Method and apparatus for generating a flow through a contained section, e.g. a pipe, below the surface of a body of water. Fields of application include coastal protection and energy production. The contained section is connected to valved mouths located beneath different points of the migrating wave profile, whereby a uni-directional flow is induced due to the differences between the hydrostatic pressures in these points. Various configurations of the system are described, including a perforated pipe enclosed by a flexible hose, which is attached at intervals to the pipe and supplied with slits.