The Discusser appreciates such an extensive investigation performed by the Authors that has provided an incentive to further the Discusser's analysis. Yet, comprehensive data analysis made by the Authors was found to be of great help to enlighten the Discusser's understanding of the issue; however, it is thought that irrefragable answer has yet to be achieved. According to the widely held perception, clear-water scour depth around a structure is a function of flow intensity, size of structure, flow depth and shape or geometry factors. Earlier, number of researchers pointed out on the fact of sediment size and grading effect, in particular for clear-water scouring at pier and abutment. Laursen (1999), suggested the consideration of the particle shear and critical tractive force. Raudkivi and Ettema (1982) defined the region in terms of the relative grain size and explained the sediment entrainment mechanism by downflow and horseshoe vortex for each region. Melville (1992) defined an auto-model region in regard to relative sediment size. Likewise, Authors put forward the argument that relative grain size has no effect on scour-hole development for its value more than 25. From the Authors' results, particularly the finer grain size does not appear to have direct effect on maximum scour. In view of inconsistence of the data for flow shallowness, namely for 0.2-2, their combined influence on temporal and maximum scour does not appear to be completely elucidated for its wide implication, e.g. for long river structures. It was found that Authors' relation overestimates the scour depth for the significant range of flow shallowness, viz. 0.02 < y/L < 2. Consequently, an attempt was made to implement the relative grain size like parameter (denoted henceforth by α) that reads as: α = (Ly)~(1/2)/d_(50) where d_(50) is the median grain size, L the length of structure and y the approach flow depth.
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