Soviet engineers were among the first to propose a stepped concrete chute design on the downstream face of embankment dams under the leadership of P.I. GORDIENKO. The choice of a stepped structure allows the use of individual blocks interlocked with the next elements and the design assists in the energy dissipation (CHANSON 1995, CHANSON 2001, Photo No. 3). For new dams, a stepped spillway made of concrete blocks may be considered as the primary flood release structure of the embankment. The design concept was more recently tested in USA and UK (Photo No. 4 and 5), although it did not prove cost-effective there.

An interesting feature of the concrete block system is the flexibility of the stepped channel bed allowing differential settlements of the embankment. Individual blocks do not need to be connected to adjacent blocks. Another advantage is the short construction time on site. In a typical design, the blocks lay on a filter and erosion protection layer. The layer has the functions to filter the seepage flow out of the subsoil and to protect the subsoil layer from erosion by flow in the drainage layer. Further the protection layer reduces or eliminates the uplift pressures acting on the concrete blocks. Usually a geotextile membrane is laid on the embankment before the placing of the layer, and another covers the protection layer before the installation of the blocks. Suction of the fluid from underneath the concrete steps can be produced by the pressure differential created by the high velocity flow over the vertical face of the step. Drains placed in areas of sub-atmospheric pressure will function to relieve uplift pressures (Photo No. 5).

Hydraulic calculations
The overflow embankment protection system is designed to operate in a skimming flow regime. The steps contribute to a substantial flow resistance and most of the energy dissipation takes place as a form drag process (CHANSON et al. 2000,2002, GONZALEZ 2005).
On the stepped chute, both the flow acceleration and boundary layer development affect the flow properties significantly. The complete flow calculations can be tedious and most backwater calculations are not suitable. CHANSON (1999,2001) proposed a pre-design calculation method which provides a general trend to be used for a preliminary design (Photo No. 6). Ideally, the maximum velocity at the downstream chute end is V_max. In practice the downstream flow velocity V is smaller than the theoretical velocity V_max because of friction losses. In Photo No. 6, the mean flow velocity is plotted as V/V_max versus H₁/d_c where H₁ is the upstream total head and d_c is the critical depth. Both developing flow calculations and uniform equilibrium flow calculations are shown. Fitting curves must be plotted to connect these lines.
In skimming flow, free-surface aeration is always significant (Photo No. 7). It occurs downstream of the inception point of air entrainment, defined as the point of apparition of 'white waters'. It is generally accepted that the inception point occurs when the outer edge of the turbulent boundary layer reaches the surface. Downstream of the point of inception, a layer containing a mixture of both air and water extends gradually through the fluid. The rate of growth of the layer is small and the air concentration distribution varies gradually with distance (CHANSON 1997, 2001). Full details are presented at {http://www.uq.edu.au/~e2hchans/self_aer.html}.

Design considerations
There are two fundamental design rules for precast concrete block systems : a skimming flow in a straight prismatic chute. The step block system was developed for a skimming flow regime : maximum block stability can only be achieved in skimming flows (e.g. BAKER 2000). All but one Russian applications were designed for relatively small discharge capacity : q ~ 3 m2/s. PRAVDIVETS urged that "the alignment of the spillway should be straight from the crest to the toe. Any curvature of the spillway in plan, or change in cross-section, will cause an uneven distribution of flows within the spillway which, in general, should be avoided" (e.g. Photo No. 3).
Usually the channel sidewalls are flat inclined slopes (i.e. trapezoidal spillway cross-section). The slopes of the sidewalls can be designed as inclined stepped surfaces (in the flow direction) and may use the same concrete block system as the main channel (Photo No. 3). Typical sidewall slopes are about 1V:3H (i.e. 18º). A known construction weakness is the joint between the chute invert and the sidewalls. At Brushes Clough, two longitudinal concrete guides were built to facilitate the installation of the blocks and the connection with the stone-pitched sidewalls. At the downstream end, the residual energy of the flow must be dissipated with a small flip bucket arrangement and a conventional concrete pool. Laboratory tests showed high risks of block uplift and failure under a hydraulic jump (BAKER 2000).
The Russian experience with overflow earth dam spillways (chute slope = 9.4 to 26.6º) showed the potential of the concept and highlighted that the quality of the drainage layer is uppermost important. Failure cases were caused by improper drainage revetment. If the drainage requirements are fulfilled, the stepped spillway can sustain large floods and discharges even ice debris. In Siberia, the Magadan experimental dam has resisted for over 15 years without accident. Additional information on design criteria can be found in CHANSON (2001) and BAKER (2000).
GONZALEZ and CHANSON (2007) presented a complete design procedure for embamkment dam stepped spillways.

