Field observation and numerical modelling of supersaturated dissolved gas at river confluence

Confluences are common habitats of ecological significance given their complicated hydraulic structures and mass transportation. In hydropower cascades, dissolved gas supersaturation downstream of a dam becomes a negative environmental impact caused by dam spilling, which may cause fish to suffer from gas bubble disease and even death. A difference in the supersaturation level of dissolved gas is consistently noted between the mainstream and tributaries mainly due to their different cascade developments and dam scales, resulting in a significant nonuniform distribution of dissolved gas at a confluence. This study focused on describing the hydrodynamics and dissolved gas distributions under unsteady inflows at an actual confluence of the Min River and Jinsha River in China. A long period of prototype observations on dissolved oxygen (DO) were conducted to demonstrate DO variation at the confluence and calibrate the DO dissipation coefficient. A depth-averaged two-dimensional numerical model was developed that reflects the nonconstant characteristics of flow and supersaturated DO at the confluence and was verified to be in good agreement with the measured data series. The supersaturated DO level at the confluence increases as the inflow DO level of the mainstream Jinsha River increases and decreases as the flow ratio of the branch Min River and Jinsha River increases, representing two aspects influencing the distribution of DO at the confluence. A comparison was performed to evaluate the DO saturation calculation method when DO was laterally mixed downstream of the confluence. The accuracy of the flow weighted average calculation based on the data of the mainstream and a tributary upstream of the confluence was acceptable, but the simulation method was recommended to obtain a refined distribution and spatial utilization of supersaturated DO at the confluence when it was necessary to pay more attention to the ecological function in this region. A two-dimensional unsteady numerical model based on valuable prototype observations was proposed, which provides technical tools for characterizing the hydrodynamics and dissolved gas variations and distributions at confluences and better evaluating the risk for fish in regions of great ecological significance.