Application of Computational Fluid Dynamics Modeling to Study Maximum Allowable Gate Openings at Spillways

This paper presents the application of computational fluid dynamics (CFD) modeling to study maximum allowable gate openings (MAGOs) in gated spillways—a critical approach for energy dissipation and safe operation of gate-controlled structures to prevent downstream scour in South Florida. Exceeding the MAGO limit may not necessarily lead to immediate catastrophic events, but can necessitate repair works following significant storms. CFD was employed in this study to evaluate the relationship between water stages, gate opening, and discharge. Hydraulic boundary conditions were selected based on MAGO curves for a prototype spillway structure, S-65A, in the Kissimmee River to obtain fully three-dimensional velocity fields. These fields were used to validate three MAGO criteria that includes (1) ensuring the hydraulic jump remains fully contained within the stilling basin, (2) maintaining an average cross-section velocity lower than the critical velocity of the riprap near the end-sill, and (3) keeping the near-bed velocity in the downstream canal below the incipient velocity of the bed particles to prevent downstream erosion. In a second case study, MAGO was incorporated into the design of a replacement structure S-49E in St. Lucie County to provide water managers with flexibility to operate the new structure without any MAGO restrictions. These CFD applications underscore the adaptability and cost-effectiveness of 3D modeling in both hydrodynamic and local scour studies. They highlight that such model applications can serve as a robust alternative and supplement to the traditional laboratory and field studies typically employed in hydraulic engineering applications.