Calculations have been performed using the CFDS-FLOW3D computational fluid dynamics code to determine the force exerted on a standing wave by gas flowing over it. It is found that the drag on the wave is mainly due to the pressure variation around it and that it is a strong function of the channel dimensions. Extension of these results to typical flooding conditions in countercurrent flow in vertical tubes shows that the gas velocity required to transport waves upwards increases significantly as the tube diameter increases. It is suggested therefore that the mechanism of flooding depends on the diameter of the tube and that flooding is induced by upward transport of waves from near the bottom of the tube in small diameter tubes whereas in large diameter tubes, it may occur due to entrainment and carryover of droplets near the liquid entry.