The wave base is the maximum depth at which a water wave's passage causes significant water motion. For water depths larger than the wave base, bottom sediments are no longer stirred by the wave motion above.
In deep water, the water particles are moved in a circular orbital motion when a wave passes. The radius of the circle of motion for any given water molecule decreases exponentially with increasing depth. The wave base, which is the depth of influence of a water wave, is about half the wavelength. At depths greater than half the wavelength, the water motion is less than 4% of its value at the water surface and may be neglected.
There are typically two wave bases, the fair weather wave base (FWWB) and the storm wave base (SWB). The fair weather wave base refers to the depth beneath the waves under normal conditions and the portion of the seafloor that is agitated by this everyday wave action is known as the Upper shoreface. The storm wave base refers to the depths beneath storm-driven waves and can be much deeper. The portion of the seafloor that is only agitated by storm-driven wave action is known as the Lower shoreface.
For instance, in a pool of water 1 metre deep, a wave with a 2-metre wavelength would barely be moving the water at the bottom. In the same pool, a wave with a wavelength of half a meter would not be able to cause water movement on the bottom.
- R. G. Dean and R. A. Dalrymple (1991). Water wave mechanics for engineers and scientists. Advanced Series on Ocean Engineering 2. World Scientific, Singapore. ISBN 978-981-02-0420-4.
- At a depth of half the wave length, the amplitude of the water particle motion by the waves has been reduced to e-π ≈ 0.04 times it value at the water surface.