Nappe flow in stepped channels – Occurrence and energy dissipation

The use of stepped spillways with nappe flow has been more closely related to the use of gabion spillways, where maximum flow velocities must be respected in order to guarantee the integrity of gabions. The increasing interest in stepped spillways built with different construction materials, either founded on the downstream slope of embankment dams, with usual slopes comprised within 1:2 to 1:3, or on mild slope chute spillways, with even lower inclination, showed the necessity to improve the knowledge on nappe flows. An experimental facility developed at the Hydraulics and Water Resources Laboratory of the Instituto Superior Técnico (IST), allowing to change the steps slope from 1:5 up to 1:2, is described. A bibliographic review on nappe flow occurrence and energy dissipation is presented. Results concerning the two previous aspects are presented and compared with published data and criteria.