Skimming Flow in Stepped Spillways

Abstract

Flow on stepped spillways can be divided into nappe and skimming flow regimes. This technical note presents an approximate method of predicting the characteristics of skimming flow on stepped spillways. Expressing the turbulent shear stress between the skimming stream and the recirculating fluid, trapped by the stream between the steps, in terms of a fluid friction coefficient and dynamic pressure (in terms of the mean velocity), a flow equation has been developed. Using the model studies of Sorensen on the new Monksville dam, this friction coefficient was found to be equal to about 0.18. An estimate has been made of the energy loss caused by the steps and it was found that this loss is significant. It was also found that the skimming flow sets in when the critical depth is greater than about 0.8 times the height of the steps.