You're asking about the wave base, which is the depth at which wave motion essentially ceases. It's not directly determined by just the wave height and wavelength. Here's why and how to determine it:

Wave Base and Factors:

* Wave base is approximately half the wavelength. This is a simplified rule, but it gives a good starting point.

* Water Depth: The wave base is also influenced by the water depth. In shallower water, the wave base will be closer to the surface.

* Wave Period: Longer wave periods (time between crests) lead to deeper wave bases.

In your case, with a 10-foot wavelength, the wave base would be approximately 5 feet.

Caveats:

* The wave base isn't a fixed boundary; wave motion gradually decreases with depth.

* This rule holds best in deep water (where depth is significantly greater than the wavelength). In shallow water, the wave base becomes shallower.

Let me know if you have any other questions!