Engineering Measures for Energy Dissipation Improvements in Existing Spillway Chutes (design development, hydraulic model tests and CFD verifications)

Project name och acronym: Engineering Measures for Energy Dissipation Improvements in Existing Spillway Chutes (design development, hydraulic model tests and CFD verifications)
Project leader: Anders Ansell, Betongbyggnad (Byggvetenskap)
Project group KTH: James Yang, Umar Farooq, Erik Nordström, Anders Ansell and Shicheng Li. Vattenfall R&D Älvkarleby: Mats Billstein, Anna Helgesson and Anders Sjödin.
Industry group: Carl-Oscar Nilsson (Uniper), Gunnar Hellström (LTU), Jonas Persson (Norconsult), Fredrik Marenius (Sweco) and Andreas Halvarsson (WSP)
Project period: 2024-08 – 2025-08
Funder: SVC Svenskt centrum för hållbar vattenkraft

The project deals with innovative chute roughness solutions for enhanced chute energy dissipation. By doing so, the flow impacts downstream are reduced at both low and high flood discharges. Note that the study deoes not deal with energy dissipation in stilling basins, but in spillway chutes. A roughened chute can be used separately or in combination with a stilling basin. Both physical testing and CFD modeling are performed, with the intention to achieve the following objectives.
 Based on previous research findings, to work out and design improved bottom roughness arrangements in spillway chutes;
 Along with a reference solution, to perform experiments and examine their similarities as well as differences in energy dissipation behaviours;
 To clarify turbulence flow structures and efficiency improvement via CFD modeling.
 To enhance the understanding of chute spillway energy dissipation;
 To provide a chance of demonstration in the laboratory;
 The project also provides unique model test data for quality and trust of CFD modeling.
Short background to the research question (a description of the importance of the project)
Reservoir water is often released in chutes, through surface spillway openings, bottom outlets or both. The use of chute bottom roughness is one way to dissipate part of the energy of the flows before discharged back into the river. It reduces the erosion potential in the tailwater and cuts down the dimension of the energy dissipator if such a structure exists in the chute (Peterka 1984, Hager 1992, Robles et al. 2015). A chute may be constructed directly on decent quality bedrock without bottom lining. If so, the natural rock surface (often reinforced with concrete) functions as roughness that acts upon the flow. However, it is a more common practice that artificial roughness elements are built, on either bedrock or concrete lining, for the purpose.

The use of roughened bed in hydraulic jumps is a conventional way to achieve effective energy dissipation on a paved apron (Chow 1959; Leutheusser and Schiller 1975; Hughes and Flack 1984). The design of a stilling basin is strongly dependent on the Froude number of the incoming flow and the tailwater conditions. Bed roughness is a means for modification of the hydraulic jump characteristics to obtain comparable or better performance in a shorter basin length. Perhaps the most common practice is the use of appurtenances of baffle piers or blocks dating from the 1950s (Peterka 1958). In experiments by Hassanpour et al. (2017), roughness elements of lozenge shape in a stilling basin with lateral expansion are studied. Their results show that the combination of the expansion with roughness reduces the required tailwater depth to form a hydraulic jump. At the same conjugate depths, the jump length is appreciably shorter than in a rectangular basin. Another type of roughness is corrugation of different configurations (Ead and Rajaratnam 2002, Akib et al. 2015). The model tests by Izadjoo and Shafai-Bejestan (2007) reveal that, compared with a smooth bed, the corrugated roughness of trapezoidal shape reduces the conjugate tailwater depth by 20% and the jump length by 50%. The effects of a sinusoidal corrugated bed with varying wave steepness are investigated by Abbaspour et al. (2009), demonstrating that the shear stress on the corrugated bed is ~10 times that on a smooth one and the corrugation dissipates extra energy. In numerical simulations performed by Ghaderi et al. (2020), three roughness shapes, i.e., triangular, square and semi-oval, are modeled, indicating that the roughness elements play a role in the reduction of the relative maximum velocity in submerged hydraulic jumps and the triangular shape is most effective in the reduction of the jump length. The study by Evcimen (2015, 2012) refers to prismatically shaped roughness appurtenances.

