The cause and inspiration of the nonlinear impact of the Three Gorges Dam operation on stage-discharge relation in downstream channels

Evaluating the effects of dam operations on downstream hydrological regimes is crucial to cope with the threat of flood. Some channels in dammed rivers have experienced erosion and counterintuitive higher flood water levels. However, the impacts of dam-induced channel changes and dam-altered streamflow on downstream hydrological regimes have been less thoroughly evaluated. Herein, we integrated the Distributed Time Variant Gain Model and a proposed grey model to thoroughly decouple the effects of TGD-altered Yangtze River streamflow and TGD-induced river channel changes on the stage-discharge relation in Jingjiang reach of the middle Yangtze River basin, from 2003 to 2014. The stage-discharge relation in Jingjiang reach is presented by the relation between water level at Chenglingji station (Zcl) and discharge at Luoshan station (Qls). The results indicate that the TGD operation increases Zcl by 0.01 ∼ 0.06 m and 0.02 ∼ 0.48 m on average when Qls is below 12,000 and exceeds 24000 m3/s, respectively. However, the TGD operation decreases Zcl by 0.01 m on average when Qls is around 18000 m3/s. Hence, the TGD operation has a nonlinear impact on downstream stage-discharge curves, which results from the TGD interception of the release, water–sediment regulation of the TGD, and the erosion of the fine-grained channel. Given that the causes of the nonlinear impact are widespread, we speculated that the resulting changes in downstream stage-discharge curves influenced by the water–sediment regulation of dams in fine-grained rivers will be nonlinear and exhibit the hook curve. Meanwhile, the purposed grey model helps to evaluate the impacts of dam-induced river channel changes on downstream hydrological regimes in data-scarce rivers. Our work helps to thoroughly evaluate the impacts of dam operations on downstream hydrological regimes and the resulting flood threat.