Numerical investigation of USBR type III stilling basin performance downstream of smooth and stepped spillways

Recent systematic studies on air-water flows have included stepped spillways. However, to date little has been investigated about how the hydraulic conditions on the stepped spillway may affect the design of traditional energy dissipation structures. In this paper, both smooth chute and stepped chute configurations terminating with the USBR type III stilling basin are tested by means of numerical modelling, allowing a qualitative comparison. Unsteady RANS equations have been employed altogether with VOF and RNG k-ε for free surface tracking and turbulence modeling, respectively. Eight different Froude numbers (F) ranging from 3.1 to 9.5 have been analyzed for a type III basin designed for F = 8, following recent studies conducted in a physical model by Reclamation. The basin flow structure is discussed for both smooth chute and stepped chute cases. Additionally, the modelled basin has been tested for design and adverse hydraulic conditions, obtaining a detailed insight of the role of each basin element and their adapting roles when insufficient tail water conditions exist