Design of Spillway Aeration Devices
to prevent Cavitation Damage on Chutes and Spillways
by Hubert CHANSON (h.chanson@uq.edu.au)
M.E., ENSHM Grenoble, INSTN, PhD (Cant.), DEng (Qld), Eur.Ing., MIEAust., IAHR Member, 13th Arthur Ippen awardee
The University of Queensland, School of Civil Engineering., Brisbane QLD 4072, Australia
Presentation
Detailed photographs
References
Footnotes
Related links
Acknowledgments

Itaipu dam spillway Presentation
On chute spillways and bottom outlets, cavitation damage may occur at clear water velocities of between 12 to 15 m/s (1). The damaging effects of cavitation erosion may be reduced or stopped by decreasing the critical cavitation number (e.g. removal of surface irregularities), increasing the cavitation resistance of the material surface (e.g. use of steel fibre concrete), using a combination of the first two methods (e.g. steel lining), directing the cavitation bubble collapses away from the solid boundaries, and inducing flow aeration. With velocities greater than 20 to 30 m/s, the tolerances of surface finish required to avoid cavitation are too severe (FALVEY 1990) and the cost of cavitation resistant materials is prohibitive. For these reasons, it becomes usual to protect the spillway surface from cavitation erosion by introducing air next to the spillway surface (Figure 2) using aeration devices located on the spillway bottom and sometimes on the sidewalls (Figure 1) (CHANSON 1997, Chap. 17).

Cavitation and air entrainment
Cavitation is defined as the explosive growth of vapour bubbles. It involves the entire sequence of events : bubble formation extending to bubble disappearance. After formation, cavitation bubbles may be carried away into regions of higher local pressures, before disappearing by collapse. Cavity collapses generate extremely high pressures in their immediate vicinity. In presence of gas content, flows may cavitate at higher static pressures and substantial quantities of air produce a large reduction in damage rate. RUSSELL and SHEEHAN (1974) suggested that entrained air is effective because : (1) the presence of air in the vapour cavities will cushion the cavity collapse and reduce the resulting water hammer pressure and (2) air bubbles present in the surrounding fluid will reduce the shock wave celerity and the magnitude of the shock waves on the material surface. The presence of air bubbles within the flow might also affect the collapse mechanisms, re-directing the water hammer jets away from the solid boundary. Tests on concrete specimens and on prototype spillways showed that cavitation damage on concrete chute spillways can be prevented if aeration provide mores than 4 to 8% of air next to the channel invert.

