: Gates as hydraulic structures are used for flow connection or In this research, characteristics of flow when downstream channel width is less than that of upstream one are investigated. Here, 193 experiments on three physical models of 405 mm width at the upstream section and 300, 240 and 200 mm width at the downstream section have been completed. Using three basic equations of fluid mechanics, namely the energy, momentum and continuity equations, four semi-theoretical models have proposed for estimate free or submerged discharges. In the estimation of discharge, the mean absolute error found to be less than 1.6% for free flow and 2.9% for submerged ones. It was found that those momentum equations which were written between the upstream and the contraction sections give the least error for both free and submerged flows. In addition, dimensionless experimental equations are given here to find the place of two vortices, establishing at the upstream section of the gate. Dimensionless equations are also given for the wake formation in the plan at the downstream section of the gate.
