Abstract: The accuracy of flow calculation at the weir sluice of water diversion project is a prerequisite for digital operation. Traditional method of calculating discharge flow in hydraulics textbooks in general is to discriminate the flow pattern first, and then to calculate discharge according to corresponding empirical formulas. The discrimination of flow conversion conditions is determined by the relative opening e/H of gate. In the present research, by experimental and theoretical analysis, it is concluded that e/H=0.65 in the traditional formula as an index of discriminating orifice flow and weir flow is only suitable for free flow condition. It would cause big error in the calculation for long-distance water transfer projects, because it is not suitable for submerged weir flow converting to submerged orifice flow. According to model test data, the fitted formula of calculating the discharge at weir sluice under specific conditions of water transfer project are obtained.

Key words: flow pattern discrimination, condition of the conversion between orifice flow and weir flow, discharge coefficient for submerged flow, water diversion project