In fluid dynamics, a wave flume is a laboratory facility for the physical modelling of water waves, in order to study their properties and their effects on coastal structures, offshore structures, sediment transport and other transport phenomena. A wave flume is a special sort of wave tank: the width of the flume is much less than its length. The generated waves are therefore – more or less – two-dimensional in a vertical plane, meaning that the orbital flow velocity component in the direction perpendicular to the flume side wall is much smaller than the other two components of the three-dimensional velocity vector. This makes a wave flume a well-suited facility to study near-2DV structures, like cross-sections of a breakwater. Also constructions providing little blockage to the flow may be tested, e.g. measuring wave forces on vertical cylinders with a diameter much less than the flume width. The waves are most often generated with a mechanical wavemaker, although there are also wind–wave flumes with wave generation by an air flow over the water – with the flume closed above by a roof above the free surface.