Determining Air Demand for Small- to Medium-Sized Embankment Dam Low-Level Outlet Works

Air vents in low-level outlet works are installed downstream of control gates or valves to mitigate subatmospheric pressures locally and reduce the potential for damaging cavitation. Published air-vent-sizing methods, which have been limited to large-dam outlet geometries (in-line vertical slide gate), are not applicable for small- to medium-sized embankment dam low-level outlet geometries in which the slide gate is installed on the upstream sloping face of the dam. The water-discharge and air-demand characteristics of a lab-scale low-level outlet works, with the inlet installed on a 3H:1V sloping floor (representing the upstream face of an embankment dam), were evaluated experimentally with respect to gate design (round or rectangular), gate opening, driving head, and outlet submergence; the results are reported herein. Discharge coefficients for predicting low-level outlet works discharge rates (vented or nonvented), and a methodology for air-vent sizing for small- to medium-sized embankment dams are also presented