Closure to “Hydraulic Performance of Step-Aerator” by Anina Schiess Zamora, Michael Pfister, Willi H. Hager, and Hans-Erwin Minor

Abstract

The authors are to be congratulated for providing a detailed study of a step aerator to counter cavitation damage for relatively large unit discharges over a stepped chute. As shown in Fig. 2 of the paper, the step aerator is provided on the first step. The potential for cavitation is along the black water reach from the first step to the point of incipient bottom aeration xi . Boes and Hager 2003 demonstrated that the distance xi increases essentially with the critical flow depth hc . Also, Semenkov and Lantyev 1973a,b proposed an average air detrainment of c=0.04 to 0.80% per meter length of chute. Based on experimental data, Chanson 1997 suggested that an average air concentration of 30% is required for obtaining a bottom concentration of about 5%. Also according to Eq. 4 , the maximum air concentration in the bottom air water boundary layer decreases with x /hc. Hence, the discussers would like to know whether a single step aerator provided on the first step is sufficient to meet the required bottom air concentration for higher discharges to avoid cavitation up to the distance xi, or will additional aerators need to be provided. We would like the authors to comment on the following points: 1. For higher discharges, the values of Cb /Cbu in the region −10 x−xi /hc 0 are very close to zero as shown in Fig. 5 b . Therefore it is doubtful whether the provision of an aerator will serve to reduce the cavitation in this region for higher values of discharges. 2. Advantages and performance of the aerator suggested in this paper compared to the aerator suggested by Pfister et al. 2006 . 3. Generally, steps are provided on spillways to increase energy dissipation so that the cost of the energy dissipator is reduced. The energy dissipation over the surface of stepped spillway decreases with the increase in unit discharge. This