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

When the water depth affects the properties of waves, we call it as feels bottom. Given the wavelength, we can have an approximation that the ocean floor must be as shallow as the half of the wavelength in order to consider it as feels bottom. Therefore, getting the half of the given wavelength, ocean floor must have an ideal depth of 10 feet. 

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