The main purpose of the construction of weirs is to raise the water level and control it in front of the weir. It is important here to know the longitudinal section of the variable flow behind the weir in the open channels. 30 experiments were tested in a horizontal laboratory channel with a length of 12 m, a width of 0.5 m, and a depth of 0.45 m. The longitudinal flow section was evaluated follow a step-by-step process under the influence of five different slopes of the channel bottom with five different discharges for each slope. After that, a MATLAB simulation model was built in order to evaluate the longitudinal section of water surface, which is a plot of the water level along the length of the structure. This method is important for the purpose of shortening the time and obtaining more accurate results. The results were compared between the two methods, and there was good agreement between the two methods.