A Study of the Free Surface Flow on Brownlee Dam

Many spillway designs form surface jets that discharge in relatively large reservoirs. It has been often observed that strong liquid entrainment is caused by these surface jets. This water entrainment can have important environmental and ecological impacts. Because of this entrainment the Total Dissolved Gas (TDG) produced in the spillway is diluted with low TDG water coming from the powerhouse. In addition, low Dissolved Oxygen (DO) waters from the powerhouse are rapidly oxygenated by mixing with the aerated spillway discharge. A numerical and experimental study of the tailrace flow on Brownlee Dam on the Snake River is presented to study this entrainment. Experimental data was obtained on a 1/48 physical model, where three dimensional velocity fields were measured using an AVD probe. Numerical models were evaluated, including the effect of turbulence models (k-epsilon and Reynolds stress model, RSM), free surface approximation (Volume of Fluid, VOF, surface capturing method or rigid lid) and single-phase or two-phase mixture. The commercial CFD code FLUENT was used. Multi-block structured grids were used to accommodate the complex geometry that includes a spillway deflector, power house units and the bathymetry. The velocity fields at different depths were compared against the experimental data taken on the physical model. The ability of the model to predict the experimental results is discussed. Both experimental and numerical results show a strong water entrainment from the powerhouse to the spillway when deflectors are used. Predicted results of the model will contribute to the understanding of the physics of this entrainment.