Two-phase air-water flows occur frequently in hydraulic engineering. Their characteristics, particularly the depth-averaged air concentrations, are of relevance for design. These must be known, for instance, to provide an adequate chute freeboard, or to avoid chocking of steep tunnel flows. Currently, two main tools are available to estimate the two-phase flow characteristics: physical modeling and numerical simulations. Both are, however, subjected to uncertainties. Physical models are prone to scale effects, whereas numerical simulations are sensitive regarding the code, calibration, as well as the validation of the results