Nowadays, numerical models are used frequently as they require less time and money compared to experimental ones. In this study, Flow 3D software was used to develop the 3D numerical model for the bottom outlet of Seymareh Dam. The 3D geometry of the bottom outlet was drawn in actual size by AutoCad 3D software. The boundary conditions of the numerical model are provided in Table 1. The fluid was considered an incompressible and single phase. The inlet head was equal to the reservoir head. The inlet boundary conditions were set to a specified pressure (P). The pressure outflow (O) and walls (W) were introduced to the outflow and walls of the model, respectively (see Figure 3). To include the Renormalization Group, rather than constant factors, the turbulent model of k–ε (RNG) was employed. The VOF method was utilized to calculate the free-surface profile of the flow downstream in the service gate. To find an optimal size for the reservoir, Jamali & Manafpour (2019) analyzed the velocity profiles at different sections of a bottom outlet. The results demonstrated no significant changes in the profiles when the length and width of the reservoir exceeded 30 and 29.53 m², respectively. Hence, these dimensions were chosen for the reservoir in the present study (Figure 3).