variables used in the method. Chao and Hennessy's method involves use of the 

 following equations: 



u„\H, 



2H„ - R r 



for H„ Z R„ 



(58) 



(H-RJ 



2 (H p /R p ) 2 - ( H p /R p ) - 1 

 2 (H D /R D ) 2 - 3 (H D /R D ) + 1 



for H„ ;> R r 



(59) 



T „ f f P<" a 



' K - Z 



(60) 



In the above, q s is the discharge through the scour hole in ft 3 /sec, and u avg 

 is the average current velocity in the flow field. Figure 17 is a plot of the 

 friction factor versus Reynolds number R defined as shown in the figure, 

 which allows determination of f f , the friction factor. In the figure, the 

 roughness factor is defined as 



Roughness factor = — p- xl0~ 

 d„ 



(61) 



with R h the hydraulic radius, approximated by H^-Rp (see Figure 16) 



tnmx. mm 



e «.<■— »o 



6 IteCsw.S'M 



-PRAKOTcs suoom-aouNOwr rojoton 



iW 4 6 8 10? 15 25 4 68(0*13 25 4 6 



.R = 



U Ry 



Figure 17. Friction factor versus grain Reynolds number 



46 



