Morphology and mechanism of the very large dunes in the tidal reach of the Yangtze River, China

High-resolution multibeam data was used to interpret the surface morphology of very large dunes (VLDs) in the tidal reach of the Yangtze River, China. These VLDs can be divided into three categories according to their surface morphological characteristics. (1) VLDs-I: those with a smooth surface and cross-section; (2) VLDs-II: those accompanied by secondary dunes; (3) VLDs-III: those accompanied by secondary dunes and numerous elliptical pits. Parameters and spatial distribution of VLDs, and bed surface sediment were analyzed in the laboratory. Overall, channel morphology is an important factor affecting the development of VLDs, and channels with narrow and straight and certain water surface slope are facilitating the development of VLDs by constraining stream power. Meanwhile, distribution density of VLDs depicts a decreasing trend from Chizhou towards the estuary, are probably influenced by channel morphology and width. Associated pits in VLDs-III change the 3D dune morphology by distributing in secondary dunes as beads. The Three Gorges Dam project (TGP) leads to the bed surface sediment activity frequently and leads to the riverbed surface sediment coarsens, which promotes the further development of dunes. Moreover, other human activities, such as river regulation project, sand mining and Deep Water Channel Regulation Project have changed the regional river boundary conditions and hydrodynamic conditions are influential on the development of VLDs.