Stepped Spillway

Stepped Spillways

The problem of dissipating the energy of the flow that passes down a spillway is important in hydraulic engineering to reduce the scour downstream of the structure. A stepped spillway can dissipate an appreciable part of this energy. A stepped spillway consists of a series of steps or drops, built into the face of the spillway (Fig. 1). When water flows down the steps, turbulent mixing and diffusion can produce more energy dissipation compared to that on a standard spillway. Currently, considerable interest is shown on stepped spillways mainly because of the technical advances in the building of RCC (Roller-Compacted Concrete) dams, where stepped spillways can be easily merged into the dam body as an economic solution.

The use of stepped spillways is not new and, in the past, it was generally used in masonry structures. The Allahverdi-Khan diversion dam (Fig. 2), built on Zayandeh-roud River in 15th century in Isfahan, Iran, is one of the oldest masonry dams featuring three dimensional steps (the steps are not continuous across the width of the river). The first step is two meters high while each of the other eight steps measures 0.35 meter in height. The line connecting the tips of steps follow a curved line that might be similar to the WES (Waterways Experiment Station) standard profile for ogee spillways.

The flow over stepped spillway can be mainly classified into two main types, jet (or nappe) flow and skimming flow (Fig. 3). In the jet regime, the flow impinges on the tread of the lower step at small discharges. Then, the sub or supercritical flow with or without the formation of a hydraulic jump will leave the step and this phenomenon repeats itself on every step. As the discharge increases, the impact point of the jet flow will move towards the end of the step thus starting the partial jet flow. In this stage, the flow does not completely strike the step surface. The upper part of the jet overshoots the tips of the steps and it disperses with considerable turbulence. At higher discharges, the steps become completely submerged and a relatively smooth current skims the steps. In this situation, the spillway may be described as a steep open channel with large roughness elements. A highly air-entrained area is formed on each step (Fig.4).

Author Papers on Stepped Spillways

Chamani, M. R. (1995), “Comparison of energy dissipation between nappe and skimming flow regimes on stepped chutes”, Journal of Hydraulic Research, 33(1), 122-125. More details...

Chamani, M. R., and Rajaratnam, N. (1994), “Jet flow on stepped spillways”, Journal of Hydraulic Engineering, 120(2), 254-259. More details...

Chamani, M. R. (1996), “An experimental study on skimming flow over stepped spillways in the developed region”, Conference on Civil Engineering by Iranian Students in Canada, Montreal, July 19-20, pp. 27-34.

Chamani, M. R., and Rajaratnam, N. (1999), “Characteristics of skimming flow over stepped spillways”, Journal of Hydraulic Engineering, 125(4), 361-368. More details...

Chamani, M. R., and Rajaratnam, N. (1999), “Onset of skimming flow on stepped spillways”, Journal of Hydraulic Engineering, 125(9), 969-971. More details...

Chamani, M. R. (2000), “Air inception in skimming flow regime over stepped spillways”, International Workshop on Hydraulics of Stepped Spillways, Zurich, Switzerland, March 22-24, 61-67. More details...

Salmasi, F., Chamani, M.R., and Farsadi Zadeh, D. (2012), “Experimental study of energy dissipation over stepped gabion spillways with low heights”, Iranian Journal of Science and Technology, 36(C2), 253-264. More details...