The Snowy 2.0 project is a large-scale pumped storage facility located in the Snowy Mountains in the SouthEastern part of New South Wales (Australia). It is an extension of the existing Snowy scheme, linking Tantangara (upper) and Talbingo (lower) reservoirs through 27 km of tunnels as well as an underground pumped hydropower station housing six reversible Francis pump-turbine and motor-generator units. The project will provide an additional generation capacity of 2'200 MW for the Snowy 2.0 scheme and is characterized by a maximal gross head of 694.35 m, a maximum design discharge of 385 m3/s in generation mode and 280 m3/s in pumping mode. The scheme includes two surge shafts: the lower and the upper ones. This manuscript presents the results of a study conducted to assess the hydrodynamic behavior of the orifice to optimize and validate the geometry of the surge shafts by means of numerical and physical modeling. It focuses on the methodology applied to first use numerical modeling to evaluate the design and the head loss, and second conduct laboratory tests on a physical model for the results comparison and validation. Results show a good agreement between numerical simulations and laboratory tests.