Q. Diffrence between Hudsons and Van der Meer formula?

Hudson's formula is usually adopted in preliminary design to attain rough initial estimate of rock size. This formula is derived from the results of regular wave tests. Though this formula doesn't take into account the following elements which Van der Meer formula does: damage level, wave period, permeability of structure and storm duration. Furthermore Hudson's formula deals with the use of regular waves only. 

Compared with Hudson's formula, Van der Meer formula is more complex and it's derived from results of a series of physical model tests. They comprise the consideration of wave period, storm duration, permeability and clearly-defined damage level of structure. The choice of appropriate formula is dependent on the design purpose (which is preliminary design or detailed design).