Prediction of the transition nappe/skimming flow on a stepped channel

On a stepped channel, two distinct flow regimes may occur : nappe flow for low discharges and flat slopes, and skimming flow for larger flow rates, A simple analytical method to predict the onset of skimming flow is presented. The method is based upon the change of momentum direction at the impact of the jet on the downstream step. The results of the analysis are compared with existing experimental data. The onset of skimming flow is expressed in terms of the initial Froude number and jet angle. The generalisation of the study results enables to predict the onset of skimming flow and hence the risk of jet deflection at the first upstream step.