Spillway Deflector Design Using Physical and Numerical Models

Elevated total dissolved gas (TDG) can be a serious threat to migrating fish. Gas supersaturation downstream of a dam occurs due to bubble dissolution in deep high-pressure regions in the tailrace. TDG production depends on both air entrainment and depth of entrained bubbles. Deflectors installed at the spillway face are designed to change the regular plunging jets into surface jets, where bubbles are transported in a thin surface layer minimizing dissolution. Distinct flow conditions may, however, occur depending on the deflector geometry, spillway flowrate, and tailwater elevation. Deflectors are commonly designed based on jet regimes observed in physical and CFD models. Since bubbles are not scaled in the physical models and most commonly used CFD models do not predict the TDG field, deflector performance cannot be fully evaluated with this methodology. This paper presents the design of spillway deflectors at Hells Canyon Dam using a 1:48 scale laboratory model and a two-phase flow model capable of predicting TDG production, dilution, and downstream mixing. The numerical model was validated against jet regimes observed in the laboratory and TDG field data. After a deflector was selected, possible fish injury due to pressure and acceleration changes near the deflector was estimated with a particle tracking technique.