Review of a scour formula

It is suggested by professor Mao and others that the scour formula used in their draft paper (1999) should be universally applicable since it is based on theoretical derivation rather than empirically based. Since such a theory concerning the scour depth near groynes and breakwaters is also of great interest for large engineering works in the Netherlands, a review was carried out of the theoretical background of this formula as presented in a publication by professor Mao and others (1995). The objective of the review is: 1) to comment the two (draft) publications of Mao and others; 2) to answer the following questions: are theoretical foundations of the scour formula of Mao sound?; is the formula universally applicable as suggested in Mao?s 1999 paper?; is the formula easy to be applied by engineers in field?; what is the predictive ability of the formula compared to those due to Breusers, Liu, and Ahmad?