The 2050 Swiss Energy Strategy aims to increase the annual production of hydroelectricity by 4% while 90% of the technically feasible potential is already used. The flexibility of high-head storage power plants (HPP), the main pillar of Swiss peak electricity production, may be improved by either increasing the installed capacity or heightening existing dams in order to concentrate the electricity production during peak demand periods and for the critical winter supply. This paper focuses on Gondo HPP that has been recently subjected to a moderate increase of installed capacity. As the upgrades of existing hydropower plants can lead to more critical mass oscillation between the upstream reservoir and the surge tank, the refurbishment of this plant required a modification of the existing surge tank by installing a throttle at its entrance. The throttle introduces asymmetrical head losses, which coefficients were identified in a previous study through a transient 1D numerical analysis involving critical load cases, followed by an optimization of the geometry by physical modeling. Hybrid modeling using a combination of 1D, 3D numerical and physical modeling is highly recommended to confirm the optimum solution, hence the need for this study to complement previous analyses throughout CFD modeling. The latter has the advantage of modeling the system in real dimensions in addition to visualizing flow patterns in internal sections, which were difficult to examine in the scaled physical model. The developed CFD model confirms the findings of the physical model regarding the head loss coefficients of the throttle. The conducted investigations provide an insight on the real behavior of a hydraulic system equipped with a throttle and are likely to improve the future design of throttled surge tanks. Additionally, they enhance the confidence in 3D numerical modeling which can provide preliminary convenient conclusions prior to physical modeling, decreasing therefore the number of likely costly modifications with the latter.