Effect of downstream contraction on side weirs discharge

Side weirs are important structures in irrigation and drainage networks that are used as a protective structure upstream of reverse siphons and road underpasses. In this research, using laboratory data, for two positions of lateral overflow placement diagonally in the width change wall and directly and in the wall without change width, a new relationship based on the critical depth of flow on the weir to determine the discharge discharge weir (QW) are provided in rectangular channels with narrowing downstream of the weir. The results showed that the discharge capacity of oblique weir decreasing width downstream is more than normal overflows and also diagonal weirs located in the width reduction wall are more than the case where the weir is in the direct wall without width reduction. Finally, by comparing the relationships obtained from dimensional analysis in two types of overflows, it can be concluded that the flow rate of side weirs located in the diagonal wall of the transition section is about 7.50 percent higher than the case where the side is located in the direct wall of the weir. The reason is less interference of flow lines. Also, the narrowed part of the downstream acts as a barrier, increasing the depth of flow and less curvature of the flow lines, and the flow passes through this weir with less drop.