Stepped Spillway Flow: Pre-Aeration With Deflector

Abstract

During floods, stepped spillways have the purpose of dissipating energy over the flow passage through the chute, minimizing stilling basins dimensions. However, stepped spillways have their utilization frequently limited to unit discharges of approximately 15 m³/s/m, due to the risk of cavitation damage. In order to increase the specific yields endured by these structures avoiding cavitation damages, some aeration elements are being studied. These elements allow a jet formation and the air can be introduced through the lower portion, in addition to the surface aeration. The present paper has the purpose of analyze the medium pressures behavior and the jet formation in a flow over stepped spillway through measures conducted on a scale reduced physical model. It was possible to notice that the jet reaches the 7 th step for the higher flows and the 9 th step for the lower. Furthermore, it was observed that, upstream of the jet impact location, the mean pressures were close to zero. These pressures were lower than the same measured at a stepped chute with natural aeration (without aerator elements) for the horizontal faces of the steps and higher for the vertical faces of the steps. Near the jet's impact location, for both faces of the steps, a peak of positive pressures was observed, exceeding the same values on a chute with natural aeration. Downstream of this location, the pressures tend to the values acquired with natural aeration.

Hydrodynamic pressures on a stepped spillway with an aerator system subjected to different air flow rates

“…This is because some of the studies already carried out are divergent, even under similar conditions, and it is not possible to identify, at the moment, whether or not air insertion is associated with increased pressures. While Marques et al (2019), Novakoski (2021) and Priebe et al (2021), for example, did not identify significant variations between the different conditions, Arantes (2007), Priebe (2020) and Ferla et al (2021) verified them in some cases -which also concurs with Peterka (1953) and Dong & Su (2006), for example.…”

“…The conclusions of Arantes (2007) differ from those of Marques et al (2019), who developed medium pressure studies in a physical model of a stepped spillway with α ≈ 53º. Marques et al (2019) compared the results obtained with the aerator system (formed by a deflector and air chamber) with those of the natural aeration condition.…”

“…The conclusions of Arantes (2007) differ from those of Marques et al (2019), who developed medium pressure studies in a physical model of a stepped spillway with α ≈ 53º. Marques et al (2019) compared the results obtained with the aerator system (formed by a deflector and air chamber) with those of the natural aeration condition. There was no significant change in the average pressure values obtained by Marques et al (2019), considering the natural and induced aeration conditions, except for the portion of the flow influenced by the launching and impact of the jet -which only happens in the induced aeration condition.…”

“…Marques et al (2019) compared the results obtained with the aerator system (formed by a deflector and air chamber) with those of the natural aeration condition. There was no significant change in the average pressure values obtained by Marques et al (2019), considering the natural and induced aeration conditions, except for the portion of the flow influenced by the launching and impact of the jet -which only happens in the induced aeration condition. Downstream of this region, the average pressures in both conditions were similar.…”

Stepped spillways with aerators: hydrodynamic pressures and air entrainment

“…Even though, in practical cases, stepped spillways have already been subject to notable floods without showing damage on the chute (Matos & Meireles, 2014;Chanson, 2015), the design practice usually restricts the specific design discharges to values in the order of 15 to 30 m 2 /s, as recommended by Pfister et al (2006a) and Amador et al (2009). Such recommendations are based on the understanding that stepped chutes are more prone to cavitation, when compared to smooth chutes, due to (i) the irregularities created by the steps protruding into the flow (Terrier, 2016) and (ii) the negative pressures that occur, especially, on the vertical faces of the steps, according to contributions of Sánchez-Juny And Dolz (2005), Sánchez-Juny et al (2007), Osmar et al (2018), Ferla et al (2019) and Marques et al (2019).…”