Dynamic Risk Evaluation of Supersaturated Dissolved Gas at a Confluence with Unsteady Flow Conditions

The confluence of two rivers gives rise to a distinctive hydrodynamic structure where numerous nutrients can be accumulated, making the confluence area a pivotal biological habitat. The threat of supersaturated dissolved gas in upstream water caused by dam discharging is indispensable for the confluence areas. Therefore, it is imperative to study and assess the risks faced by fish in these areas, particularly when the merging of main and tributary streams leads to varying degrees of supersaturation and non-uniform distribution of dissolved gases. The spatial and temporal distribution of supersaturated dissolved gas at the natural confluence in the Yangtze River utilizing continuous prototype monitoring and unsteady flow numerical simulation was investigated, and a dynamic evaluation method that classifies risk into three levels (high, medium, low) was proposed. The confluence area exhibited evident non-uniform distribution characteristics, wherein the primary factors influencing this distribution were identified as the dissolved gas saturation of inlet flow, and flow ratio. The presence of non-continuous discharge intensity resulted in an increase in medium-risk areas within the river, thereby mitigating the risk to fish from the supersaturation of the discharge. A velocity barrier was formed at the confluence, with the velocity range being 1.8 similar to 3.4 m/s, exceeding the fish's burst speed of 1.7 m/s. Because of the barrier, there was no direct access to the low-saturation areas, and potential avoidance routes are primarily located in the medium-risk zones downstream. The paper offers a useful method to assess risk distribution in critical ecological regions characterized by non-constant characteristics. Additionally, it recommends dam discharging strategies to expand the scope of fish risk avoidance.