Stepped Spillway Prototype Operation and Air Entrainment: Toward a Better Understanding of the Mechanisms Leading to Air Entrainment in Skimming Flows

A spillway is a conveyance structure designed to pass flood waters. The construction of steps down the steep chute may contribute to some energy dissipation, in turn reducing the size of the downstream energy dissipator. A unique opportunity for field observations was provided at the Hinze Dam Stage 3 (Gold Coast, Australia) between 2013 and 2021. Detailed observations were conducted for six overflow discharges within $20.9{\mathrm{m}}^3/\mathrm{s}<\mathrm{Q}<334{\mathrm{m}}^3/\mathrm{s}$ with Q the volume discharge, corresponding to dimensionless discharges $0.44<{\mathrm{d}}_{\mathrm{c}}/\mathrm{h}<2.82$ with $d_c$the critical depth and h the step height and Reynolds numbers ranging from $0.68\times {10}^7$ to $10.8\times {10}^7$. Some uniquely novel aspects of the research included a series of systematic observations of a full-scale prototype stepped spillway, operating with a relatively wide range of unit discharges $1.71{\mathrm{m}}^2/\mathrm{s}<\mathrm{q}<27.3{\mathrm{m}}^2/\mathrm{s}$ with q the unit discharge. The observations provided new information on the basic hydraulic flow patterns, inception of free-surface aeration, and surface velocity field. Overall, the current study detailed some unique insights into the mechanisms leading to air entrainment in skimming flows in high-velocity prototype stepped spillways. While surface velocity measurements were achieved, the limitations are discussed and future enhancements are proposed.