THE IRRIGATION AGE. 



269 



opening is usually 

 made sufficiently 

 large to require 

 as little loss of 

 head as is prac- 

 ticable. Any 

 error in reading 

 this loss of head 

 is thus a larger 

 percentage of the 

 whole than it 

 would be for 

 greater total dif- 

 ferences. 



In the use 

 of the submerged 

 orifice two gage 

 readings are re- 

 quired, one above 

 and one below 

 the orifice. The 

 reading above 

 the orifice should 

 be taken back 

 from the edge of 

 the orifice. In 

 the type of struc- 

 ture shown in 

 figures 13 and 

 14 this can be 

 taken on the 



Fig. 15 Drawings of submerged orifice used by the United States Reclamation Service. 



side wing wall. 



The measurement below the orifice should be taken 

 at least two feet below it, and farther if the dis- 

 charging water is rough. A convenient method of 

 obtaining the difference in the elevation of the water 

 above and below the orifice is to set marks at equal 

 elevations above and below the orifice or to set a 

 board with its top level extending above and below 

 the orifice sufficiently far to give good points for 

 m e a surements. 

 The difference 

 in measurements 

 from this level 

 board to the sur- 

 face of the water 

 above and below 

 the orifice gives 

 the head or 

 pressure under 

 which the water 

 is passing through 

 the orifice. 



The type of 

 orifice described 

 above and illus- 

 trated in figures 

 13 and 14 has 

 been adopted by 

 the U.'S. Recla- 

 mation Service 

 for use where 

 sufficient loss of 

 head is not avail- 

 able for weirs. 

 The data- given 

 here regarding 



the sizes of the 

 structures, and 

 the table of dis- 

 charges (Table 

 4) are taken 

 from the publi- 

 cation of the 

 R e c 1 a m a tion 

 Service on the 

 measurement of 

 irrigation water 

 and from their 

 standard plans 

 for submergud 

 orifices. The cost 

 of one of these 

 devices installed 

 will vary from 

 about $5 to about 

 $15. 



One of the 

 orifices described 

 above, 2.0 feet 

 wide and 0.5 foot 

 high, has been 

 installed at Davis 

 and a series of 

 tests made with 

 d i s c h arges of 

 from 1 to 2.2 

 cubic feet per 

 second. The 



mean of all tests gave a coefficient for use in the 

 formula given with the table of 0.61, which is the 

 same as has been found in other experiments. 



When properly installed this type of sub- 

 merged orifice should give dependable results if the 

 difference in head is correctly measured. Care 

 should be taken to prevent silting in front of the 

 orifice or the catching of drift. 



The sub- 

 merged orifice 

 headgate (Figs. 

 17 and 18) has 

 been used to a 

 large extent on 

 systems where 

 the small loss 

 of head avail- 

 able makes a 

 combination of 

 head gate and 

 measuring device 

 necessary. While 

 all such devices 

 have many points 

 of similarity, dif- 

 ferent canal com- 

 panies have 

 adopted slightly 

 different forms 

 as their stand- 

 ard. 



The accuracy 

 of measurement 

 of water through 

 a s u b m e T ged 



