Sand waves in the shelf area off Xiamen Bay

Based on the multi-beam and single-channel seismic data, sediment grain size distribution data and current data collected from the period of 2014–2017, seafloor sand waves are discovered on the shelf off Xiamen Bay. This paper deals with the morphological features, distribution patterns and sediment composition of the sand waves. Hydrodynamic conditions and their bearing on sand waves are discussed. The results show that the sand waves are mainly developed in the offshore area of 10～60 m in water depth with a slope of 0～1 degree, where the sea bottom is relatively gentle and open. Crests of sand waves show a parallel pattern extending in a NW-SE direction. The axis of the sand wave ridge is SW-NE. Most of the sand waves are 2～12 m high and 0.12～0.8 km long with large sand wave index (＞30). In the seismic profiles, there are three types of waves i.e. symmetric sand waves, asymmetric sand waves and superimposed sand waves. The symmetrical sand waves are characterized by larger wave height and smaller wave index, while the asymmetrical sand wave by longer wave length and larger sand wave index. The stable sand wave is a kind of superimposed sand wave reformed and reactivated by the flow in later stage. The offshore shelf of Xiamen Bay and its adjacent waters are rich in sand. The bottom sediments are dominated by silt and fine-medium sand, controlled by hydrological dynamics of the area. Affected by the Zhejiang-Fujian coastal current, the surface current of the South China Sea and the branch of Kuroshio current, the current system of the study area is strong and complicated. With regard to the tidal system, the study area is dominated by irregular semi-diurnal tides with a velocity of 0.3～0.7m/s. Tidal currents play an important role in the development and transformation of sand waves.This paper clarified the distribution pattern, and sediment characteristics of the seafloor sand wave, and provided useful foundation for submarine engineering construction in the study area.