Sediment deposition and erosion during the extreme flood events in the middle and lower reaches of the Yangtze River

Abstract

Floods often are accompanied by intensified soil erosion and thus high sediment supply from the catchment into the main channel of the river. This paper examined the sediment processes, especially deposition and erosion, during the two extreme flood years in 1954 and 1998, in the middle and low reaches of the Yangtze River. The total amount of sediment supplied to the middle and lower reaches from Yichang–Hankou and Hankou–Datong, was 1095 Mt in 1954 and 855.9 Mt in 1998, or 176% and 137% of the averaged 622 Mt/a from 1950 to 2006. A high deposition occurred in the floodplains, lakes and channels along the section Yichang–Hankou, both in 1954 and 1998. The total deposition reached 635 Mt and 499 Mt in 1954 and 1998 or 138% and 124% of the sediment load measured at the Datong station, respectively. However, different processes in the downstream section Hankou–Datong during the two flood events were found. Severe erosion (−140 Mt) happened in 1954, but only a slight erosion occurred (−22 Mt) in 1998. Thus, the floodplains and channels in the middle and lower reaches of the Yangtze River play an important role in regulating sediment discharge during extreme flood events. The heavy deposition in the middle reaches effectively reduced the amount of sediment discharged into the river mouth and thus acted as a major depocenter in the extreme flood year. The results also suggested that the Yichang–Hankou tended to shift from a depositional section to an erosional section, whereas the Hankou–Datong changed from an erosion-dominated section to a deposition-dominated section even before the Three Gorges Reservoir infilling in 2003. It is anticipated that this tendency would be accelerated and the historical role of sediment regulation by the two reaches could eventually be changed due to the reduced sediment supply from the Upper Yangtze.