Flow aeration, cavity processes and energy dissipation on flat and pooled stepped spillways for embankments

Abstract

The design floods of several reservoirs were recently re-evaluated and the revised spillway outflow could result in dam overtopping with catastrophic consequences for some embankment structures. Herein a physical study was performed on flat and pooled stepped spillways with a slope typical of embankments ( = 26.6°) and four stepped configurations: a stepped spillway with flat horizontal steps, a pooled stepped spillway, and two stepped spillways with in-line and staggered configurations of flat and pooled steps. The focus of the study was on the flow aeration, air-water flow properties, cavity flow processes, and energy dissipation performances. The results demonstrated the strong aeration of the flow for all configurations. On the in-line and staggered configurations of flat and pooled steps, the flow was highly three-dimensional. The residual head and energy dissipation rates at the stepped chute downstream end were calculated based upon the detailed air-water flow properties. The results showed that the residual energy was the lowest for the flat stepped weir. The data for the stepped spillway configuration with in-line and staggered configurations of flat and pooled steps showed large differences in terms of residual head in the transverse direction. Altogether the present results showed that, on a 26.6 slope stepped chute, the designs with in-line and staggered configurations of flat and pooled steps did not provide any advantageous performances in terms of energy dissipation and flow aeration, but they were affected by three-dimensional patterns leading to some flow concentration.

Free-Surface Aeration, Turbulence, and Energy Dissipation on Stepped Chutes with Triangular Steps, Chamfered Steps, and Partially Blocked Step Cavities

“…For all discharges and locations, the void distribution at step edge follows an S-shape, characteristic of an inter-diffusion process about the free surface. The same trend has been observed in stepped chutes with flat, roughened, pooled and gabion steps (Chanson and Toombes 2002a, Felder and Chanson 2011a, Guenther et al 2013, Wuthrich and Chanson 2014a. The present data compare well with the analytical solutions except next to the pseudo-bottom, where Equation (6.4) consistently underestimated the void fraction.…”

“…In more recent studies the analyses of flow resistance in skimming flows were extended onto non-standard geometries including non-uniform, pooled and gabion steps (e.g. Felder and Chanson 2011b, Guenther et al 2013, Wuthrich and Chanson 2014a. Skimming flows on stepped chutes bear some similarities to those on conventional chutes.…”

“…Recent investigations of non-standard geometries (e.g. Felder and Chanson 2011b, Guenther et al 2013, Wuthrich and Chanson 2014a showed that uniform flat steps were the most advantageous in terms of energy dissipation performance and flow stability.…”

Air entrainment and energy dissipation on Gabion stepped weirs

“…New experiments were conducted in a large size stepped spillway model at the University of Queensland, previously used by Felder and Chanson (2013) and Guenther et al (2013). The test section consisted of a broad-crested weir followed by ten steps with step height h = 0.1 m and step length l = 0.2 m. The chute width was W = 0.52 m. A pump controlled with an adjustable frequency AC motor drive delivered the flow rate, allowing an accurate discharge adjustment in a closed-circuit system.…”

Embankment dam spillways and energy dissipators

“…Most modern stepped spillways consist of flat horizontal steps, although different step configurations may be considered (Andre et al 2004, Gonzalez and Chanson 2008, Guenther et al 2013. The preferred construction method is the placement of roller compacted concrete (RCC) overlays on the downstream embankment slope (Ditchey andCampbell 2000, Gonzalez andChanson 2007).…”