126 
Journal of Agricultural Research 
Vol. IX, No. 4 
in the practical operation of this device, and it is probable that this limit 
is about at the reversing point on the discharge curves made from the 
formula. It was found by experiment that for any given H a after 
a certain H & had been obtained, a further lowering of the water surface 
in the diverging section had no influence upon the elevation of the water 
at the throat gage, H b . 
Table III .—Comparison of theoretical and experimental discharge values of V-notch 
Venturi flume 
H a 
Hb 
m 
{ 
Experi¬ 
mental. 
2 
Com¬ 
puted.** 
C 
0-35 
0.05 
0. 102 
0. no 
°* 93 
•30 
. 10 
. 107 
. 114 
.94 
•95 
•°5 
.805. 
.832 
•97 
.90 
. 10 
•999 
1.052 
•95 
. 80 
. 20 
1.124 
1.165 
•97 
1*55 
•°5 
2. 300 
2. 302 
1.00 
1. 50 
. 10 
2. 925 
3- 023 
•97 
1.40 
. 20 
3 - 525 
3- 669 
.96 
1. 30 
•30 
3. 802 
3- 830 
•99 
a Discharges computed by equation (4), p. 122. 
TRAPEZOIDAL VENTURI FLUME WITH SIDE SLOPES OF i TO i 
This is a special type which was developed to meet a condition com¬ 
mon in some sections of the irrigated West, where a ditch is used to carry 
a small head of water for orchard irrigation at one time and a flow of 
approximately 10 second-feet for alfalfa irrigation at another time. This 
requires a quite flexible measuring device, and therefore called for the 
design shown in figure 11. The side slopes for this type of Venturi flume 
are i to i, and it is expected that it will be built only in the one size; 
that is, with a 6-inch bottom throat width. The discharges through this 
Venturi flume are given in graphic form in figure 12. 
The discharge through the Venturi flume with trapezoidal cross 
section, having side slopes of 1 to 1 in a plane normal to the axis of the 
flume and with a bottom throat width of 6 inches, is represented by the 
following equation, which was derived in a manner similar to that given 
for the V-notch Venturi flume: 
p_ R(#d —Q -°9 #o— 0 - 005 )* + 0 - 0 QI Hg + O. 274 H ,, . ' 
« |_ 0.30 
I 29 H d 
L (¥+H b yH b > 
x (¥+"•)»«■ 
