Saturation recovery coefficient of nonuniform suspended sediment in the Jingjiang reach of the Middle Yangtze River

Sediment discharge in the Jingjiang reach has sharply reduced since the Three Gorges Project operation, and the channel has been scoured by the clear water in order to recover the sediment transport capacity. However, the recovery rate of nonuniform suspended sediment was different with the different bed material compositions along the Middle Yangtze River. Therefore, the equation of settling distance of nonuniform sediment is derived, based on the theories of Markov stochastic process and sediment motion. Then, according to the definition of saturation recovery coefficient (α) based on the diffusion theory of suspended sediment, the formula of grouped saturation recovery coefficient (α_i) is modified using the equation of settling distance; However, it ignores the effect of bed material composition (BMC) on suspended sediment recovery. For the actual process of sediment transport in the Jingjiang reach, a formula of α_i is then proposed with the effect of the BMC being considered. These results show that: ① The value range of α_i in this study was 0.12—0.27 at Shashi and Jianli when the influence of the BMC was not taken into account, but it became to the range of 0.000 3—0.171 8 and 0.003 5—0.157 9 respectively when considering the BMC effect; ② Among α_i of different sediment groups, the relationship of α_if for fine sediment > α_im for medium sediment > α_ic for coarse sediment existed when the BMC effect was not considered, but the relationship of α_if < α_im < α_ic existed when considering the influence of BMC; and the recovery rate of suspended sediment at Shashi was higher than that at Jianli except for the recession period; ③ The variations of α_i at Shashi and Jianli were sensitive to different groups during different periods, in which α_i during the flood period was higher than the value during other periods. Through comparation and analysis, the proposed formula in this study can predict the value of α_i for the Middle Yangtze River well, and it also providing a basis for further research on the recovery mechanism of suspended sediment in the reach downstream of the Three Gorges Dam.