Operation of a spillway aeration device
Aeration devices (2) are designed to introduce artificially air within the flow upstream of the first location where cavitation damage might occur (Figures 3 and 4) . Aerators are designed to deflect high velocity flow away from the chute surface (Figure 4). The waters taking off from the deflector behave as a free jet with a large amount of interfacial aeration. The basic shapes of aerators are a ramp, an offset and a groove. The ramp and the offset tend to deflect the spillway flow away from the chute surface. In the cavity formed below the nappe, a local subpressure is produced by which air is sucked into the flow (e.g. LAALI and MICHEL 1984, CHANSON 1990). Usually a combination of the three basic shapes provides the best design : the ramp dominates the
operation at small discharges, the groove provides space for air supply, the offset enlarges the jet trajectory at higher discharges.
The main flow regions above a bottom aeration device are : (1) the approach flow region which characterises the initial nappe flow conditions, (2) the transition region which coincides with the length of the deflector, (3) the aeration region, (4) the impact point region and (5) the downstream flow region (Figure 5). CHANSON (1989a,b) presented detailed measurements of free-surface aeration along a spillway aerator model and in the downstream flow. Air bubbles are redistributed downstream of an aeration device as in self-aerated flows and there is a complete analogy between the flow downstream of an aerator and self-aerated flows. This similarity was demonstrated first by CHANSON (1989b) and it is now well recognised (e.g. FALVEY 1990).
Foz do Areia dam spillway
Discussion
The quantity of air supplied by the air ducts is not always an important design parameter in term of aerator efficiency. In fact the total quantity of air entrained above an aerator is related to the interfacial aeration at both upper and lower nappes, rather the air supply. CHANSON and TOOMBES (2002) suggested that a stepped invert may be more efficient on flat chutes.
The practice to design a large number of air inlets is completely empirical and un-economical. In one instance (i.e. Nurek tunnel spillway), aerators had to be shut down to reduce the excessive aeration in the tunnel. In another case, calculations suggest that two aerators instead of three would have protected the entire spillway length from cavitation damage and at a cheaper cost.
The contribution of the downstream free-surface aeration is an uppermost important parameter, often neglected by design engineers. The optimum location of the first aerator and the required aerator spacing depend essentially upon the free-surface aeration potential. In the downstream flow region the air content tends to the uniform equilibrium air concentration for the channel slope (see Self-aeration studies). Basic design recommendations (CHANSON 1989b) include :
+ On a steep spillway (slope > 20 deg.), the air concentration distribution downstream of an aerator will tend to the equilibrium air concentration distribution (Cmean > 30 %). If the average air concentration at the start of the downstream flow region is high enough, all the length downstream of the first aerator will be protected  and no additional aerator will be required as long as the invert slope is greater than 20 degrees.
+ If the spillway slope becomes lower than 20 degrees or for a flat spillway (slope < 20 deg.), the flow may be de-aerated and an additional aerator will be required when the depth-averaged air concentration becomes lower than 30 %.
These results are important and they suggest the following design recommendations :
(A) For steep spillways, the first aeration device must be located near the upstream end of the channel to 'trigger' the free-surface aeration process and to use self-aeration in the downstream flow region to maximise air entrainment. All the spillway length downstream of the first aerator is protected from cavitation damage and no additional aerator is required as long as the channel slope is larger than 20 degrees.
(B) On small-slope chutes, the first aerator must be located immediately upstream of the potential cavitation inception location (in absence of aerators). Additional aerator(s) are required when the bottom air concentration downstream of the aerator falls below 4-8 % (i.e. Cmean < 0.30).

Practical considerations
The designers of aeration devices must : (1) avoid the aerator submergence (or cavity filling), (2) limit the cavity subpressure to reasonable values and (3) limit the air velocities in the air inlets. CHANSON (1995) developed a method to predict the risk of cavity filling. FALVEY (1990) suggested that the cavity subpressure should be less than one tenth of the critical pressure ratio for sonic velocity to prevent excessive noise. To avoid the effect of compressibility, the air velocities in the vents should be less that 100 to 120 m/s. Altogether these considerations may be more important when designing an aeration device than the maximisation of the quantity of air supplied by the air ducts.
 

Footnotes

(1) Examples of major spilway damage by cavitation include Aldea-Davilla dam (Portugal, 1966), Yellowtail dam (USA 1967), Tarbela dam (Pakistan, 1974), Karun dam (Iran, 1977-1993), Glen Canyon dam (USA, 1983). In most cases, a spillway aeration devices was installed as part of teh remdial measures. FALVEY (1990) presented detailed case studies.
(2) Spillway aeration devices are also called air slots in North-America.
 

