Small and Medium Size Dam Air Vent Sizing for Low-Level Outlet Works

Limited information pertaining to sizing air vents for low-level outlet works, specific to geometries consistent with small-medium size embankment dams, which feature an included slide gate at the pipe inlet, is available in the literature. Current air vent sizing methodologies available in the literature are specific to large-dam outlet geometries. Air venting is used to increase the pressure downstream of the slide gate, which reduces the risk of cavitation. Air demand is related to the water flow rate through the outlet. In the absence of an independent flow measurement facility (e.g., a weir or flume installed in the downstream discharge channel), the water discharge through a low-level outlet works could be estimated using slide gate discharge coefficient and an appropriate driving head. No published discharge coefficient data for included slide gates were found in the literature. A lab-scale model of a low-level outlet works was constructed and tested in the lab to determine vented and non-vented square slide gate discharge coefficient data as a function of gate opening and driving head. Air demand data were also collected. Designer can use the Cd and air demand data as a guide for sizing air vents for small to medium size embankment dams.