ENERGY DISSIPATION AND RESIDUAL ENERGY ON EMBANKMENT DAM STEPPED SPILLWAYS

Stepped spillways are designed to increase the rate of energy dissipation on the chute and to reduce the size of the downstream energy dissipator. It is essential to predict accurately the turbulent dissipation above the steps for large discharges per unit width corresponding to the skimming flow regime. New measurements were conducted in a large facility with a channel slope of 21.8o and a step height of 0.05 m. The experiments were performed with dimensionless discharges dc/h between 1.17 and 3.16, and flow Reynolds numbers up to 7.2E+5. The waters were highly turbulent and they dissipated a major proportion of the flow kinetic energy. Taking into account the free-surface aeration, the present results showed a decreasing rate of energy dissipation on the steps with increasing discharge: from about 80% for small discharges to less than 60% for medium to large flow rates. The residual energy data were compared with earlier studies conducted with step heights from 0.025 to 0.143 m, and invert slope between 3.4 and 26.6°. The results implied that the dimensionless residual head was about 2.7 < Hres/dc < 3.1 with a median value of 3.0 for a slope = 21.8 and 26.6°, and between 3.7 < Hres/dc < 5 with a median value of 4.5 for a slope = 3.4 and 15.9° independently of the step height and discharge. Altogether the comparative analysis yields some simple and basic design guidelines for embankment dam stepped spillways.