Trend Analysis of Discharge and Water Level Changes in the Fluctuating Backwater Area

Under the operation of the large reservoir, the variation law of water level in the fluctuating backwater area is complex, which causes river protection engineering to lack a theoretical basis. The changing trend of daily water level in the fluctuating backwater area of the Three Gorges Reservoir (Cuntan hydrological station) was calculated, based on the relationship between daily discharge and water level, and the flow duration curve method. From 2002 to 2021, the daily water level processes had a distinct plateau stage after the flood season since 2008. The water level processes were composed of two parts, including the natural period (2002–2008) and the response period (2009–2021). The average daily discharge increased from 10214.93 m³/s to 10893.38 m³/s, and the average water level increased from 163.87 m to 169.03 m since 2008. The coefficient parameter of the relationship between daily discharge and water level decreased from 0.041 to 0.026, which indicates that the effect of daily discharge variation on the water level change was weakened. The maximum flood discharge and water depth increased by 29.82% and 27.21%, respectively, which led to a higher flood risk in the fluctuating backwater area. In this study, we proposed a novel approach to test trend change in the relationship between daily discharge and water level, which can be generalized to rivers influenced by human activities. Combining the trend test method and flow duration curve method, the characteristic daily discharge and water level can be calculated to guide engineering projects.