High-head power plants are the main pillar of Swiss peak electricity production. With the 2050 Energy Strategy approved by Swiss voters, the annual production of hydroelectricity should increase by 4%, while 90% of the technically feasible potential is already used. The flexibility of high-head storage power plants may be improved by 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,. These upgrades of existing hydropower plants can lead to more critical mass oscillations between the upstream reservoir and the surge tank, which is a hydraulic device allowing dampening of the fast change of discharge and reducing the consequences of water hammer in the pressure tunnel of high-head power plants. A simple way to reduce the amplitudes of the mass oscillations is to place an orifice at the entrance of the surge tank. Three different orifice geometries, chamfered, rounded orifices with a sharp side or two chamfered orifices, were systematically studied in conduits with laboratory experiments and numerical simulations in order to gain deeper knowledge of the behavior of orifices such as the steady and transient head losses, influence and reattachment length and the incipient cavitation number. Steady head losses were evaluated with both approaches. A catalog summarizes the produced head loss coefficients in the two different flow directions as a function of all the geometries. Furthermore, three different empirical relationships were developed in order to predict the head loss coefficient for a sharp, chamfer or rounded approach flows and to design orifices as throttle. On the one hand, the length of the zone disturbed by the orifice has been experimentally evaluated and increases with the orifice opening area. On the other hand, reattachment length has been numerically estimated and does not depend on the presence of a chamfer. Empirical formulas were derived to predict the two characterizing lengths. Transient experiments were performed on chamfered orifices and revealed a clear transient behavior that could account for up to 20% of the steady head losses. The global head losses were higher for accelerate flow and less for decelerate flow than the corresponding steady head losses. The incipient cavitation number was evaluated for chamfered orifices with single-phase computational fluid dynamics (CFD) simulations in order to develop predictive empirical relationships. This allowed for the assessment of the risk of cavitation. A cavitation number predicting the cavitation of the vena-contracta was also determined. A graphical view of the cavitation risk is suggested to evaluate the cavitation risk for surge throttles during mass oscillations. Finally, one-dimensional (1-D) numerical simulations were conducted with the numerical software Hydraulic System on an existing high-head power plant to determine the throttling effects on the whole waterway.