THE FLOW OF WATER IN DRAIN TILE. 



47 



COMPARISON OF VARIOUS FORMULA. 



Since from a practical standpoint we are interested only in the 

 tile flowing full, velocities as computed by formula 34 are compared 

 with velocities as taken from figures 1 to 12, Plate IX, for the tile 

 flowing full. These results are shown in Tables 8 and 9. The differ- 

 ence is shown, rather than the ratio, between each two determina- 

 tions of velocity, in order that the variations in the low velocities shall 

 be given only equal weight with equal variations in the high veloci- 

 ties. To get the average differences given in Tables 8 and 9, the arith- 

 metical sum of the differences is divided by the number of items. 



5.5 



5.0 



4.5 



B4.0 

 r* 



$|3.0 

 $2.5 



£2.0 

 1.5 

 1.0 

 .5 



Slope m Feet per 100 Feet 

 J> 6 7 .8 .9 1.0 



I -1000 ft. 

 K - 3 ft. 

 Cw= /20 



Williams- Hazen. V= C w FT 



a C/jezy-Kutter.&ZvRs, 



n Elliott. V=48/r?i4D 



* Derived. V= 1 3 8 R * s 2 



Fig. 5. — Comparison of velocities computed by various formulae. 



Both the Poncelet and the Beardmore formulae gave greater dif- 

 ferences when applied to the experimental data than does the ten- 

 tative formula, No. 13. The velocities from the curves in Plate IX 

 were substituted in the Williams-Hazen formula, and the average 

 value of C w , was found to be approximately 120. Using this value 

 in the Williams-Hazen formula, recomputing the velocities, and 

 comparing them with the velocities from the curves in Plate IX, it 

 was found that the average differences were practically the same as 

 the average differences stated at the bottom of Tables 8 and 9. 



A comparison of the velocities computed by the various formula3 

 for one size of tile may be obtained from text-figure 5. This figure 

 shows the velocities for 8-inch tile as computed by the Poncelet, the 

 Williams-Hazen, the Chezy-Kutter (with the coefficient of roughness, 

 n, taken equal to 0.013), the Elliott, and formula 13 herein derived 

 from the experimental data. The observed experimental velocities 

 are also shown. 



