GATE STRUCTURES FOR IRRIGATION CANALS. 45 



the dredge would be liable to tear out the gate end of the con- 

 struction. 



Various modifications in the kind of pipe will suggest themselves, 

 depending on the relative cost and the use. The principle will be 

 the same. 



CHECK GATES. 



In a flat country where there is very little fall to the laterals and 

 to the head ditches leading from them, it may be necessary to raise 

 the water at the point of diversion from the canal and raise the upper 

 end of the diverting ditch above the surface of the surrounding coun- 

 try. In this manner grade enough is developed for the diverted water 

 to maintain some semblance of velocity. This condition is met by 

 building a check gate or " check," as it commonly is called, across the 

 canal supplying the water, below the lateral or delivery headgate. 

 This check serves as a bulkhead to check up or completely stop the 

 water in the canal and turn it* through the side gate. 



Some companies maintain a system of deliveries, especially in the 

 smaller canals and laterals, by which the ditch tender makes the first 

 delivery to the consumer farthest down the canal. When his run of 

 water is completed it is desired to turn all the water to the consumer 

 next above him. This is accomplished by the use of checks in the 

 supplying canal below the points of diversion to the consumers. 



Another use for this device is found in the latter part of the season, 

 when there is comparatively little water in the canals. The vents in 

 the check are closed enough so that the level of the water above them 

 is raised and it is possible to deliver the desired volume of water. 



Where the supply of water in the canal from which water is being 

 diverted is subject to great fluctuation, it is possible to assure the level 

 of the water always reaching a known height at least by the insertion 

 of flashboards in a check. This is sometimes accomplished by closing 

 some of the panels with flashboards and some with solid gate shutters, 

 allowing water to flow under the latter. The level of the water above 

 a check which is discharging water over a crest is held more nearly 

 constant in spite of fluctuations than is possible in a check where the 

 water is discharged under gate shutters, for the reason that the dis- 

 charge over the crest varies as the cube of the square root of the head 

 on the crest, while the discharge through the openings under the shut- 

 ter varies as the square root of the head over the opening. In other 

 words, a given fluctuation in the supply will be more quickly cared 

 for over a crest than through an orifice, and the elevation of the water 

 will change less. 



This is a rather important question in the operation of a system, 

 so a case in point may not be out of order. Assume a discharge of 

 3.5 second-feet to be passed through a check. With a submerged ori- 



