Hydraulic performance of stepped spillway aerators and related downstream flow features

Roller compacted concrete gravity dams are built since several decades. In combination with such dams, stepped spillways are very often used as efficient flood release structures. More recently stepped spillways are also excavated into rock along the abutments of embankment dams. Over the last years, the specific design discharge over stepped spillways has increased significantly. Specific discharge higher than 30m3/sm for skimming flow regime are not rare anymore. For such high specific discharges risk of cavitation may occur at the beginning of the stepped chute. This risk can be mitigated by implementing in the first step of the stepped chute a specially designed aerator. In order to prepare design guidelines for such aerators, Dr. Stéphane Terrier carried out for the first time a systematic experimental study regarding the hydraulic performance of a deflector aerator by varying the chute angle, the step height, the approach flow Froude number, the approach flow depth, the deflector angle as well as the deflector height. He systematically analyzed the lower and upper surfaces of the jet issued by the deflector and could derive empirical equations for the lower and upper effective takeoff angles. Together with an equation to obtain the takeoff velocity, the candidate could then describe the lower and upper jet surfaces with ballistic equations. Then the maximum jet elevation, the jet length and the jet impact angle on the pseudo-bottom can then be obtained, which are the most important parameters to predict the aerator performance. Dr. Terrier demonstrated that the air entrainment coefficient of the aerator could be derived from the relative jet length. Finally, he proposed an empirical relationship, which allows obtaining the air entrainment coefficient as a function of the Froude number and the deflector geometry. As main result, he gives relations estimating the average and bottom air concentrations at significant locations along the flow, proving a sufficient value to counter cavitation. With this information Dr. Terrier could present for the first time a procedure for the design of a stepped spillway aerator.