Should the Morison equation or diffraction analysis be adopted in determining wave force on piles?

The choice between the Morison’s equation and diffraction analysis in determining the wave forces on piles depends on the ratio between the diameters of piles to wavelength. If the ratio between the diameter of piles to wavelength is less than 0.2, the Morison equation is usually
recommended. The reason behind this is that the effect of viscosity and separation is significant below this ratio. On the contrary, if the ratio between the diameter of piles to wavelength exceeds 0.2, the waves are scattered with negligible occurrence of separation. As such, diffraction analysis is adopted to calculate the wave forces on piles

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.