Impact of water-sediment diversion and afflux on erosion-deposition in the Luoshan-Hankou reach, middle Yangtze River, China

It is not yet fully understood how water-sediment diversion and afflux along a mainstream reach of a river affect erosion-deposition in downstream reaches. This study focuses on the Luoshan-Hankou mainstream reach of the middle Yangtze River, China. The Luoshan-Hankou reach is vitally important for flood control, being located downstream of three diversion mouths and an afflux outlet along the Jingjiang reach. We establish empirical formulae for sediment transport rates at boundary cross-sections, and hence estimate the amount and proportion of erosion-deposition and its relative increase (termed erosion-deposition promotion) in the Luoshan-Hankou reach. We then propose critical net water supplies from Dongting Lake to Luoshan-Hankou reach based on maxima and equilibria of erosion-deposition and its promotion. It is found that net water supply partly drives erosion-deposition in the Luoshan-Hankou reach where maximal proportions of deposition and deposition-promotion may be approximated by 0.01c(-37.67) and 0.01c(-37.67) + c-1, in which c is a dimensionless parameter representing the erosion-deposition condition in Luoshan-Hankou reach for no water-sediment exchange. At Zhicheng hydrological station, the critical ratio of net water supply to overall water discharge is 0.418c(-33.33)-1, and critical net water supply ratios for equilibria of erosion-deposition and its promotion are 1 (or c(-33.33)-1) and 0 (or (0.06 + 3.257c(54.61))(-1)-1). A chart based on net water supply and c is devised representing four types of erosion-deposition and its promotion for the Luoshan-Hankou reach. Historical data over the past 65 years demonstrate that erosion-deposition and its promotion in the reach are respectively governed by c and net water supply; there is a remarkable shift from alternate erosion-deposition to monotonic erosion whilst the erosion-deposition effect remains consistent. The foregoing are in agreement with observed data, and comparable with data for the Jingjiang reach (affected by the three water-sediment diversion mouths). Satisfactory flood-control conditions in the convergence zone between the Yangtze mainstream and Dongting Lake accompanied by increasing erosion in the Luoshan-Hankou reach are predicted for the future.