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

Jorge Matos,Carolina Kuhn Novakoski,Rute Ferla,Eder Daniel Teixeira,Alba Valéria Brandão Canellas,Mauricio Dai Prá,Marcelo Giulian Marques

doi:10.3390/w14030306

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.

Highlights

Flood-control structures designed to release excess water safely from a reservoir, with the aim of preventing water from spilling over the dam crest, are relevant to major hydropower schemes.Since the 1980’s, techniques such as roller compacted concrete (RCC) have greatly renewed the interest in using stepped spillways, namely on large dams
In order to evaluate the development of mean and fluctuating pressure along the chute near the outer edges of the horizontal and vertical step faces, the following parameters were analyzed: (1) mean pressure pm ; (2) root mean square value representative of the pressure fluctuations p0 ; and (3) extreme pressures characterized by px%, x% being the probability of non-exceedance, that is, the probability of occurrence of lower pressure values
Evaluating the fluctuating pressure field on the step faces, along with the conditions leading to the onset of cavitation and protection against cavitation damage, is of utmost importance for a sound design of steeply sloping stepped spillways on large dams

Summary

Introduction

Flood-control structures designed to release excess water safely from a reservoir, with the aim of preventing water from spilling over the dam crest, are relevant to major hydropower schemes. Since the 1980’s, techniques such as roller compacted concrete (RCC) have greatly renewed the interest in using stepped spillways, namely on large dams. Some notable examples of prototype applications can be found elsewhere, namely in [1,2,3]. Various stepped spillways have been incorporated in hydropower schemes, such as the Stagecoach (USA), Petit Saut (French Guayana), Dachaoshan and Shuidong (China), Pedrógão (Portugal), and Dona Francisca, Santa Clara-Jordão, and Anta dams (Brazil).

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