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Geosciences 2018, 8(9), 333; https://doi.org/10.3390/geosciences8090333

Re-Aeration on Stepped Spillways with Special Consideration of Entrained and Entrapped Air

1
Hydraulic Engineering Section (HES), FH Aachen University of Applied Sciences, Bayernallee 9, 52066 Aachen, Germany
2
Department of ArGEnCo, Research Group of Hydraulics in Environmental and Civil Engineering (HECE), University of Liege (ULg), 4000 Liège, Belgium
*
Author to whom correspondence should be addressed.
Received: 4 August 2018 / Revised: 28 August 2018 / Accepted: 3 September 2018 / Published: 5 September 2018
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Abstract

As with most high-velocity free-surface flows, stepped spillway flows become self-aerated when the drop height exceeds a critical value. Due to the step-induced macro-roughness, the flow field becomes more turbulent than on a similar smooth-invert chute. For this reason, cascades are oftentimes used as re-aeration structures in wastewater treatment. However, for stepped spillways as flood release structures downstream of deoxygenated reservoirs, gas transfer is also of crucial significance to meet ecological requirements. Prediction of mass transfer velocities becomes challenging, as the flow regime differs from typical previously studied flow conditions. In this paper, detailed air-water flow measurements are conducted on stepped spillway models with different geometry, with the aim to estimate the specific air-water interface. Re-aeration performances are determined by applying the absorption method. In contrast to earlier studies, the aerated water body is considered a continuous mixture up to a level where 75% air concentration is reached. Above this level, a homogenous surface wave field is considered, which is found to significantly affect the total air-water interface available for mass transfer. Geometrical characteristics of these surface waves are obtained from high-speed camera investigations. The results show that both the mean air concentration and the mean flow velocity have influence on the mass transfer. Finally, an empirical relationship for the mass transfer on stepped spillway models is proposed. View Full-Text
Keywords: gas transfer; stepped spillways; skimming flows; self-aeration; air-water interface gas transfer; stepped spillways; skimming flows; self-aeration; air-water interface
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Bung, D.B.; Valero, D. Re-Aeration on Stepped Spillways with Special Consideration of Entrained and Entrapped Air. Geosciences 2018, 8, 333.

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