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ABSTRACT:The development of the roller compacted concrete as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewal interest in stepped spillways.Previous research has focused in studying the air-water flow down the stepped chute with objective of obtaining better design guidelines. The non aerated flow region enlarges as the flow rate increases and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the Von Karman's integral momentum equation.The shear strain, vorticity and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudo-bottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudo-bottom. Finally the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.

[1] A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical-chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities-Á/ 0 % O(10 À5 )-and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Á/ 0 of O(10 À3 ), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.

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En el presente artículo se presenta un conjunto de criterios para el diseño de aliviaderos escalonados con pendientes típicas de presas de hormigón compactado con rodillo. El texto se basa en Informa­ción recogida en la bibliografía e incorpora los resultados del trabajo de Investigación desarrollado en Amador (2005). Se aborda el diseño de la cresta, la selección del ancho del aliviadero y la geometría de los peldaños. Se proponen criterios para determinar el tipo de flujo existente sobre la estructura y, en el caso del flujo rasante, se presentan expresiones que permiten obtener las principales caracterís­ticas del flujo a lo largo de la rápida. Con ha.se en los resultados del análisis del campo de presiones se proponen un conjunto de ecuaciones para estimar las solicitaciones que el vertido ejerce sobre los peldaños a lo largo del aliviadero. Finalmente se hacen algunas consideraciones sobre los aspectos constructivos de los aliviaderos escalonados.