Bottom Aeration of Stepped Spillways

The upstream reach of stepped spillway flows may become prone to cavitation damage for large specific discharges because of the absence of air close to its bottom, until the point of bottom self-aeration is reached. This study considers the effect of two aerator types located at the first vertical step face to add air to the chute bottom. Compared to standard stepped spillway flow, considerable differences may be observed closely downstream of the aerator, whereas no significant deviations occur in the far-downstream chute reach. The characteristics of bottom air concentration curves on stepped chutes are investigated with an experimental approach. The results are then compared with flows on both smooth chutes and standard stepped chutes. The data analysis results in design equations that may be applied to usual stepped spillways of chute angles around ${50}^{\circ }$ . In addition, a sinusoidal variation of air concentration about the average value as a novel phenomenon is described relating to a local instability in the minimum bottom air concentration reach.