Prototype spillway observations: self-aeration on a smooth chute

Dams and reservoirs constitute an essential means of water security and flood protection. The hydraulic structure must be equipped with a spillway system to pass safely excess flood waters during natural disasters. To date the literature remains very limited on prototype observations of spillway chute flows. The current contribution presents field observations conducted between 1997 and 2025 at a large dam equipped with a smooth-invert spillway. For all investigated flood events, the overflow consisted of an upstream non-aerated flow region followed by a self-aerated flow region with significant air-water mixing. The location of the onset of free-surface aeration presented some three-dimensional features. Downstream, the air-water flow region exhibited some rapid time variations in air-water surface structures. Long-exposure photography and stacked imaging highlighted the extremely complicated nature of the air-water surface region in the high-Reynolds number flows. The air-water surface was a highly turbulent region corresponding to the outer edge of the boundary layer region, while further self-aeration took place in the hydraulic jump at the downstream end. • Prototype measurements at large dam spillways are rare. • Field observations were successfully recorded at the Chinchilla Weir over three decades. • The field experience highlights the complexity of the turbulent and multiphase flow. • The air-water surface was highly turbulent and corresponded to the outer edge of the boundary layer region.