Detailed photographs

Photo No. 1 : Foz do Areia dam spillway (Brazil), H = 160 m, spillway slope: 14.5 deg. slope, design discharge: 11,000 m3/s, chute length: 400 m. Spillway in operation (flow from bottom to top) (Courtesy of Prof. N. PINTO).
Photo No. 2 : Itaipu dam spillway (Brazil/Paraguay), H = 196 m, Parana river, completed in 1982, spillway slope: 10 deg., W = 345 m, design discharge: 61,400 m3/s (Courtesy of Prof. N. PINTO). Spillway in operation (flow from bottom to top). Note the air duct intakes on the left wall although the air slots were not built across the spillway invert. Strict tolerance criteria were applied to the concrete surface lining during construction and aeration devices were deemed unnecessary.
Photo No. 3 : Clyde dam spillway (NZ) during construction in March 1988. Note the scaffolding in the spillway aeration device.
Photo No. 4 : Spillway aeration device model : 1/15 scale model of the Clyde dam spillway aeration device; flow conditions : Fr = 6.5, V=5.8 m/s, d =85 mm. (See also CHANSON 1989a, 1989b, 1990)
Photo No. 5 : Sketch of a spillway aeration device (after CHANSON 1997).
Photo No. 6 : Chungju dam, Korea. Completed in 1985, the concrete gravity dam is 97.5 m high, 447 m long and it is equipped with 4 100-MW turbines. Located on the South Han river, the reservoir is multipurpose: flood control, hydropower and water supply. Photo No. 6A : details of the Chungju dam spillway on 14 Sept. 2005. Photo No. 6B : Details of the spillway aeration devices; the second bottom slot is unnecessary. Photo No. 6C : Detail of the first aeration slot, located at the downstream end of the gate piers.
 

Related links

{http://www.uq.edu.au/~e2hchans/photo.html} Gallery of photographs
{http://www.uq.edu.au/~e2hchans/self_aer.html} Self-aeration in chutes and spillways
{http://www.itaipu.gov.br/homeing.htm} Itaipu dam

Air bubble entrainment in turbulent shear flows References

[1] CHANSON, H. (1997). "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows." Academic Press, London, UK, 401 pages (ISBN 0-12-168110-6).
[2] FALVEY, H.T. (1990). "Cavitation in Chutes and Spillways." USBR Engrg. Monograph, No. 42, Denver, Colorado,USA, 160 pages.
[3] CHANSON, H. (1989). "Flow downstream of an Aerator. Aerator Spacing." Jl of Hyd. Res., IAHR, Vol. 27, No. 4, pp. 519-536. (PDF Version at EprintsUQ)
[4] CHANSON, H. (1989). "Study of Air Entrainment and Aeration Devices." Jl of Hyd. Res., IAHR, Vol. 27, No. 3, pp. 301-319. (PDF version at EprintsUQ)
[5] LAALI, A.R., and MICHEL, J.M. (1984). "Air Entrainment in Ventilated Cavities : Case of the Fully Developed 'Half- Cavity'." Jl of Fluids Eng., Trans. ASME, Sept., Vol. 106, p.319.
[6] CHANSON, H. (1990). "Study of Air Demand on Spillway Aerator." Jl of Fluids Eng., Trans. ASME, Vol. 112, Sept., pp. 343-350. (download PDF file)
[7] RUSSELL, S.O., and SHEEHAN, G.J. (1974). "Effect of Entrained Air on Cavitation Damage." Can. Jl of Civil Engrg., Vol. 1, pp. 97-107.
[8] CHANSON, H. (1995). "Predicting the Filling of Ventilated Cavities behind Spillway Aerators." Jl of Hyd. Res., IAHR, Vol. 33, No. 3, pp. 361-372. (Download PDF file)
[9] CHANSON, H. (1999). "The Hydraulics of Open Channel Flows : An Introduction." Butterworth-Heinemann, London, UK, 512 pages (ISBN 0 340 74067 1). CHANSON, H. (2002). "Hidraulica Del Flujo De Canales Abiertos", McGraw Hill Interamericana, División Universidad,  Columbia (ISBN: 958-410-256-7) (in Spanish).
[10] CHANSON, H., and TOOMBES, L. (2002). "Energy Dissipation and Air Entrainment in a Stepped Storm Waterway: an Experimental Study." Jl of Irrigation and Drainage Engrg., ASCE, Vol. 128, No. 5, pp. 305-315 (ISSN 0733-9437). (Download PDF File)

Video movies on YouTube
Physical Modelling of Air Bubble Entrainment in Vertical Planar Plunging Jets - {https://youtu.be/GcAiBD4LpwM}
Stepped Spillway Research - {https://youtu.be/j_AsUXD4D3M}

