Effective discharge variability for suspended sediment transport in the middle Yangtze River

Effective discharge is the flow rate corresponding to the maximum suspended sediment transport within a certain period in a natural river, which can reflect the medium- to short-term bed-forming effect. The middle Yangtze River has been in a long-term unsaturated state since 2003 (the impoundment of the Three Gorges Dam (TGD)) due to altered water and sediment regimes. Based on the prototype data from 1991 to 2016 on the sandy bed (from Shashi to Hankou), the magnitude and duration of the effective discharge (Qe) before and after impoundment of the Three Gorges Dam (TGD) corresponding to the suspended sediment grouped transport are investigated via a magnitude-frequency approach, and then, its geomorphic effectiveness is discussed. The results indicate the following: (1) The magnitude of the effective discharge corresponding to particle-size group (> 0.062 mm) increases, but the frequency of it decreases after the TGD. (2) The Qe magnitude is 1.3–1.4 times and 1.6–2.0 times of the annual average discharge before and after the TGD. The effective discharge (Qe) is smaller than the bankfull discharges (Qb), while the values of Qb/Qe become smaller after TGD and gradually decrease from Shashi to Hankou. (3) The geomorphic effectiveness at each hydrological station after the TGD is generally less than that before the TGD. The effective discharge corresponding to particle-size group (0.125–0.25 mm) has the largest geomorphic effectiveness after the TGD. It is also found that the geomorphic effectiveness of Qe corresponding to the particle-size group (0.062–0.125 mm) increases along the distance from Shashi to Hankou, but that to the group (0.125–0.25 mm) remains basically unchanged after the TGD. It highlighted the increase of effective discharge after the TGD essentially reflects the decrease of sediment carrying capacity, which is mainly affected by the coarse particle-size groups (> 0.125 mm).