Free-flow discharge characteristics of an overshot gate: A non-hydrostatic numerical modeling approach

Abstract

Overshot gates, such as the Lay-flat gate, have been used extensively as a flow-measuring structure in open-channel irrigation conveyance systems. Despite their simple geometric shape, the free flow over such structures possesses a substantial curvature of streamline and a steep free-surface slope, thereby making the assumption of a hydrostatic pressure distribution invalid. Accordingly, the shallow-water approach becomes inapplicable for analyzing their discharge characteristics. Using the depth-averaged Boussinesq-type model, the critical flow conditions based on this lower-order approach were extended, leading to an equation for the free-flow coefficient of discharge that implicitly incorporates the flow’s dynamic effects. The developed model was tested for free-flow cases, with a satisfactory agreement between computational results and experimental data. Overall, it was shown that the proposed model is capable of accurately simulating a sharply-curved flow over an overshot gate. The study found that the relative overflow depth prominently affects the characteristics of the curvilinear transcritical flow and hence the free-flow coefficient of discharge. Furthermore, the angle of inclination has a moderate influence on the discharge characteristics of a full-width overshot gate with a face slope flatter than 56°.