Discussion on earth dam with precast concrete stepped spillway
Embankment overflow stepped spillways have common features with stepped storm water systems and sabo channel systems (Photo No. 9), which differ significantly from concrete dam stepped spillways. Namely the channel has often a trapezoidal cross-section, the bed slope is moderate (i.e. less than 30 deg.) and the step height ranges from about 0.05 to 0.3 m. Futher strong interactions may occur between seepage and overflow (e.g. CHANSON and TOOMBES 2001, pp. 41-50). Therefore steep stepped chute results cannot be applied (CHANSON and TOMBES 2001, GONZALEZ and CHANSON 2004a,b). CHANSON and TOOMBES (2001, pp. 46-50) further discussed a number of key issues, including the design of the downstream energy disispator. GONZALEZ and CHANSON (2007) developed the particular case of small dams.

Practical considerations
In one case (Brushes Clough dam spillway, Photo No. 4 and 5) numerous acts of vandalism were reported, including destruction of concrete blocks. In practice it is recommended to use concrete blocks heavy enough to avoid their displacement by individuals and to enhance their strength against acts of vandalism. At the limit another construction technique (e.g. RCC overlays) should be selected if man-made destruction cannot be prevented.
In Russia, the cheapest construction method uses precast concrete slabs which are made for the building industry. The slabs are rectangular (3-m long, 1.5-m wide and 0.160-m thick typically) and they are installed in an overlapping arrangement with mild steel spacers. A long-term issue would be the corrosion of the spacers.

Gabion stepped weirs are commonly used for embankment protection, river training and flood control; the stepped design enhances the rate of energy dissipation in the channel, and it is particularly well-suited to the construction of gabion stepped weirs (WUTHRICH and CHANSON 2014). For very-low flow, a porous seepage flow regime may take place, when the water seeps through the gabion materials and there is no overflow past the step edges. At larger flow rates, strong air-water exchanges between seepage and stepped cavity flows are observed, with a complex bubbly seepage motion in the gabions, leading to a modification of the step cavity recirculation and lesser flow resistance (ZHANG and CHANSON 2016). The discharge properties of capped gabion stepped weirs are typically intermediate between the flat impervious and un-capped gabion stepped chute flow properties (WUTHRICH and CHANSON 2015).

Detailed photographs

Photo No. 2 : Chinchilla weir (Chinchilla QLD, Australia 1973) on 8 Nov. 1997 during low overflow. Designed with the assistance of Professor Gordon McKAY. Weir height: 14 m, Crest length: 410m, Spillway capacity: 850 m3/s, Condamine river. The Chinchilla weir is listed as a "large dam" by the International Commission on Large Dams (1984). For further techncial data, see CHANSON, Butteworth-Heinemann 1999, pp. 417-421 & 316. More information at : {http://www.uq.edu.au/~e2hchans/mel_weir.html}.

Photo No. 3 : Zaraysk dam (also called Laraisky), Russia (Courtesy of Prof. Y. PRAVDIVETS). Overflow embankment spillway made of precast concrete blocks.

Photo No. 4 : Brushes Clough dam (1859-1991). 26-m high embankment dam refurbished with a new overflow spillway system in 1991. General view (Courtesy of Mr GARDINER, NWW).

Photo No. 5 : Brushes Clough dam (1859-1991). Details of the precast blocks (120 kg each) and of the drainage holes (Courtesy of Mr GARDINER, NWW).

Photo No. 6 : Relationship between the flow velocity at the end of the chute V, the ideal fluid flow velocity V_max (at end of chute), the total head above spillway toe H₁ and the critical flow depth d_c for a stepped chute (f = 0.2) (after CHANSON 1999,2001 - see also http://www.uq.edu.au/~e2hchans/reprints/errata.htm).

