Air Entrainment and Energy Dissipation on Embankment Stepped Spillways

During the 19th century, overflow stepped spillways were selected frequently with nearly one third of dams built in USA equipped with a stepped cascade. More recently, the 1980s and 1990s were marked by a regain of interest for that type of spillway design. During the last two decades, a number of embankment dam spillways were designed with concrete overtopping protection shaped in a stepped fashion (Chanson, 2002; Chanson and Gonzalez, 2004; Chanson and Toombes, 2001). Although some overflow systems were made of timber cribs, sheet-piles, riprap gabions and reinforced earth, recent structures were designed with concrete slabs and formed concrete. During the 1990s, the construction of secondary stepped spillways for embankments accounted for nearly two-thirds of dam construction in USA (Ditchey and Campbell, 2000). The preferred construction method was roller compacted concrete overlays placed on the downstream slope. Most stepped spillway structures had flat horizontal steps, but some were equipped with devices to enhance energy dissipation. Some spillways had pooled steps with vertical walls (Sorpe dam, 1932) or rounded edges (Le Pont dam, 1882). Recently, Andre et al. (2001) suggested an alternation of pooled and flat steps for maximum energy dissipation. Peyras et al. (1992) demonstrated greater energy dissipation with inclined upward steps. Newer designs were introduced and tested in laboratory by adding baffle row or blocks at the downstream end of step, or by using turbulence manipulators (Andre et al., 2004; Chanson and Gonzalez, 2004). In the present study, detailed air-water flow measurements on 16deg. and 22deg. stepped slopes (h = 0.1 m) are re-analysed, and the rate of energy dissipation is presented. Results obtained in flat, smooth steps are compared with results corresponding to rough faced step chutes and chutes with added turbulence manipulators. The results yield new design guidelines that are especially applicable to embankment dam stepped chutes with bed slopes between 15deg. and 25deg..