Hydrodynamic Characteristics of the Mainstem–Tributary Confluence in a Thermally Stratified Reservoir and the Effects of Selective Withdrawal

Abstract Hydrodynamics characteristics in the mainstem–tributary confluence profoundly affect the local environment and ecology. Thermal stratification widely occurs in large reservoirs, and the need to investigate hydrodynamics of mainstem–tributary confluences is urgent. Moreover, selective withdrawal is often used to regulate the outflow temperature, and its effects on the hydrodynamics of confluence are still unknown. Focusing on the stratified Sanbanxi Reservoir, a three‐dimensional (3D) hydrotemperature–scalar numerical model is constructed and validated using the prototype observations, and the flow regimes in the reservoir–tributary confluence are systematically explored. Different scenarios of withdrawal elevation, inflow rate and temperature of tributary, temperature stratification, and tributary location and width were analyzed. This study revealed the following: 1) Affected by the withdrawal layer flow of the mainstem and density flow from the tributary in the stratified reservoir, the flow structures in the mainstem–tributary confluence presented as a 3D sheared inverted cone‐shaped circulation, with the vorticity and kinetic energy got more concentrated in the lower layers. 2) The low withdrawal elevation, high tributary flow rate, high tributary inflow temperature, strong temperature stratification, long distance from the dam and small width of the tributary strengthened the 3D sheared inverted cone‐shaped circulation. 3) In a reservoir with obvious temperature stratification, the velocity ratio ( Vr ) at different layers was more useful for determining circulation than the vertically averaged momentum flux ratio ( Mr ) was. 4) A high selective withdrawal elevation favored epilimnetic fish migration, and a low withdrawal elevation decreased hypolimnetic sediment deposition and nutrient accumulation, and vice versa. These results provide support for the withdrawal regulations and environmental improvement in reservoirs.