River bed erosion and deposition responses to sediment reduction in the Chengtong reach of the Yangtze River

To examine the erosion and deposition response mechanisms to river basin sediment reduction in different areas of a tidal reach, this study investigates the Chengtong reach of the Yangtze River. The reach is first divided into two sections based on their hydrodynamic characteristics, namely the Jiangyin-Tianshenggang section and the Tianshenggang-Xuliujing section. Then, the differences between the two sections' erosion and deposition responses to river basin sediment reduction are compared by using and integrating hydrological and sedimentological data during 1950-2014 and topographic data during 2005-2014. The results demonstrate that the upstream Jiangyin-Tianshenggang section is sensitive to river basin sediment reduction and switches from deposition to erosion relatively quickly. The downstream Tianshenggang-Xuliujing section was affected by the reduced wet-season average runoff and the enhanced uplift action by tide during 1998-2004; thus, the erosion rate decreased. After 2004, the wet-season average runoff increased while the uplift action by tide weakened, and sediment from the river basin further reduced. These changes jointly led to an increase in the erosion rate. The critical wet-season average runoff, at which tidal dynamics no longer promote deposition and instead cause erosion in the Tianshenggang-Xuliujing section, is 36 000 m³/s. This is also the critical runoff threshold at which the insignificant uplift action by tide in the Jiangyin-Tianshenggang section becomes significant. At present, river basin sediment reduction has intensified the erosion of the entire Chengtong reach.