Prototype and Laboratory Low-Level Outlet Air Demand Comparison for Small-to-Medium-Sized Embankment Dams

Abstract

Low-level outlet works through dams are typically used to provide irrigation, reservoir sediment flushing, reservoir draw down, and/or minimum downstream flows to meet biological needs. To reduce the likelihood of negative pressure development downstream of the control gate, which may lead to cavitation noise, vibration, and damage, air vents are typically installed downstream of the gate. Undersized air vents may allow vapor pressure and cavitation to occur; oversized air vents have a negative economic impact. Air demand design criteria have been developed for vertical control gates (common to larger dams). The guidance for sizing air vents for an inclined slide gate, which are common for small-to-medium-sized embankment dam geometries, however, is limited. This study compares laboratory-scale and prototype-scale air-demand data. The influences of conduit slope, hydraulic jump presence, and air delivery system geometry on air demand were found to influence air demand. A method for estimating the required air vent diameter for a low-level outlet with an inclined slide gate at the inlet is presented.