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 5.7° up to 55° 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.