Boundary of Transition Flow Regime on Stepped Spillways by Physical Modeling

In a stepped spillway, the spillway face is provided with a series of steps from near the crest to the toe. The energy dissipation caused by the steps reduces the required size of the energy dissipater which is generally provided at the toe of the spillway. Stepped spillways have been built in the past and there is interest in them because of significant cost savings. The purpose of this study is to determine the hydraulic conditions that govern ther transition flow regime. For this purpose, 11 physical models of stepped spillways were built with 15, 25, and 45 degree slopes and different step numbers from 3 to 50. The width of flumes for installing the stepped spillways were 50 and 25 cm. Results showed that dimensionless parameters h/l (spillway slope) and yc/h (critical depth to step height ratio) are effective at influencing the occurrence of a transition flow regime. Two boundaries that are important are the “upper limit for the nappe flow regime” and the “lower limit for the skimming flow regime”. Boundary defined in this study are recommended for future applications and design of hydraulic structures.