The effect of cross-waves in physical stepped spillway models.

Pedro Lopes, Daniel B. Bung, Jorge Leandro, Rita F. Carvalho

Monday 29 june 2015

14:35 - 14:50h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 2F. Engineering - Instrumentation

Stepped spillways are hydraulic structures with high energy dissipation potential widely used at dams to safely convey water from upstream to downstream during floods. Their study, in terms of general conception and hydraulic efficiency, in laboratory scales are being progressively detailed since the 1980s. The focus has been essentially on centre-line of the stepped spillways and in the uniform flow region. Nonetheless, the influence of the walls in the flow behaviour remains unexplored and a complete description of the flow depths at the stepped spillways has not been dealt with in depth. This paper aims to analyse the effect of cross waves in a physical stepped spillway model. Flow depths were measured in an experimental installation where cross-waves at surface are present, and compared with another experimental structure where cross-waves do not occur. Three ultrasonic sensors were used to rasterise the flow surface over the stepped spillway and discover the influence of the walls in the flumeā€™s centre-line. High-speed videos from the flow field were captured in order to recognise the phenomena of flow pattern alternation between adjacent steps, previously observed by other authors. This work demonstrates the unequivocal existence of cross-waves in the larger stepped spillway, and their tentative influence on the flow depths and on the well-known alternating flow pattern over adjacent steps. It was observed that, by reduction of the spillway width from 0.5 to 0.3 m, where the cross-waves do not exist, the flow pattern alternation was choked down. Regimes SK1 and SK2 were found in the wide spillway whereas just the regime SK2 was found on the narrow structure. Comparison in terms of velocities can just be made at steps with identical flow regimes.