Prototype spillway experience during large floods: extraordinary high Reynolds number overflow events

During major floods, large dams can discharge very high-velocity free-surface flows with extraordinary high Reynolds number (Re > 108). Quantitative observations of large flood events are extremely rare and constitute a major knowledge gap. In this contribution, the operation of three large dam spillways is reviewed for flow conditions at extraordinary high Reynolds numbers. All the spillways were un-gated, equipped with an ogee crest, followed by a steep chute and energy dissipator. On the spillway chute, the flow accelerated, transient elongated vortices were observed, and the water surface became choppy, before becoming self-aerated. At the inception region of free-surface aeration, dynamic interactions between large-scale turbulent structures and the free-surface were seen. The inception region was well-marked and a robust correlation for the aeration inception location was derived at extraordinary high Reynolds numbers. Intense free-surface aeration was observed downstream of inception, with complex three-phase coupling between air, water and sediments, as well as air–water surface waves in the energy dissipator.