Hydraulics of the Developing Flow Region of Stepped Spillways. I: Physical Modeling and Boundary Layer Development

Abstract

© 2016 American Society of Civil Engineers. On a stepped spillway, the steps act as macroroughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. The flow properties in the developing flow region were documented carefully in a large stepped spillway model (1V:1H; h = 0.10 m). The upstream flow was controlled by a broad-crested weir and critical flow conditions were observed along most of the weir crest, although the pressure distributions were not hydrostatic at the upstream and downstream ends. Downstream of the broad crest and upstream of the inception point, the free surface was smooth, although some significant free-surface curvature was observed for all discharges. The boundary layer growth was faster than on a smooth chute for identical flow conditions. The inception point of free-surface aeration was observed when the boundary layer thickness reached 80% of the flow depth: δ=d i ≈ 0.8. The location of the inception point of free-surface aeration and the flow depth at inception were compared successfully to previous laboratory and prototype results.