Spillway deflector designs for mitigating supersaturated total dissolved gases

Abstract

Supersaturated total dissolved gas (TDG) produced by dams can have negative ecological consequences and development of appropriate mitigation requires facility specific information. In this study, a computational fluid dynamics (CFD) model and an analytical model for air entrainment were used to investigate how different flow deflector designs influence TDG production. Specifically, an evaluation was conducted to assess the influence of various flow deflector designs and installation positions within the spillways at the Hugh Keenleyside Dam, B.C, Canada, on both the production of TDG supersaturation and energy dissipation. Model results indicate that the CFD model incorporating an air entrainment model reliably predicts the production of TDG supersaturation. While the installation of a straight submerged deflector can successfully reduce TDG production by up to 16%, it also results in a significant decrease in energy dissipation rate, which can be up to 25%. After installing baffle blocks below water surface, the production of TDG supersaturation can be decreased by about 9%, with little decrease of energy dissipation (7%). The effectiveness of flow deflectors in enhancing energy dissipation and reducing TDG production depends on their depth placement, with optimal results achieved at a specific depth below the water surface. Nevertheless, beyond this depth, both TDG production and energy dissipation exhibit a significant decline. This study provides an advanced tool capable of accurately predicting TDG production under different operational conditions and mitigative alternatives. Results from this study offer insights for spillway design or retrofitting to optimize the mitigation of TDG supersaturation while maintaining energy dissipation.