ABSTRACT
Reservoir dams play a key role in modern society, water resources management and economy. Low-level outlets (LLOs) represent important safety structures for regulating the water level in the reservoir, and for its fast drawdown in case of scheduled maintenance or emergency situations. The flow in LLO tunnels is characterized by high velocities and turbulence levels, leading to air entrainment and transport. This results in sub-atmospheric air pressures, which may induce and aggravate serious issues such as gate vibration and cavitation. An adequate flow aeration via an air vent can mitigate these problems and is key to good performance. While many studies focused on the effects of hydraulic parameters, tunnel geometry and air vent design on the air demand of LLOs, the influence of the tunnel wall roughness is still unclear. To this end, physical model tests were carried out to investigate the effects of invert, soffit, and sidewall roughness on the LLO performance, for various combinations of gate opening, energy head at the gate and air vent properties. The roughness modelled in this study represents unlined rock, and it was implemented by attaching expanded aluminum plates to the inner sides of the outlet tunnel. Air velocities in the air vent were measured to estimate the air demand, and pressures along the tunnel were recorded to assess cavitation potential. For rough wall conditions, both the air demand and the cavitation risk were found to increase compared to the smooth tunnel conditions (i.e., acrylic-made invert, walls, and soffit in the model). In conclusion, the study represents a preliminary analysis of the effects of LLO tunnel roughness on air demand and cavitation occurrence, and future research is needed to enable a more quantitative assessment of the differences in air demand between model and real-world prototypes.
