The replacement of existing classical weirs with labyrinth weirs is a proven techno-economical solution and a means to increase the discharge capacity when rehabilitating existing structures. However, flows exiting a labyrinth weir are complex, three-dimensional, and aerated; additional information is needed regarding energy dissipated by such weirs. In this study, three labyrinth weirs with different crest lengths were simulated with FLOW-3D HYDRO. Reynolds-averaged Navier-Stokes (RANS) modeling with the use of finite-volume method and Re-Normalisation Group (RNG) k-ε turbulence closure were employed. An attempt was made to more precisely account for the flow field in the downstream region. Consequently, the velocity head was determined with both depth-averaged and section-averaged velocities. Additionally, the kinetic energy correction coefficient was considered. A comparison of the computational fluid dynamics (CFD) results with prior experimental data showed that the residual energy was influenced by factors such as probe position and geometric parameters. A minor influence was observed for the kinetic energy correction coefficient. Overall, the high amount of energy dissipation was underlined and an acceptable agreement between simulated and literature data was documented.