The surge tank located at the upstream head of penstocks and shafts of hydropower plants is an effective measure to protect the headrace tunnels against high pressure fluctuations. It reflects the transient pressure waves that are generated by the variation of the flow discharge inside the powerhouse. The up- and down-surges inside the tank, as well as, the percentage of the transmitted pressure waves towards the headrace tunnel are significantly affected by the local hydraulic head losses situated at the entrance of the tank. The circular orifice diaphragm generates such a local head loss by contracting the flow inside the connection tunnel between the surge tank and the headrace tunnel. The diameter of the orifice is an important parameter to adjust in order to fulfill the hydraulic design criteria of the projects.The construction works to double the actual 240 MW capacity of the existent pumped-storage power plant FMHL (Forces Motrices Hongrin-Léman SA) in Switzerland are now ongoing. The capacity used will be 420 MW with 60 MW as reserve. This plant belongs to the shareholders: Romande Energie SA, Alpiq SA, Group E and the city of Lausanne. A consortium called “GIHLEM” composed of three international engineering companies Stucky Ltd, EDF and Emch+Berger have been mandated to carry out the final design and the tender documents and to assist the owner during the constructions works. These works include the construction of a new powerhouse and a surge shaft of 7.2 m of diameter connected to the headrace tunnel by mean of a tunnel 28.5 m long with 2.2 m of internal diameter. The numerical calculations of transient flows carried out on the waterway system have shown that additional losses are needed at the entrance of the new surge shaft to keep the transient pressures inside the latter above the minimum value of 10 m W.C. The preliminary value of the interior diameter of a thin steel diaphragm has been determined by using analytical formulations. This diameter has been then optimized by series of physical tests carried out on a model at the geometrical scale factor of 1/18.2 operated respecting the Froude similarity within acceptable Reynolds limits for turbulent flow. The model covers a short reach of the headrace tunnel, the connection tunnel and the surge shaft. Four splits of flow directions at the T-junction between the headrace and the connection tunnels were modeled. For each split, different discharge distributions between the three branches were tested.The first part of this paper presents a short description of the pumped-storage power plant. In the second part, the numerical model and its main results are presented. The third part provides the preliminary design of the diaphragm’s diameter and the fourth part presents the results of the diameter optimization using the physical model tests. The conclusions are given in part five.