Bibliography

  CHANSON, H. (1988). "A Study of Air Entrainment and Aeration Devices on a Spillway Model." Ph.D. thesis, Dept. of Civil Engineering, University of Canterbury, Christchurch, New Zealand. (PDF version at EprintsUQ) Order Form
   CHANSON, H. (1991). "Aeration of a Free Jet above a Spillway." Jl of Hyd. Res., IAHR, Vol. 29, No. 5, pp. 655-667 & Vol. 29, No. 6, p. 864 (ISSN 0022-1686). (PDF Version at EprintsUQ)
   CHANSON, H. (1993). "Velocity Measurements within High Velocity Air-Water Jets." Jl of Hyd. Res., IAHR, Vol. 31, No. 3, pp. 365-382 & No. 6, p. 858 (ISSN 0022-1686). (Download PDF file)
   CHANSON, H. (1993). "Model Studies of the Aeration Device of the Clyde Dam Spillway." ANCOLD Bulletin, No. 94, pp. 29-45. (Record at UQeSpace) (PDF file)
   CHANSON, H. (1994). "Aeration and De-aeration at Bottom Aeration Devices on Spillways." Can. Jl of Civil. Eng., Vol. 21, No. 3, June, pp. 404-409 (ISSN 0315-1468). (Download PDF file)
   CHANSON, H. (2004). "Environmental Hydraulics of Open Channel Flows." Elsevier-Butterworth-Heinemann, Oxford, UK, 483 pages (ISBN 0 7506 6165 8).
   CHANSON, H. (2004). "Air Entrainment in Hydraulic Engineering." in "Fluvial, Environmental & Coastal Developments in Hydraulic Engineering", Balkema, Leiden, The Netherlands, Proc. International Workshop on State-of-the-Art Hydraulic Engineering, 16-19 Feb. 2004, Bari, Italy, M. MOSSA, Y. YASUDA and H. CHANSON Ed., pp. 17-63 (ISBN 04 1535 899 X). (PDF version at EprintsUQ)
   CHANSON, H. (2013). "Hydraulics of Aerated Flows: Qui Pro Quo?" Journal of Hydraulic Research, IAHR, Invited Vision paper, Vol. 51, No. 3, pp. 223-243 (DOI: 10.1080/00221686.2013.795917) (ISSN 0022-1686). (Postprint at UQeSpace) (Reprint) (PDF file)
   HAGER, W.H. (1992). "Spillways, Shockwaves and Air Entrainment - Review and Recommendations." ICOLD Bulletin, No. 81, Jan., 117 pages.
   KRAMER, K. (2004). "Development of Aerated Chute Flow." Ph.D. thesis, VAW, ETH-Zürich, Switzerland, 178 pages. (also Mitteilungen der Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie, ETH-Zurich, Switzerland, No. 183.)
   LOW, H.S. (1986). "Model Studies of Clyde Dam Spillway aerators." Research Report No. 86-6, Dept. of Civil Eng., Univ. of Canterbury, Christchurch, New Zealand.
   PINTO, N.L. de S. (1984). "Model Evaluation of Aerators in Shooting Flow." Proc. Intl Symp. on Scale Effects in Modelling Hydraulic Structures, IAHR, Esslingen, Germany, H. KOBUS Editor, pp. 4.2-1/4.2-6.
   VISCHER, D., VOLKART, P., and SIGENTHALER, A. (1982). "Hydraulic Modelling of Air Slots in Open Chutes Spillways." Intl. Conf. on the Hydraulic Modelling of Civil Engineering Structures, BHRA Fluid Engineering, Coventry, UK., pp. 239-252.
   VOLKART, P., and CHERVET, A. (1983). "Air Slots for Flow Aeration." Mitteilungen der Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie, ETH-Zurich, Switzerland, No. 66.
   VOLKART, P., and RUTSCHMANN, P. (1984). "Air Entrainment Devices (Air Slots)." Mitteilungen der Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie, ETH-Zurich, Switzerland, No. 72.
    TOOMBES, L., and CHANSON, H. (2007). "Surface Waves and Roughness in Self-Aerated Supercritical Flow." Environmental Fluid Mechanics, Vol. 5, No. 3, pp. 259-270 (DOI 10.1007/s10652-007-9022-y) (ISSN 1567-7419 (Print) 1573-1510 (Online)). (PDF file at UQeSpace)

Acknowledgments

The writer acknowledges the assistance of Professor N.L. de S. PINTO and Companhia Paranaense de Energia, and Yann CHACHEREAU.

