Free-flow discharge estimation method for Piano Key weir geometries

Currently, many existing spillways located throughout the globe require rehabilitation due to dam safety issues. Consequently, there is an increasing need for design tools that facilitate better, more economical spillway designs. Piano Key weirs (PK weirs), the focus of this study, represent one option to address spillway deficiencies. Two design approaches used by practitioners for estimating PK weir discharge were examined: (1) empirical prediction methods ranging from simple to sophisticated and (2) computational fluid dynamics. This evaluation included five published empirical design methods and CFD simulations featuring two different turbulence models (i.e., LES and RNG k-epsilon). The results indicate that, depending on geometry, differences exceeding 30% can exist between predictive method and experimental results. Also, CFD discharge results were relatively independent of turbulence closure schemes and showed very good agreement with experimental data (mean relative errors of 3-4%). Finally, recommendations are presented to designers that include a new empirical equation for Type-A PK weir geometries and rectangular labyrinth weirs.