The calibration of the water level in a hydraulic model experiment is a time-consuming task. In this study, the authors proposed a guide to adjust the water level in the fixed-bed hydraulic experiment, by establishing a connection between the water level increase (ΔZ) in the model with other factors such as roughness diameter (d), roughness density (s), and flow velocity (v). Based on the results of 105 model experiments with different d, s, and v, the study also suggested a process to design a model experiment. The results of the study were used to build a fixed-bed hydraulic experiment for a river section passing through the Ialy hydropower plant in Vietnam. The results showed that after 01 time of implementation, the water level in the experiment was close to the observed water level. The differences between the calculated and measured water levels have been significantly reduced, from 0.027–0.036 m to 0.003–0.008 m. This finding shows that the approach of the study saves time and effort in the process of setting up a hydraulic experiment.