You're asking about the wave base, which is the depth at which water movement caused by a wave is negligible.

Here's the key point: The wave base is approximately half the wavelength of the wave.

Therefore, for a wave with a wavelength of 15 feet, the wave base would be approximately:

15 feet / 2 = 7.5 feet

Important Note: This is a simplified explanation. The actual wave base can be influenced by factors like:

* Water depth: In shallower water, the wave base is closer to the surface.

* Wave period: Longer wave periods result in deeper wave bases.

* Seabed topography: Irregularities in the seabed can affect the wave base.

Let me know if you have any other questions!