In air-water flows, the presence of bubbles hinders the application of traditional velocimetry techniques, such as particle image velocimetry, acoustic Doppler velocimetry and laser Doppler velocimetry, etc. Image-based velocimetry recently received much attention, particularly the optical flow (OF) technique, which detects the apparent motion between consecutive images. In the present study, the optical flow techniques were applied to different high-velocity air-water flows, including stationary hydraulic jump, steady plunging jet flow, and unsteady breaking bore. All experiments were conducted in large-size facilities. A set of parameters was found to work for different air-water flows. The two-dimensional averaged interfacial velocity distributions were obtained, and compared favourably with the intrusive phase-detection probe measurements.