Photo No. 7 : Air entrainment in skimming flow down a 22 degree slope (1V:2.5H) for dc/h = 1.1 (h = 100 mm). Research experiments at the Hydraulics/Fluid Mechanics Laboratory of the University of Queensland.

Photo No. 8 : Nappe flow down a 16 degree slope (1V:3.5H) for dc/h = 0.64 (h = 100 mm). Research experiments at the Hydraulics/Fluid Mechanics Laboratory of the University of Queensland.

Photo No. 9 : Stepped storm waterway East of Okazaki city, Aichi prefecture (Japan) in the middle of a residential area. Photograph taken on 10 Nov. 2001. 

Photo No. 10 : Salado 10 embankment dam and secondary stepped spillway (Courtesy of Craig SAVELA and USDA, Natural Resources Conservation Service; National Design, Construction and Soil Mechanics Center, Fort Worth, Texas).

Photo No. 11 : Choctaw 8A embankment dam and secondary stepped spillway (Courtesy of Craig SAVELA and USDA, Natural Resources Conservation Service; National Design, Construction and Soil Mechanics Center, Fort Worth, Texas).

Photo No. 12 and 13 :  Melton dam overflow stepped spillway (Melton VIC, Australia 1916). The Melton dam is an earthfill structure. Completed in 1916, the dam was heightened twice because of the rapid reservoir siltation. During the last refurbishment in 1994, an overflow stepped spillway was added. The overflow stepped spillway is considered to be the world's largest embankment overflow stepped spillway in terms of total discharge capacity (CHANSON 2001). Photo No. 12 : general view (30 Jan. 2000). Photo No. 13 : details of the dam overflow spillway (30 Jan. 2000).  More about Extreme reservoir siltation ...

Photo No. 14 and 15 : Pedrogao dam, Moura (Portugal, 2006). Completed in March 2006, the Pedrogao dam is a RCC gravity dam (H = 43 m, L = 473 m) with an uncontrolled overflow stepped spillway (h = 0.6 m, 1V:0.75H). The dam is equipped also witha  fish lock/lift. The reservoir is located immediately downstream of the Alqueva dam which is multipurpose reservoir for irrigation (326 km of open channels, 9 main pump stations) and hydropower (2 * 130 MW pump-turbines). Photo No. 14 : view from right bank on 4 Seopt. 2006. Photo No. 15 : view from left bank on 4 Sept. 2006.

Photo No.16 to 18: Joe Sippel weir (Murgon QLD, Australia) - Completed in 1984, the 6.5-m high stepped weir is used for irrigation and water regulation purposes. The structure was built of steel sheet piles and concrete slabs. It is located upstream of the Silverleaf weir. Photo No. 1: in November 1997. Photo No. 2:  on 5 March 2013. Photo No. 3: details of the plung point on 5 March 2013.

References

Additional bibliography

Internet links

CIRIA {http://www.ciria.org.uk/about.htm} CIRIA Report : http://www.ciria.org.uk/publications/pubscat/sp142.htm
USBR Concrete Step Overtopping  Protection{http://www.usbr.gov/wrrl/steps/}

Video movie on YouTube
Stepped Spillway Research - {https://youtu.be/j_AsUXD4D3M}

 

Acknowledgments

License

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 1200 international refereed papers and his work was cited over 10,000 times (WoS) to 26,000 times (Google Scholar) since 1990. His h-index is 47 (WoS), 51 (Scopus) and 79 (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), "Applied Hydrodynamics: an Introduction" (CRC Press, 2014). He co-authored three further books "Fluid Mechanics for Ecologists" (IPC Press, 2002), "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006) and "Fish Swimming in Turbulent Waters. Hydraulics Guidelines to assist Upstream Fish Passage in Box Culverts" (CRC Press 2021). 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), 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 2020 Outstanding Reviewer Award, and the 2021 Outstanding Reviewer Award. The Institution of Civil Engineers (UK) presented him the 2018 Baker Medal. In 2018, he was inducted a Fellow of the Australasian Fluid Mechanics Society. 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 co-chaired the Organisation of the 22nd Australasian Fluid Mechanics Conference held as a hybrid format in Brisbane, Australia on 6-10 December 2020.
His Youtube channel is: {https://www.youtube.com/@Hubert_Chanson}. 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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