عنوان مقاله [English]
The submergence threshold of a structure is investigated to determine the boundary between free and submerged flows. Knowing the Submergence threshold of the structure, can help to estimate the discharge in free and submerged conditions with reasonable accuracy. Lopac gates are control structures used to regulate the flow in irrigation canals which has limited reported studies. In this study, the submergence threshold of the lopac gate in different gate opening was investigated. For this purpose, the lopac gate was studied under different hydraulic and operation conditions. Experiments were conducted on a laboratory model at Water Research Institute of Iran. Using the interpolation, the submergence threshold of the lopac gate is obtained for different values of opening angle and flow discharge based on the criteria of one, two and three percent of the depth variation. The graphs of the submergence threshold can be presented as a functional guide. The most important non-dimensional parameters affecting the submergence threshold of the lopac gate are the ratio of the gate opening to the gate width, and upstream Froud Number ( and ). Accordingly, a relationship was found for the submergence threshold of the lopac gate using experimental results in two discharge of 0.025 and 0.030 cms and based on the affecting non-dimensional parameters. The resultant relationship estimates the submergence threshold of the lopac gate with the maximum relative error of ± % 10 which is practically acceptable. In addition the derived figures of the submergence ratio are practical guidelines for determining the boundary between free and submerged flow for different conditions.