Mar. 13,1916 
A New Irrigation Weir 
1141 
Table III .—Difference between discharges computed from the formula 
Q—I3S3— o.o 7 L]LH( 1 * 62+0 * 01L ) and those obtained by experiment , 
for the new type of weir —Continued 
4-foot weir 
Head. 
Observed Q 
corrected true 
for length. 
Computed Q. 
Difference in Q. 
Percentage of 
difference. 
Feet . 
0. 200. 
I. 148 
I -153 
+0. 005 
+O.44 
. 3 QI . 
2. 188 
2. 182 
— . 006 
- .27 
•399 . 
3-417 
3 - 387 
- .030 
- .88 
. 500. 
4. 806 
4.817 
+ . Oil 
+ -23 
. 601. 
6. 427 
6. 417 
— . 010 
— . 16 
. 700. 
8.158 
8.141 
— . 017 
— . 21 
• 799 . 
10. 045 
10.006 
" -039 
“ -39 
. 900. 
12. 081 
12. 047 
- .034 
- .28 
1.000. 
14.194 
14. 200 
+ . 006 
+ . 04 
1.100. 
16. 426 
16. 476 
+ 
0 
cn 
O 
+ • 3 ° 
SPECIFICATIONS FOR CONSTRUCTION AND USE OF THE NEW 
IRRIGATION WEIR 
A plan and elevation of the standard weir is shown in figure 16. The 
weir notch is rectangular in form, with sharp crest and sides. The floor 
of the weir box must be level with the crest, and it is therefore convenient 
to use an angle iron for the crest, embedding one face of the angle until 
flush with the surface of the floor, the other face of the angle extending 
downward. The sides of the weir notch may also be made of angle iron 
placed in a vertical position, with one end extending below the crest and 
one face of the angle against the angle-iron crest. The angle can then 
be attached to the weir bulkhead through holes placed in the other face. 
This arrangement is durable and inexpensive and will meet the require¬ 
ment of sharp crest and full lateral expansion for the escaping stream 
of water. The grade of the canal downstream from the weir must be low 
enough to give free fall and complete aeration to the nappe. 
The floor of the weir box must be level throughout, and there must be 
no sudden or decided differences in elevation between the floor and the 
grade of the channel of approach. The weir box must be placed in the 
center of the ditch, so the axial line of the box corresponds with the axial 
line of the canal, in order that the water may enter the weir box in straight 
lines. The width of the weir box must be twice the length of the weir 
crest (2 L) at the plane of the weir, and two and a half times the length 
of the weir crest (2 % L) at a distance of 20 feet upstream from the plane of 
the weir. The standard tests were made with a weir box 32.5 feet long, 
except for the 4-foot weir, No. 34, Table I, and the sides were extended 
at the angle indicated above. However, from Table I, Nos. 7, 8, and 9, 
