Evaluation of operational strategies to minimize gas supersaturation downstream of a dam

Bubble dissolution downstream of spillways may create zones of high total dissolved gas (TDG) concentration, which can be detrimental to fish. This paper presents the use of a numerical model to identify dam operational strategies that mitigate elevated TDG production. A mixture model takes into account the effect of the bubbles on the hydrodynamics. The model calculates bubble dissolution considering bubble size change due to dissolution and pressure. The model is validated using field velocity and TDG data.Several simulations are performed to understand the physical phenomena leading to supersaturated water under different operational configurations. According to the model, concentrating the spillway flow in one bay causes bubbles to travel closer to the free surface and thus lower TDG production and more degasification. In order to obtain the lowest TDG concentration, it is best to concentrate most of the flow in a central spillway bay. If additional water needs to be spilled, the use of a western bay is recommended. An additional simulation using compliance conditions indicates that, using the proposed configuration, the dam meets TDG water quality standards. Moreover, statistical analysis performed on inert particles released in the spillway and turbines demonstrates that, after 2 h, 83.9% of the time particles are exposed to TDG values lower than 110%. If hydrostatic compensation is considered, particles are in undersaturated water 70% of the time.

► A two phase flow model is used to predict the hydrodynamics and TDG in a hydropower dam.
► Several operational configurations were analyzed.
► Concentrating the spillway flow in a central spillway bay minimizes TDG production.
► Statistical analysis was performed on inert particles.
► TDG exposure changes with hydrostatic compensation.