Self-aeration on large dam spillways during major floods

In a spillway chute flow, the upstream flow is typically non-aerated and the flow becomes self-aerated when the turbulent stresses acting next to the water surface exceeds the combined resistance of gravity and surface tension. The inception region of air entrainment is a rapidly-varied region characterised by the transition from a monophase water to two-phase air–water flow. In this contribution, field observations were conducted at large dam spillways during major flood events, with a focus on prototype data for discharges between 100 m³/s and 6,000 m³/s and Reynolds numbers between 2.6 × 10⁶ to 1.1 × 10⁸. The onset of self-aeration was a complicated three-dimensional transient process, and the dimensionless location of the inception region was a function of the Reynolds number. Surface velocities obtained with an optical technique showed that the streamwise surface velocities were close to theoretical estimates, and the streamwise surface turbulent intensities in excess of 100 %, consistent with self-aerated measurements in laboratory. The current findings yield a couple of seminal questions: (a) what do we know about prototype spillway operation during major floods? (b) how large the Reynolds number of a prototype flow needs to be truly representative of large dam spillway self-aerated flows during major flood events?