270 



THE IRRIGATION AGE. 



opening depends 

 on the measure- 

 ment of the loss 

 of head, the area 

 of the opening, 

 and the selec- 

 tion of the co- 

 efficient for use 

 in the formula 

 of discharge. 



Measurements 

 of the pressure 

 for such gates 

 are made in the 

 same way as de- 

 scribed for the 

 submerged orifice 

 with fixed open- 

 ing. The best 

 method is to 

 make the meas- 

 urements suffi- 

 ciently far from 

 the gate to 

 avoid any suck- 

 ing of the water 

 on the upper 

 side or rough 

 water below. 

 The pressure 

 sometimes is de- 

 termined by 



measuring down to the water sur- 

 face on the upper and lower sides 

 of the gate. This is a poor method, 

 as the water is liable to be drawn 

 down below its true level above the 

 gate and to shoot out from the gate 

 below. The area of the opening is 

 generally measured between a fixed 

 mark on the gate stem and the top 

 frame of the gate, the mark being 

 placed so that it is even with the 

 top of the frame when the gate is 

 closed. The width is the same for 

 any height of opening. 



The proper coefficient to use in 

 computing the discharge through 

 such orifices is uncertain. Within 

 general limits for any fixed set of conditions, the 

 coefficient is probably nearly constant, but the 

 actual coefficient to use may depend on many varia- 

 bles. For this reason discharge tables are not in- 

 cluded. The values for sharp-edged orifices 0.61 or 

 0.62 have been used by some canal companies. It is 

 certain that these values are too low for the orifices 

 made of either 1-inch or 2-inch lumber. The Yolo 

 Water and Power Co. have adopted a standard 

 form of headgate 3 feet wide with spreading wing 

 walls, the bottom of the gate being 1 vinch above 

 the floor. Experiments made with this gate under 

 the direction of Professor B. A. Etcheverry gave a 

 mean value for the coefficient of 0.73, which has 

 been adopted by the company for use in the de- 

 livery of water. 



Tests of a submerged orifice gate under two 

 conditions were made at Davis. One gate 3 feet 

 wide with the bottom 6 inches above the floor was 



Fig. 17 Drawing of a submerged orifice head gate. 



set at the upper 

 end of the turn- 

 out box divert- 

 ing from a con- 

 crete flume. 

 This gate is sim- 

 ilar in type to 

 the one shown 

 in Figure 15. It 

 was tested with 

 d i s c h arges of 

 from 0.50 to 4.25 

 cubic feet per 

 second. The 

 height of the side 

 walls prevented 

 the use of larger 

 d i s charges, al- 

 though the gate 

 can handle much 

 larger heads. 

 The loss of head 

 was determined 

 by m e a s uring 

 down from a 

 level board above 

 and below the 

 gate and also on 

 the gate. The 

 mean of all meas- 

 urements using 



the level board gave a mean coeffi- 

 cient of 0.80 ; the measurements on 

 the gate gave a mean of 0.72 but 

 were more variable than the others. 

 A similar gate set across the 

 3-foot wide concrete flume of the 

 field laboratory was also tested, giv- 

 ing a mean value of 0.79 for the co- 

 efficient with level board readings. 

 The discharges varied from 1.6 to 

 6.9 cubic feet per second in these 

 experiments. 



From these results it is seen 

 that the coefficient for such meas- 

 uring gates varies with the type of 

 gate. It is possible that the coeffi- 

 cient would have been lower for 

 higher rates of discharge at Davis if such tests 

 could have been made. 



There are several types of these gates in use 

 in California. The gate used by Imperial Co. No. 1 

 is set 4 feet back from the front of the box. In the 

 box used by the Kern County Land Co. the gate 

 is set at the front, flush with the side of the canal. 

 Where the lack of sufficient fall for the use of 

 a better measuring device makes the use of this 

 type of submerged orifice necessary, a standard 

 size and structure should be adopted, and special 

 discharge tables prepared. This should then be 

 rated under the condition in which it will be used. 

 As long as the conditions of use can be maintained, 

 fairly satisfactory measurements can be made. Care 

 should be taken to prevent the deposit of silt or 

 sand near the gates, as this will change the condi- 

 tions of discharge and affect the rating. The 



Fig. IS A submerged orifice head gate 

 in action. 



