Characteristics of skimming flow over stepped spillways

This paper presents the results of a laboratory study on the characteristics of fully developed skimming how in a large model of a stepped spillway for two slopes, for a range of discharges with y(c)/h in the range of 0.7-4.4. Fully developed aerated flow on a stepped spillway can be divided into lower and upper regions, similar to those for self-aerated flow in steep chutes. The air concentration distributions in these two regions agree with the equations developed by Straub and Anderson for flow in steep chutes. It was found that the depth at which the air concentration is equal to 90% can be considered as the depth of aerated flow on stepped spillways. In the lower region, the velocity profiles were described by the Karman-Prandtl equation for rough turbulent flow when an equivalent bed roughness was used. A correlation was developed for the skin friction coefficient to predict the Reynolds shear stress at the virtual bed of the stepped spillway. It was found that the relative energy loss in the stepped spillway is in the range of 48-63%. It was also found that the mean air concentration on a stepped spillway is larger than that in a corresponding chute.