Underpass gates are hydraulic structures in which water moves under the gate. Here, different models of gate position including vertical, deflection to upstream and downstream of the channel with respect to the vertical axis and also oblique position concerning the transverse axis at angles of 10°, 20° and 30° was investigated numerically with FLOW-3D software. Also its effect on discharge coefficient (Cd) and hydrodynamic force applied to the gate was checked. The nested mesh was used for modeling. The results indicated that the statistical indices such as Absolute Error (AE), percentage Relative Error (RE%) and Root Mean Square Error (RMSE) in the RNG turbulence model were less than those of the k-ε, k-ω and Large Eddy Simulation (LES) and were more accurate compared to the experimental results. Cd in the same discharges increases with the increasing angle to the upstream of the channel compared to the vertical position. Cd is inversely related to increasing the angle of the gate relative to the vertical axis in the downward position. In the oblique position, Cd decreases with increasing angle to the transverse axis of the channel due to the increase in the area of flow through the gate. This increase in angle and area of the flow passing through the gate leads to a decrease in the upstream fluid depth, therefore, less hydrodynamic force is applied to the gate. The upward inclined gate tolerates less hydrodynamic force than the downward and oblique positions. The maximum value is related to the downward position.
