In Switzerland, during the last decade, the development of the new renewable energies (small hydro, solar, wind…) has been promoted by a specific feed-in tariff. However, the regulator schedules to remove this specific measure to force the producers to match the demand. Regarding the small hydropower plants, this change requires to increase the flexibility of the power plants. The present work focuses on the hydro power plant of Gletsch-Oberwald (KWGO, Valais, Switzerland), a run-of-river power plant equipped with two six-jet Pelton turbines. An extension of the flexibility of the power plant is investigated in the framework of the SmallFlex project by using one-third of the headrace tunnel as a storage volume. The use of this new storage capacity would require emptying the headrace tunnel and to operate the Pelton turbines at a lower head than the one for which they have been designed. By lowering the head, the efficiency of the Pelton turbines will decrease abruptly if the Pelton turbines make faces to the so-called “falaise” effect. Before performing on-site measurements, unsteady two-phase flow simulations of the interaction between the jet and the Pelton runner have been carried out for four different heads with the Ansys CFX software. At the best efficiency point, the efficiency is well predicted by the simulation. For lower heads, the interaction between two consecutive jets is observed but the decrease in efficiency is underestimated compared to the on-site measurements.