Discharge and water level fluctuations in response to flow regulation in impounded rivers: An analytical study

To analyze the hydrodynamics in an impounded river in response to discharge regulation at upstream and downstream dams, we developed a one-dimensional analytical model. The model provides analytical solutions for discharge and water level fluctuations along the river for periodic discharge boundary conditions at both dams. As a case study, the model was applied to a 38 km long section of the Yangtze River located between the Three Gorges Dam and Gezhouba Dam. The outflow discharge at both dams fluctuated diurnally due to varying demand of hydropower electricity during day and night. In good agreement with observational data, the model results showed significant diurnal variation of water level and discharge along the river. Using the model, we analyzed the sensitivity of the amplitude of diurnal fluctuation of water level and discharge to the parameters describing diurnal discharge regulation. Rapid discharge changes at the dams induced large amplitudes of diurnal variation of water level and discharge in the river. The amplitude of the diurnal wave in the river depends strongly on the lag time between discharge changes at both dams and was lowest if the lag time corresponded to the propagation time of a gravity wave along the river (0.6 h in the case study). In addition to the diurnal wave, high frequency waves were generated, with a period corresponding to the eigenperiod of the impounded river system (1.2 h in our case study). The standing wave is generated by rapid discharge changes at both dams and its amplitude is strongly dependent on the lag time between these changes. The high frequency eigenwave has little influence on water level variation but relative strong influence on discharge variation in the river.