128 



BENDS, TRANSITIONS, AND OBSTRUCTIONS 



the most contracted section. In all of Yarnell's comparisons, the value 

 of 6 was taken as 0.3. (It is likely that 6 should vary, approaching 

 zero as the contraction percentage approaches zero, and assuming 

 higher values if the flow is in Class 3.) The coefficient (3, which corrects 



2.2 



2.0 



1.8 

 <x». 



"o 

 $1.6 



3 



1.4 



1.2 



1.0 



10 20 30 40 50 60 70 80 90 100 

 a = Percentage of Channel Obstructed by Bridge Piers 



Fig. 1006. Values of /3 in Nagler Bridge-Pier Formula. 



for the velocity of approach, is considered to vary as shown in Fig. 1006, 

 which is taken from Nagler's paper. 



Values of K in the Nagler formula, for Class 1 and Class 2 flow be- 

 tween various types of bridge piers with difi^erent percentages of con- 

 traction, are shown in Fig. 1007. The values shown are for common 



1.10 



.20 .30 



Channel Contraction, oc 



Fig. 1007. Yarnell's Data on Variation of K in Nagler Bridge-Pier Formula. 



Average values for Class 1 and Class 2 flow. 



types of piers, with length equal to four times the width, set parallel 

 with the current. Tests on longer piers showed that lengths up to 

 thirteen times the width gave values for K less by only 2 per cent. 

 Setting standard piers at an angle of 20 degrees with the current de- 



