Apr. » 3 t 1917 
Flow through Submerged Rectangular Orifices 
“3 
The computed discharges agree with the discharges taken from the 
•experimental curves within a maximum error of less than 1 per cent, 
except on the 2 by 4 foot and the 2 by 4.5 foot orifices, where the experi¬ 
mental control was not entirely dependable and somewhat in error. 
The computed discharges agree with the experimental discharge data 
within 1 per cent, except for the large orifices noted above. It is there¬ 
fore safe to assume that this type of orifice and the discharge formula for 
it will give results within 2 per cent of the truth for all cases, and prob¬ 
ably within 1 per cent for the majority of cases. 
Notwithstanding the fact that this type of orifice will permit a meas¬ 
urement of flow with an accuracy well within practical demands and has 
other previously enumerated practical advantages, it must be borne in 
mind that a correction factor will have to be applied to the tables unless 
the depth of water in the channel of approach is 2.5 feet, which is the 
depth upon which the formula and tables are based. Such correction fac¬ 
tors are not only bothersome, but often are a source of grave error; and 
therefore it is desirable to maintain the standard depth of water if pos¬ 
sible. The correction factor is made necessary for a change in depth of 
water in the channel of approach because of a changed velocity of ap¬ 
proach and also because of a changed contraction distance at the top of 
the orifice. Where the standard depth of water, 2.5 feet, can not be 
obtained, or where there is a considerable fluctuation in the depth of 
water, the use of the modified submerged orifice should be discouraged. 
SUPPLEMENTAL TESTS ON SUBMERGED ORIFICES 
Some unusual results were obtained from the orifice experiments made 
in the hydraulic laboratory in the summer of 1914, and because of the 
revolutionary character of those original data, special care has been taken 
to insure their accuracy. Every part of the apparatus and every phase 
of the results that offered a very probable source of error have been 
questioned and examined. The data were consistent with themselves, 
but did not conform to the somewhat arbitrary theory that has grown 
piecemeal from isolated parts of experiments. It was thought probable 
that the gate placed at the end of the channel of recession to control the 
submergence on the orifice was so close to the orifice as to have a marked 
effect upon the flow through the orifice. 
A series of control experiments was made during the summer of 1916, 
in which the orifice structures were duplicates of those used in the original 
experiments, the heads were the same, the same general methods of 
experimentation were used; but the 1916 check experiments were made 
in the concrete rating channel outside the laboratory. The orifice struc¬ 
ture was placed near the middle of the channel, which is 200 feet long, 5 
feet wide, and 3.5 feet deep, and there is no gate or other obstruction in 
the channel within approximately 100 feet of the orifice. Although it 
78372°—17-2 
