Due to the European electricity market liberalization and the appearance of other renewable electricity producers, an increase of installed peak power capacity is relevant to provide larger amount of electricity in a shorter turbine duration. When the discharge increase is not too large, it is frequently efficient to place an orifice at the entrance of the existing surge tank. Actually, the surge tank modifications have to be designed case-by-case. The introduction of head losses helps to manage maximum and minimum water level following the management of downstream discharge control and transient events. It allows to keep the same safety level. The placed orifice should commonly produce asymmetric losses. It is important to note that target head losses are evaluated with a unidimensional numerical model which performs transient simulation for relevant flow directions in the whole water way system and hydropower plant. A previous study performed by the authors focused on the effects of sharp-edged orifice parameters. This research carries out the analysis of corresponding elliptical orifices to tested sharp-edged orifices. The goal is to evaluate the head loss modification in both directions due to the shape change.