibu Gulf, we have identified five typical dune asymmetry transitions at the depths of shallower than 50 m basing on the latest bathymetric data. Those regions are separated into two classes according to their different shapes and locations. Sand transport directions are deduced by dune migrations between surveys, simulated regional currents, and sand size trends. The two classes of the regions are found to experience opposing net sand transports from the north and south. Around the asymmetry transitions, the dunes have impressive steep sections, relatively symmetrical shapes, and almost straight crests. They have a general Height-Length (H-L) relation: H=0.0575 L^1.0836 (R²=0.834), which is steeper than dunes in other seas. Some of the dunes have extraordinary large heights exceeding a quarter of water depth, which is attributed to the preserved sand body, bidirectional sand accretion, erosion in troughs, and absence of significant erosion on crests. The steepness and straight crests of dunes adapt to the nearly symmetrical or weak asymmetrical reversing currents, which induce flow separations and reversals on both flanks of the dunes. It also suggests that dunes develop differently under sufficient and limited sand supply. Dunes will gradually grow in size when being continually supplied with sand. However, if sand supply is restricted, dunes may separate or turn into more isolated individuals to sustain the H-L relation. The H-L relation of dunes can be maintained during either growth or separation. Overall, the nearly symmetrical or weak asymmetrical reversing currents maintain the steep shapes of dunes while variations in sand supply affect their sizes. To work out the detailed processes of sediment transport in the dune asymmetry transition area on the shallow shelf, we also encourage more observations of flow structures and related models of such two-sided steep dunes.