Variations in the effective and bankfull discharge for suspended sediment transport due to dam construction

A varied class method is applied to calculate the effective discharges and their variations after the Three Gorges Dam (TGD) construction based on the mean daily flow discharge and suspended sediment concentration field data from 1981 to 2016. For comparison, the bankfull discharges are also determined according to the cross-section profiles and flow discharge-stage relations. Our results show that a bimodal effective discharge curve usually exists at the fixed sites, which generates two effective discharges (Qe1 and Qe2) within the moderate flow range. Under the quasi-equilibrium circumstances of the pre-dam period, effective discharges are closely related to the mean annual runoff, with a narrow range of regional variations in occurrence frequency. Our analyses draw the conclusion that the relatively higher unsaturation degrees of the pre-dam effective discharges caused by dam interception and riverbed coarsening are the primary cause of the increase in effective discharges from Yichang to Shashi, while the more frequent low and medium discharges due to flow regulation drive the decrease in effective discharges from Jianli to Datong. The slightly elevated flood levels and descending bankfull levels collaboratively result in the decrease of bankfull discharges from Yichang to Shashi, while the lowered bed elevation causes the increase in bankfull discharges from Luoshan to Datong. Overall, the bankfull discharge in the Middle and Lower Yangtze River is larger than effective discharge and approaches the 1.5- year recurrence interval discharge.