Extreme Pressures and Risk of Cavitation in Steeply Sloping Stepped Spillways of Large Dams

Abstract

Stepped spillways have been increasingly used to handle flood releases from large dams associated with hydropower plants, and it is important to evaluate the fluctuating pressure field on the steps. Hydraulic model investigations were conducted on three 53° (1V:0.75H) sloping and relatively large-stepped chutes to characterize the mean, fluctuating, and extreme pressures acting on the most critical regions of the step faces, near their outer edges. The pressure development along the chutes is presented, generally indicating an increase of the modulus of pressure coefficients up to the vicinity of the point of inception of air entrainment, and a decrease further downstream. The extreme pressure coefficients along the spillway are fitted by an empirical formula, and the critical conditions potentially leading to cavitation on prototypes are calculated. The correlation between the cavitation index and the friction factor is also applied for predicting the onset of cavitation on prototypes, and the results are compared with the pressure data-based method. Generally, the results obtained from those methods yield typical values for the cavitation index in the vicinity of the point of inception, varying approximately from 0.8 to 0.6, respectively. In light of these results, maximum unit discharges of about 15–20 m2/s are considered advisable on 53° sloping large-stepped spillways without artificial aeration, for step heights ranging from 0.6 to 1.2 m. For much higher unit discharges, a considerable reach of the spillway may potentially be prone to the risk of cavitation damage.

Analysis of hydraulic parameters in determining the occurrence of cavitation in the spillways of the Furnas, Luiz Carlos Barreto de Carvalho and Batalha hydroelectric power plants

“…Besides the estimate performed using SpillwayPro for the cavitation indices based on Equation 3, it was decided, for comparative purposes, to determine the index σc by utilizing the friction factor " f " as described in Equations 4 and 5 (Arndt & Ippen, 1968;Falvey, 1990;Frizell et al, 2013;Kermani et al, 2013;Matos et al, 2022). (5)…”

“…It is known that index c, estimated from the friction factor proposed for roughnesses uniformly distributed in turbulent boundary layers was widely used in the studies of Arndt (1981), Falvey (1990), Frizell et al (2013) and Matos et al (2022). Frizell et al (2013) and Matos et al (2022) used Equation 4to estimate the occurrence and location of greatest activity of the cavitation phenomenon in spillways with steep slope steps (1V:0.75H).…”

“…Lee & Hoopes (1996), Wahl et al (2019) and Wahl & Falvey (2022) elaborated a model to predict the occurrence of damage by cavitation utilizing factors that influence the phenomenon: cavitation index, velocity, air concentration, resistance of the material and time of exposure to the action of flow. Many scale model studies and damage analysis at prototype scale attribute the damage generated by the phenomenon of cavitation and/or erosion in hydraulic structures to maximum velocities and minimal extreme instantaneous pressures (Bhate et al, 2021;Matos et al, 2022;Schleiss et al, 2023). Lopardo (1996Lopardo ( , 2013, Hampe et al (2020), Mousavi et al (2020a, 2020b) and Steinke Junior et al (2021 recommends the analysis of pressures with a 0.1% probability of non-exceedance to look at tendencies to cavitation in structures submitted to macroturbulent flows at high velocities.…”

“…Lopardo (1996Lopardo ( , 2013, Hampe et al (2020), Mousavi et al (2020a, 2020b) and Steinke Junior et al (2021 recommends the analysis of pressures with a 0.1% probability of non-exceedance to look at tendencies to cavitation in structures submitted to macroturbulent flows at high velocities. Sanagiotto (2003), Amador et al (2009), Conterato et al (2015), Dai Prá et al (2016, Osmar et al (2018), Canellas (2020), Priebe et al (2021 and Matos et al (2022) analyzed minimum extreme pressures in stepped spillway chutes considering differents ranges of specific discharges, chutes with a slope and step widths. Kermani et al (2013) developed a risk classification for cavitation damage in the stepped spillway of dam of Shahid Abbaspour, in the Iran, based on the flow velocity and the cavitation index from tests on a reduced scale model of 1V: 62.5H.…”

Study on the Hydraulic Characteristics of the Trapezoidal Energy Dissipation Baffle Block-Step Combination Energy Dissipator

“…In terms of the water flow pattern, Chanson [2] and Ohtsu et al [3] proposed the boundary equations of the step nappe flow and skimming flow regime. In terms of step pressure characteristics, Mator et al [4], Sanchez-Juny et al [5], and Amador et al [6] analyzed the pressure variation law on the horizontal and vertical surfaces of the steps, and found that negative pressure is generated on the vertical surface of the step near the convex corner position, and this area of negative pressure is prone to producing cavitation hazards, which was verified in the experiments of Frizell et al [7]. In actual projects, cavitation has also occurred to varying degrees in step spillways, such as the New Croton Dam in the United States, the Dona Francisca Dam in Brazil [8], the Danjiangkou Hydropower Station [9], and the step spillway in the Suofengying Hydropower Station [10] in China.…”