Previously, in the construction of low-head dams, gabions are used to form step chutes for energy dissipation (Stephenson 1979, Leutheusser and Chisholm 1980, Peyras et al. 1992, Maynord 1995). With the development of roller compacted concrete (RCC) dams, the stepped spillway becomes a common practice as a flood release structure in large dams. To reduce the flow discharge per unit width, its chute can be wide, in some cases close to the length of the dam body. In such a chute, the steps are unique roughness elements that modify flow pressure conditions and enhance air entrainment efficiency, thus effectively improving the energy dissipation and confining the cavitation potential (Sorensen 1985, Chanson 2001; Boes and Hager 2003). Zare and Doering (2012) examine the effects of baffles and sills on steps on flow characteristics, showing that the baffle-edged chute dissipates more energy than the sill-edged one and to shift baffles or sills from the sharp edges deteriorates the energy dissipation. Even step chamfer and cavity blockage, step face inclination angle and step planform affect the flow pattern and turbulence structure, which have bearing on energy dissipation (Chinnarasri and Wongwises 2004). With a milder downstream slope, it is even incorporated over embankment dams (Felder et al. 2012; Felder 2013, Felder and Chanson 2013). Depending on the discharge per unit width and water head, cellular concrete blocks are often used. The precast wedge-shaped blocks form a step-overlay protection and use the steps for energy dissipation and the hydrodynamics to achieve immense stability in high-velocity conditions.

The use of a baffled chute spillway eliminates a costly stilling basin, which allows for a more simplistic overall design. In a broader sense, a stepped spillway is a baffled chute, albeit its steps may appear in a variety of shapes. The most common type is perhaps the Bureau of Reclamation Type IX chute spillway with impact baffle blocks (Bureau of Reclamation 1987). In the Maple River chute spillway, a total of 12 rows of baffle blocks; each row has 7 or 8 blocks with rectangular front. Similar layouts are found in a number of low-head dams in the U.S. (Tullis and Bradshaw 2015). In the U.K. Yeoman Hey Reservoir spillway, a great number of rows of staggered block baffles are erected (Figure 1a). They extend over the whole chute length, with 5 or 6 baffles in each row, producing considerable energy dissipation along the waterway (Rennison 1996). In the study by Robles et al. (2015), several rows of staggered circular pipes are incorporated in a diverging chute spillway to dissipate energy and render the flows to acceptable velocities. Another purpose of those vertically placed risers is to supply air to the offset aerators. In the experiments by Nugroho et al. (2019), multiple rows of cubical blocks are placed on a 45° overflow apron immediately downstream of the spillway threshold. They are equally spaced along its length, with varying lateral and streamwise spacings. They claim that the optimum energy dissipation corresponds to the flow depth equal to the block height and the baffled chute gives rise to a shorter hydraulic jump length. With a stilling basin preceded by a baffled chute, Stojnic et al. (2020) examine the influences of the roughness elements on the terminal flow energy and reveal the design principles for the stilling basin differ from those for a smooth chute (Figure 1b).

Figure 1. Examples of baffled chutes. (a) Yeoman Hey Reservoir spillway (© Paul Anderson). (b) Siah-Bishe upper dam spillway grounded on the bedrock in the abutment of the embankment dam (Courtesy Anton J. Schleiss).

Concerning energy dissipation and ways and means to enhance it, studies of baffled chute spillways have been surprisingly limited, an opinion that is shared by Tullis and Bradshaw (2015). There has not been any project within SVC that deals with energy dissipation in channels. For an existing spillway chute, a number of bottom beams were tested in the Älvkarleby laboratory and implemented successfully in the prototype channel (Yang and Andreasson 2023, Figure 2). The rib width and height finally chosen were 0.4‒0.5 m. They covered the whole chute length and width.
 

Figure 2. The recent successful use of bottom ribs as roughness element in refurbishment of a Swedish spillway
(Yang, James & Andreasson, Per 2023. Rebuilding of a spillway chute for effective energy dissipation. Int. Journal on Hydropower & Dams, Vol. 30, Issue 2).

With this as background data and reference, roughness element layouts that imitate labyrinth and piano key weirs are devised as bed roughness elements. Both physical tests and numerical modeling are performed, and evaluations are made in terms of flow pattern, turbulence features, energy dissipation, etc. The purpose is to explore whether they provide more effective energy dissipation suitable for use in existing spillway chutes.
Compared with many other countries where high dams dominate, we have a large number of low- and medium-head facilities. To blast and excavate in an existing chute is never preferred; to add is much easier. To add chute bottom roughness to existing chutes should be an effective manner to dissipate energy. This can be used separately or in combination with a stilling basin (energy dissipator).