Study on effective discharge in the Lower Jingjiang River before and after construction of the Three Gorges Dam

Effective discharge is the flow rate corresponding to the maximum suspended sediment transport within a certain period of time in a natural river, which can reflect the medium to short term bed-forming effect. Flow frequency and features of suspended sediment transport of different particle size groups before and after the construction of the TGD are analyzed, based on the prototype data from 1991 to 2016 in the Jianli hydrological station of the mean monthly flow rate and suspended sediment discharge and the suspended sediment gradation. The magnitude, return period and duration of the effective discharge corresponding to the suspended sediment grouped transport in the Lower Jingjiang River are investigated via theoretical analysis and magnitude-frequency approach. The results showed that, as influenced by factors such as coming flow and sediment, sediment transport capacity and bed material replenishment, the effective discharge decreases with the increase of particle size. The deviation of effective discharge between various particle groups is increased after the construction of the TGD. The effective discharge corresponding to the particle group with 0.062 mm < d < 0.25 mm increases, and decreases for the other particle groups and for all suspended load after the construction of the TGD. The return period of the effective discharge corresponding to the particle group with d<0.125 mm increases, while decreases with d>0.125 mm. After the construction of the TGD, the cumulative discharge and cumulative duration of the cumulative transport of 50% sediment decreases with the increase of the particle size. Because of the severe unsaturation of fine sediment and relatively large transport capacity of coarse sediment, roughening of the suspended sediment gradation is resulted in. 50% of the cumulative sediment transport requires 43%—82% cumulative discharge and 62%—90% cumulative duration. Studying the changes in effective discharge before and after the construction of the Three Gorges Dam (TGD) is of significant importance to analyze the riverbed evolution in the Lower Jingjiang River under erosion conditions.