Seiche and clapotis demonstrated

YouTube link. 

A standing wave is created in a wave tank by fixing a wall at one end and moving the other wall—the wave generator—at a frequency such that the outgoing waves are superposed on those reflecting back from the wall. This doubles the amplitude of the wave. In the standing wave (also called clapotis), the surface rises and falls in a mirrored pattern: troughs become crests become troughs and so on. When the wave generator is turned off, the standing wave’s energy dissipates and eventually the tank stills. The sloshing motion that persists in the meantime is known as a seiche, which commonly occurs in nature in lakes, seas, bays, and any partially enclosed body of water. 

Not a very exciting or interesting video, unless you have a little interest or curiosity about fluid dynamics.  From the appropriately named Fuck Yeah Fluid Dynamics blog.