Journal of Water Resource and Protection, 2012, 4, 1051-1060 doi:10.4236/jwarp.2012.412121 Published Online December 2012 (http://www.SciRP.org/journal/jwarp) Modeling the Mangla Dam Spillway for Cavitation and Aerators Optimization * Mohammad Rafi 1 , Akhtar Ali 2# , Ghulam Qadir 3 , Rafaquat Ali 1 1 Water Resources Division, National Engineering Services of Pakistan (NESPAK), Lahore, Pakistan 2 Asian Development Bank, Manila, Philippines 3 WAPDA Model Study Cell, Irrigation Research Institute, Irrigation Secretariat, Lahore, Pakistan Email: # akhtar_rn@yahoo.com Received September 15, 2012; revised October 16, 2012; accepted October 23, 2012 ABSTRACT This study evaluated the effects of increased reservoir conservation level by 40 ft (12.2 m), on spillway velocities; it’s discharging capacity and associated cavitation risk. The study optimized the aerators size and shape to avoid cavitations. The mathematical model was used to estimate the flow velocities and cavitation risk, when scale model study assessed the spillway discharging capacity and optimized the performance of the aerators for modified conditions. The mathe- matical model simulations showed increased flow velocities and damage index for modified conditions. The damage potential was 2 - 3 times higher with modifications and falls within the major to catastrophic region. The scale model study showed that discharging capacity of the spillway can effectively be restricted to original design by raising spill- way crest by 5.0 ft (1.52 m). The scale model study also showed that the two aerators near sluice and at the chute with an air duct pipe of 3.0 ft diameter can improve the free surface flow profile reducing the risks of cavitation. Simulations for several configurations demonstrated clearer affect of aerators ramps on flow trajectory and gate opening. It also de- picted that the height of the ramp of sluice aerator has a positive effect on the flow performance to about 7.5 inches (19 cm), when further increase in the ramp height reduced the flow performance. Keywords: Spillway; Model Studies; Discharging Capacity; Cavitation Risk; Aerators Optimization 1. Introduction High flow velocities can induce cavitations and cause serious damages to the spillways of high dams. Forma- tion of flow bubbles indicates spillway surface deforma- tion [1]. Increasing flow velocities and as a result de- creasing pressures may pass through a critical value ini- tiating cavitation, which is known as incipient cavitation. Conversely, decreasing velocity resulting in increasing pressure may arrive to a point to disappear cavitation and is called desinent cavitation. Surface roughness of spill- way floor and water impurities aggravate cavitations, accelerate damages and can result is spillway failure. Interactions between the flowing water and the at- mosphere may lead to significant air-water mixing and complex multiphase flow situation [2,3]. The cavitation can be prevented either reducing the flow velocity or increasing the flow pressure or with combination of both. The studies on the effect of variable spillway width and invert curvature on the flow pressure for Amaluza dam spillway in USA indicated that dispersion of a small amount of air through water prism can significantly re- duce for the risks of cavitation damage [4]. It was found that about 7.5% of air by volume was needed to stop cavitation damages in a 28-day concrete surface with a compressive strength of 17 mega-Pascals [5]. The re- quired air quantity to protect a spillway surface from cavitation increases with decrease in surface strength [6]. Application of aerators to prevent cavitation damage was successfully tested for Grand Coulee Dam in USA [7]. Bottom aerators are provided when natural aeration of the high velocity spillway chute does not satisfy the minimum air concentration requirements to develop posi- tive pressures. The aerators for the first time were tested at Yellowtail dam following high discharges in 1967 [8]. The minimum air concentration is function of Froude Number [9]. 0 min 90 min 0.015 2 C F F C         (1) * Disclaimer: The ideas and finding presented in this paper are of the authors and do not necessarily reflect the views and policies of the organizations, they belong. # Corresponding author. where Ć 90min is minimum air concentration, F o inflow Froude number and F Cmin is Froude number at the incep- tion point. Copyright © 2012 SciRes. JWARP