Factors Influencing Distribution Characteristics of Total Dissolved Gas Supersaturation at Confluences

Total dissolved gas supersaturation (TDGS) produced by flood flows over a high dam has negative consequences on downstream fish. Reducing the total dissolved gas (TDG) levels and/or providing adequate low-TDGS fish shelter areas remains a challenge. The presence of a low-TDG-saturation (LTS) zone at river confluences can protect the fish from harmful high TDG levels, but the effects of confluence hydrodynamics on the TDGS distribution are not well understood. In this study, three-dimensional (3D) hydrodynamic simulations are performed to understand the influence of flow ratio and junction angle on the TDGS distribution downstream of river confluences. The simulation results show that the TDGS distribution is mainly affected by (a) inflow TDGS that determines the supply, (b) secondary flow that leads to rapid renewal of surface water for degassing, and (c) increased turbulence intensity that promotes dissipation of TDGS. Relationship between the LTS area, the flow ratio and the junction angle is constructed via regression analysis. The regression model is validated against a 3D numerical simulation that is compared to field observations at two river confluences along the Jinshajiang River (China). This study provides scientific basis for potential hydraulic modifications at confluences to enlarge an LTS zone and reduce fish fatality.