Air Demand of Low-Level Outlets for Large Dams

Abstract

Low-level outlets are key safety elements of reservoir dams, especially for structures with high heads. Their main purpose is the regulation and—if required—rapid drawdown of the reservoir water level in case of maintenance works or structural damage to the dam. A common outlet configuration for high-head structures uses a high-pressure vertical slide gate discharging into a free-flow tunnel. The high-speed water jet in the outlet tunnel leads to considerable air entrainment and transport, resulting in negative air pressures, which can aggravate problems with gate vibration, cavitation, and slug flow. Sufficient air supply via an air vent mitigates such problems. However, current methods for estimating the required air demand do not incorporate all factors affecting design of air vents for low-level outlets. Therefore, tests were conducted in a 20.6-m-long hydraulic scale model at heads up to 30 m, to improve the general understanding of aeration processes, to determine the governing parameters affecting air demand, and to formulate a new air demand design equation. The results show that air demand is mainly a function of the Froude number at the vena contracta. Furthermore, the new design equation enables quantifying the effects of the air vent loss coefficient, air vent size, tunnel slope, and tunnel length on air demand.