Hydraulics of skimming flow on modeled stepped spillways

TWO sets of models of stepped spillways of slope 1 upon 0.6 with the same crest-shape and with a range of step sizes (0.25-2.0 m on the 1:10 scale and 0.5-2.0 m on the 1:20 scale models) were built and studied. In the range of prototype unit flows considered (0.8-3.8 m(2)/s on the 1:10 scale models and 1.8-21.7 m(2)/s on the 1:20 models), the residual specific energy is independent of these step sizes, at positions where measurements were made on the spillways (from 30 m below the crest) once fully aerated skimming flow is established. The residual specific energy at the toe of a 50-m-high (or higher) stepped spillway, within the range of step heights tested, is <60% of the residual specific energy at the same level on a similar smooth spillway experiencing flows up to 20 m(2)/s; at these flows and at this height, the stepped spillway was found to be in equilibrium. The reduction of specific energy is lower than that expected from previous studies; these findings are likely to impact on the design of stepped spillways.