At early stage of a hydroelectric project, 1D transient simulations are performed to determine the basic layout of power plant. In this phase, the design of surge tanks is decisive to achieve good dynamic performances of the power plant, with respect to water hammer and mass oscillations induced by the hydraulic machines for normal, exceptional, and accidental operation. As the head losses between the gallery and the surge tank have strong influence on the transient behavior of the hydraulic system, they are usually optimized by means of 1D transient simulation to avoid low pressure in gallery or surge tank overflow. An asymmetric diaphragm is often placed at the surge tank inlet to achieve the optimum inlet and outlet head losses. Thus, the design of such diaphragm is a challenging task usually performed through an iterative process on reduced-scale model. In this context, 3D CFD simulations can significantly improve the design process to select the appropriate geometry of the diaphragm. In this chapter, head losses coefficients of a surge tank scale model are derived from CFD simulations performed with ANSYS CFX. Results are compared with measurements on reduced-scale physical model and analytical approach. The good agreement of CFD computations with measurements demonstrates that a design optimization with 3D flow simulations can be performed preliminary to scale model tests in order to reduce the number of geometries to be tested to achieve the expected head losses.