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Total dissolved gas (TDG) supersaturation downstream has a negative environmental effect on fishes. It is caused by discharge from high dams and increases the incidence of gas bubble disease and fish mortality. Downstream of a high dam, there is an area with low TDG saturation due to the gradual mass exchange of gases between the separation zone and the mainstream and the long retention time in the confluence, which contributes to the dissipation of saturated TDG at the confluence of the mainstream and its tributaries. This area can provide a temporary shelter for fish to avoid the effects of TDG supersaturation during dam discharge. A depth-averaged, two-dimensional model of TDG dissipation at a river confluence was established. The concentration field was verified by a flume experiment. A numerical simulation of the TDG at the confluence of the Zumuzu River and its tributary, the Mozigou River, was conducted. The simulation showed that the convergence of the tributary, which had a low TDG saturation level, could reduce the TDG saturation level of the mainstream. However, the low-saturation area was not large enough for fish to avoid the negative effects of TDG saturation due to a sharp river slope and a large flow ratio between the mainstream and its tributary. To expand the suitable shelter area with low TDG supersaturation levels in order to provide a suitable shelter for fish, some engineering measures were explored, including the excavation of the riverbed and the construction of resistance obstacles. After the engineering measures were introduced, we observed a 30-fold increase in the size of the area with low TDG saturation, which was as high as 10005 m 2 at 110% of the TDG saturation. Combined with the comprehensive analyses of the flow velocity and the water depth, the 3 confluence region was thought to be suitable to protect the fish from the effects of TDG supersaturation. This study provides an important reference for protecting fish during high dam discharge.

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The recent construction and operation of high dams have greatly changed the natural flood process. To meet the ecological demands and flood control requirements of rivers, dams discharge flow through the flood discharge facility, always accompanied by total dissolved gas (TDG) supersaturation in the water, which is harmful to fish. The purpose of this paper is to explore the dissipation characteristics and prediction methods of supersaturated TDG in water flowing through a floodplain covered with vegetation. A three-dimensional two-phase supersaturated TDG transportation and dissipation model considering the effects of vegetation was established. Using existing mechanism experimental results, the inner dissipation coefficient kin of TDG in vegetation-affected flows was studied, and the quantitative relationships between the inner dissipation coefficient kin and the average flow velocity, average water depth, average water radius, Reynolds number, and vegetation density were characterized. Based on the simulation results, the distribution characteristics of the supersaturated TDG in water around vegetation and in the vertical, lateral, and longitudinal directions of the flume under different flow and vegetation densities were analyzed. A supersaturated TDG transportation and dissipation model for vegetation-affected flow is proposed and can be used to predict the impact of TDG in a floodplain.