Onset of Air Entrainment and Self-Aeration Development on Spillways: Reassessment of Turbulence and Slope Effects

Abstract Air entrainment on spillways of reservoir dams is critical to avoid cavitation, quantify flow bulking, and determine the inflowing momentum to the energy dissipator. The determination of the inception point remains uncertain, and the developing aerated flow is usually assessed based on experimental findings that allow limited physical insight into the driving mechanisms of this complex multiphase flow process. In this study, physical experiments were conducted in a large-scale spillway model with two inclination angles (30° and 50°), and turbulence was enhanced through three different equivalent invert roughness levels. Using state-of-the-art instrumentation and large sampling campaigns, we investigated the onset of air entrainment and the self-aeration development. We show that the intersection of the turbulent boundary layer with the free water surface is the most consistent criterion to define the inception point location, which leads to stable depth-averaged air concentrations of roughly 0.1, independent of slope and turbulent flow conditions. A key novelty is that roughness-driven turbulence strongly affects the self-aeration processes downstream of the inception point. For the first time, air concentration measurements as close as 1 mm above the invert roughness tops—closer than in any study before—revealed considerably smaller close-to-bottom air concentrations than predicted by existing models, a finding of critical importance for cavitation protection. This investigation provides revisited and advanced equations for spillway hydraulics.