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Hubert CHANSON is a Professor in Civil Engineering, Hydraulic Engineering and Environmental Fluid Mechanics at the University of Queensland, Australia. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations. His publication record includes over 850 international refereed papers and his work was cited over 4,500 times (WoS) to 16,000 times (Google Scholar) since 1990. His h-index is 35 (WoS), 40 (Scopus) and 62 (Google Scholar), and he is ranked among the 150 most cited researchers in civil engineering in Shanghai’s Global Ranking of Academics. Hubert Chanson is the author of twenty books, including "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995), "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows" (Academic Press, 1997), "The Hydraulics of Open Channel Flow : An Introduction" (Butterworth-Heinemann, 1st edition 1999, 2nd editon 2004), "The Hydraulics of Stepped Chutes and Spillways" (Balkema, 2001), "Environmental Hydraulics of Open Channel Flows" (Butterworth-Heinemann, 2004), "Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: Theory And Observations" (World Scientific, 2011) and "Applied Hydrodynamics: an Introduction" (CRC Press, 2014). He co-authored two further books "Fluid Mechanics for Ecologists" (IPC Press, 2002) and "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006). His textbook "The Hydraulics of Open Channel Flows : An Introduction" has already been translated into Spanish (McGraw-Hill Interamericana) and Chinese (Hydrology Bureau of Yellow River Conservancy Committee), and the second edition was published in 2004. In 2003, the IAHR presented him with the 13th Arthur Ippen Award for outstanding achievements in hydraulic engineering. The American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) presented him with the 2004 award for the Best Practice paper in the Journal of Irrigation and Drainage Engineering ("Energy Dissipation and Air Entrainment in Stepped Storm Waterway" by Chanson and Toombes 2002) and the 2018 Honorable Mention Paper Award for  "Minimum Specific Energy and Transcritical Flow in Unsteady Open-Channel Flow" by Castro-Orgaz and Chanson (2016) in the ASCE Journal of Irrigation and Drainage Engineering. The Institution of Civil Engineers (UK) presented him the 2018 Baker. Medal. Hubert Chanson edited further several books : "Fluvial, Environmental and Coastal Developments in Hydraulic Engineering" (Mossa, Yasuda & Chanson 2004, Balkema), "Hydraulics. The Next Wave" (Chanson & Macintosh 2004, Engineers Australia), "Hydraulic Structures: a Challenge to Engineers and Researchers" (Matos & Chanson 2006, The University of Queensland), "Experiences and Challenges in Sewers: Measurements and Hydrodynamics" (Larrate & Chanson 2008, The University of Queensland), "Hydraulic Structures: Useful Water Harvesting Systems or Relics?" (Janssen & Chanson 2010, The University of Queensland), "Balance and Uncertainty: Water in a Changing World" (Valentine et al. 2011, Engineers Australia), "Hydraulic Structures and Society – Engineering Challenges and Extremes" (Chanson and Toombes 2014, University of Queensland), "Energy Dissipation in Hydraulic Structures" (Chanson 2015, IAHR Monograph, CRC Press). He chaired the Organisation of the 34th IAHR World Congress held in Brisbane, Australia between 26 June and 1 July 2011. He chaired the Scientific Committee of the 5th IAHR International Symposium on Hydraulic Structures held in Brisbane in June 2014. He chairs the Organisation of the 22nd Australasian Fluid Mechanics Conference in Brisbane, Australia on 6-10 December 2020.
 His Internet home page is http://www.uq.edu.au/~e2hchans. He also developed a gallery of photographs website {http://www.uq.edu.au/~e2hchans/photo.html} that received more than 2,000 hits per month since inception.

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