Stepped spillways are hydraulic structures that have regained a recent renewal in interest, that is associated with the development of new construction methods like the roller compacted concrete (RCC) and gabions. An important number of stepped spillways have been observed in RCC dams, these hydraulic structures are compatible with the construction method and allow much higher energy dissipation along the chute, reducing the size of the downstream energy dissipator. The macro-turbulent flow over the stepped spillway causes hydrodynamic loads on the steps that are of interest to study and affect the design and safety of the structure. The present study is based on a significant set of experimental data gathered in three different 1v:0.8h sloped stepped chutes. The developing flow region is characterised with the growth of the boundary layer being focused. The mean and turbulent velocity field is studied. Also the dynamic pressures on the steps are investigated. Pressure field on the horizontal and vertical faces are studied for skimming and transition flow regimes. The scale effects on the pressure field are evaluated by comparing the results from geometrically similar models with different scales. The risk of cavitation on stepped spillways is analysed. Some design guidelines are presented for stepped spillways over RCC dams. Key-words: stepped spillways; RCC dams; skimming-flow; velocity; turbulence; pressure; cavitation; air entrainment.

Comportamiento hidraulico de los aliviaderos escalonados en presas de hormigon compactado

To protect spillways against cavitation damage, adding a small air concentration to the flow close to the invert is efficient. Aerator performance on smooth chutes was therefore well studied in terms of air entrainment and downstream air concentration development. Since bottom aerators are built upstream of regions exposed to cavitation, no damages have been observed on spillways. The introduction of roller compacted concrete (RCC) dams in the 1980s promoted the use of stepped spillways rarely used until then. Compared to conventional smooth spillways, they have the advantage of a higher energy dissipation rate, and of a self-aeration point located higher upstream. However, the non-aerated flow upstream of the inception point is exposed to an increased cavitation risk due to flow separation on the steps. Until recently, the uncertainty about the conditions required for cavitation inception motivated conservative unit discharges. Today, the cavitation potential on stepped spillways is better known and is significant, so that techniques are necessary to safely use stepped spillways under increased unit discharges. This research includes a physical model investigation of a deflector aerator on a stepped spillway. The key parameters influencing aerator performance and stepped spillway flow are systematically varied. The air concentration downstream of the aerator is measured at regularly spaced profiles by means of a fiber optical probe. The flow field downstream of the stepped spillway aerator can be described in three main zones: (i) thejet zone where air is entrained on the lower and upper surfaces, (ii) the spray and reattachment zone where spray is produced by the jet impact and where there are rapid variations of the average and bottom air concentrations, and (iii) the far-field zone where the flow depth as well as the average and bottom air concentrations gradually tend towards quasi-uniform conditions. The lower and upper surfaces of the jet issued by the deflector were considered to derive the effective takeoff angles. With the takeoff velocity, it allows to describe the lower and upper jet surfaces with projectile motion trajectories. The maximum jet elevation, the jet length, and the jet impact angle on the pseudo-bottom can then be determined. Similarly to smooth spillways, the air entrainment coefficient of the aerator is described as a function of the relative jet length. Besides, a relation for the air entrainment coefficient in function of the Froude number and the deflector geometry is presented. The average and bottom air concentration developments show a minimum shortly after the jet impact, followed by a maximum in the spray zone. These extrema are quantified and are related to the relative jet length. In the far-field zone, unlike smooth chutes, no continuous detrainment is observed for the bottom air concentration. Both the average and the bottom air concentrations gradually converge to quasi-uniform flow values. Tests with an increased approach flow bottom roughness showed a large increase of air entrainment due to the higher flow turbulence, but only small average and bottom air concentration differences downstream of the jet impact result. A pre-aerated approach flow leads to slightly higher average and bottom air concentrations downstream of the aerator. The design of a stepped spillway aerator is presented in the end to summarize the results obtained and their practical application.

Hydraulic performance of stepped spillway aerators and related downstream flow features

Investigations showed that aeration of the flow by jump-off aerators is a simple, reliable, and economic means of protecting the overflow surface against cavitation erosion or suppressing it on an operating structure. Flow aeration can be regulated by the size of the jump-off aerators and change of the capacity of the air-supply system.

A. G. Oskolkov

Papers

Cavitation in elements of hydraulic structures and methods of controlling it