Abstract: Based on the Shields diagram of sediment incipient motion the similar experimental curves given by different scholars are discussed with comparison. The common characteristics of these curves are existing the easiest scour grain diameter of 0.1～0.2 mm and that the grain smaller or greater than this diameter the threshold velocity increased. Then by practical view the empirical formulas of Shields diagram and the similar experimental curve are found out. By comparison of the fine grain （<0.1～0.2mm）formulas of these experimental curves the formula of cohesive soil which the cohesion or shear stress are converted to equivalent grain diameter shows that the calculating results of threshold velocity are conformity each other. But by comparison of the coarse grain（>0.1～0.2）formulas of these curves with the general formula and practical data the discrepancy becomes large. And yet the larger the grain size the inclined large error increased. Finally the method to find the threshold velocity of bed material under abnormal flow pattern is suggested and the formula of threshold velocity under bottom flow pattern for scour defense of riprap stone is given as an example.

Key words:  silt-sand and gravel-stone  ,   threshold velocity  ,  Shields diagram  ,   empirical formula  ,  coarse and fine grains  ,  abnormal flow pattern