Improvement on numerical modeling of total dissolved gas dissipation after dam

Abstract

Discharge from high dam causes total dissolved gas (TDG) supersaturation, which dissipates slowly along the downstream river and induces fish bubble disease. The dissipation process is closely related to the hydraulic condition. In most TDG prediction models, the hydraulic condition is lumped in a parameter called the dissipation coefficient (k), whose determination usually contains large uncertainty. In this study, the property of k was experimentally investigated under three water depths and three flow velocities. The results demonstrated that the k value increased with v/H ratios. Based on the experimental results, an equation was proposed to calculate k value. The equation was validated by the observed data from field survey in Xiangjiaba Reservoir in the Jinshajiang River, the upstream of Yangtze River. Meanwhile, the proposed equation was compared to two typical equations, a dissolved oxygen (DO) reaeration equation and an empirical equation, using the same datasets. The comparisons indicated that DO reaeration equation was not suitable for k value estimation, particularly under shallow water conditions. The limitation of empirical equation was that the k value was determined by data from a specific case so that was not applicable to other conditions. The proposed equation showed best performance in calculating k value and hence estimating the TDG levels, especially under the condition of low sediment concentration. In addition, the proposed equation was more generically applicable compared to the existing formulas. Our findings could help to draw up governing measures to minimize the risk of supersaturated TDG and achieve river ecological restoration.