Self-Aeration and Surface Turbulence at a Large Dam Spillway with a Smooth Converging Chute

On a spillway chute, the upstream section is nonaerated, and a strong air–water mix develops downstream of the onset region of free-surface aeration. During a series of flood events, between 1997 and 2024, qualitative and quantitative observations were undertaken at a large hydraulic structure, the Chinchilla Weir (Australia), with a converging spillway chute and 1V:5H invert slope. Downstream of the crest, the waters accelerated, and the boundary friction generated the development of a turbulent boundary layer along the invert. At some distance, the free-surface started to present free-surface waves and instabilities, evidences of the interactions of coherent structures produced in the turbulent boundary layer with the water surface through the ideal fluid region. The free-surface instability onset appeared at a location Ls≈1/3×LI, where LI is the mean distance to the inception of free-surface aeration. Theoretical prediction of the boundary layer growth was derived based upon the von Karman momentum integral equation for the smooth converging chute. The prototype observations showed that the inception of free-surface aeration occurred for a relative boundary layer thickness δ/dI≈0.8. Downstream of the inception of self-aeration, a combination of high-shutter-speed photography, long-duration pictures, and image stacking suggested that the upper free-surface of the air–water flow was an extremely fragmented region corresponding to the outer edge of the developing turbulent boundary layer region. Theoretical calculations of the longitudinal free-surface and surface velocity profiles were created for the developing and fully developed flow regions, and the results were validated against optical flow surface velocity data measured during flood events.