Modeling total dissolved gas downstream of spillways with OpenFOAM

One environmental concern of hydropower is the supersaturation of Total Dissolved Gas (TDG), which may cause gas bubble disease in affected fish. The severity of the disease depends on the supersaturation level, exposure time and compensation depth. As part of the relicensing process, hydropower utilities in the Columbia River Basin are required to meet water quality standards, including the numerical standards for TDG, to protect aquatic life. The main source of TDG downstream of dams is the dissolution of entrained bubbles during spill events. Air dissolution downstream of a spillway depends not only on the number and size of bubbles entrained but also on the depth reached by those bubbles in the tailrace. A mechanistic model to compute entrained bubbles and TDG distribution downstream of a spillway was implemented in interFOAM. The air entrainment model is based on the physical processes involved in the entrainment of bubbles including formation at the free surface, transport to depth against buoyancy, and interaction with vortices that break it up into smaller bubbles, or with other bubbles by coalescence. A source term in the momentum equation was included to account for the effect of the bubbles on the hydrodynamics. The velocity of the bubbles was computed considering drag, pressure and turbulent dispersion forces. A bubble number density transport equation predicted bubble size, which can change due to mass transfer and pressure. A realizable k-epsilon model is used for turbulence closure. A transport equation for TDG concentration taking into account bubble size changes caused by dissolution and compression was included in the model. The entire powerhouse and about 1000 m of the tailrace of Hells Canyo Dam (HCD) was simulated with the model. Modeling challenges and validation against TDG measurements downstream of the dam will be presented and discussed.