Short-crested trapezoidal-sectioned weirs fitted with vertical lift gates are extensively used as river diversion structures in the Indian subcontinent. While the downstream slopes of the weir are known to influence the discharge coefficient, the present work investigates the increase in discharge capacity of the structure with the addition of a semicylindrical lip upstream of the gate. The lip helps straighten the streamlines at the gate opening, as confirmed from flume experiments, supplemented by numerical simulations carried out with the computational fluid dynamics (CFD) software package ANSYS-Fluent. The addition of a semicylindrical lip, even with a small radius, improves the discharge coefficient (Cd) by up to 28%–36% compared to the sharp-edged gate. A steeper downstream slope of the weir accelerates the flow, further increasing the coefficient under free and submerged flow conditions, although at the cost of a lower pressure over the weir crest. Compared with a flat bed, a weir with a 1V:2H downstream slope increases the Cd by approximately 13%–16% under free-flow and 48%–56% under submerged-flow conditions for a semicylindrical lipped gate. The modular limit increases significantly with the addition of a lip, indicating that submergence occurs at higher tailwater levels, whereas a milder downstream slope further delays weir submergence. For a given discharge under free-flow, a sharp-edged gate exhibits 65%–70% higher energy dissipation and a longer downstream roller than a semicylindrical lipped gate. Pressure dips at weir-crest edges also show significant variations with both weir slope and lip diameter.
Singh et al. (Tue,) studied this question.