A two-dimensional Reynolds type equation is derived and justified for a three-dimensional viscous incompressible flow, taking place between two smooth fixed adjacent curved walls, under intensive percolation. Resulting from the dimension reduction, the model is proved to admit fluxes of order 1 = h 0 with respect to the relative thickness h, a small parameter. Thus, velocities are allowed to be of order h −1 , which is just the case where linear and nonlinear terms have the same asymptotic powers O h −3 . Boundary layers are also taken into account and estimates obtained for the remainder terms in the asymptotic representation formulae.