Spatial distribution and response of dunes to anthropogenic factors in the lower Yangtze River

Spatial distribution and morphological scales of dunes in the lower Yangtze River (YR) have not been investigated in detail because of the lack of large-scale high-resolution bathymetric data. However, they are critical for understanding bedload transport and flow resistance, thereby predicting river channel evolution. In this study, to determine the characteristics and spatial distribution of dunes, high-resolution bathymetry surveys were conducted in 2015–2016 in a 235-km channel of the lower YR using a multibeam echo sounder. Overall, dunes were found to be commonly developed, with bottom-bed velocity and bed shear stress ranging between 0.47 and 0.84 m·s⁻¹, and 2.11–4.86, respectively. Laterally, the bathymetric data from 22 cross-sections showed that dunes pervasively covered over 63.6% (even reaching 100% in some cross-sections) of the investigated areas. Longitudinally, bathymetric data along the deep-water channel showed that dunes developed over nearly 84% of the area. Moreover, a power function relationship between dune length and height was established: H=0.0979L^0.737 (R² = 0.84, n = 1,157). The maximum dune length and height were found to reach up to 441 and 10 m in the lower YR, respectively. The operation of dams in the upper and middle YR significantly decreased the sediment supply to the lower YR, leading to bed sediment coarsening, which promotes the development of dunes. In addition, deep-water channel regulation, sandbank regulation, and the enclosure of mudflats in the lower YR narrowed the channel width, resulting in an increase in flow strength, which could further enhance the development of larger dunes. Therefore, a re-evaluation of the impact of dunes on river channel evolution, as well as regulation projects for the lower YR are urgently needed.