This paper is devoted to a theoretical analysis of counter-current gas-liquid wavy film flow between vertical plates. We consider two-dimensional nonlinear waves on the interface over a wide variation of parameters. We use the Navier-Stokes equations in their full statement to describe the liquid phase hydrodynamics. For the gas phase equations, we use the Benjamin-Miles approach where the liquid phase is a small disturbance for the turbulent gas flow. We find a region of the superficial velocity where we have two solutions at one set of the problem parameters and where the flooding takes place. We calculate the flooding dependences on the gas/liquid physical properties, on the liquid Reynolds number and on the distance between the plates. These computations allow us to present the correlation for the onset of flooding that based on the fundamental equations and principles. (C) 2010 Elsevier Ltd. All rights reserved.