Flow resistance in skimming flows in stepped spillways and its modelling

Dams and weirs must be equipped with adequate flood-release facilities for a safe dissipation of the kinetic energy of the flow. With stepped spillway design, it is essential to accurately predict the flow-resistance contribution of the steps. The authors investigate the flow resistance of skimming flows and associated form losses. Recent laboratory experiments were systematically performed with channel slopes ranging from 5.7degrees up to 55degrees and the results are compared with existing laboratory and prototype data. The results provide a better understanding of the basic flow patterns and flow-resistance mechanisms and emphasize that form loss is dominant. Simple analytical models provide a reasonable order of magnitude of the pseudo-boundary shear stress and the recirculation cavity ejection frequency. Altogether, more than 38 model studies and four prototype investigations (for a total of more than 700 data points) are reanalysed.