How shallow must the ocean floor be for waves to start to "feel bottom" for a wave set with a wavelength of 20 feet?

According to the rules of wave magnitude speed and motion, the magnitude of the orbit of the water molecules is proportional to depth and orbital motion a depth equal to 1/2 of the wavelength. Hence in order to feel bottom, the wavelength necessary should be less than or equal to the 1/2 of the wavelength.

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