GATE STRUCTURES FOR IRRIGATION CANALS. 5 



Large cast gates are lifted by powerful one or two man gate stands, 

 or, in some cases, by electric or water power. These stands are 

 equipped with ball-bearing wheel and screw, bevel gearing and screw, 

 or worm and wheel and screw. The waste gates on the Milner dam 

 in Idaho are regulated by a traveling hoist equipped with electric 

 power and running on a track set back of the gate guides. This device 

 is used in many cases where there is a battery of gates, but one hoist- 

 ing apparatus being provided. This is transferred from one stem to 

 another and the adjustments made. The hoist may then be removed 

 entirely and locked in a near-by tool house, thus preventing passers-by 

 from tampering with the gates. 



FLASHBOARDS. 



The flashboard is the form most used for an overflow gate, such as 

 most check and some types of waste gates. With boards a definite 

 crest is maintained over which any excess water will pass. Altera- 

 tions in this crest height may be made in steps gaged by the width of 

 the boards. Where there are several panels in a gate structure the 

 general water level may be regulated by flashboards and more delicate 

 adjustments made by a solid gate shutter of either the groove or 

 radial form. 



Primarily a set of flashboards should be used only where leakage is 

 of little or no consequence unless measures to prevent leakage are 

 adopted. In California double sets of flashboards sometimes are used 

 for floodgates and the spaces between them are packed with mud dur- 

 ing such times of the year as they are not being adjusted. The water 

 issues from a structure regulated by flashboards with less velocity 

 than it does from an undershot gate, and the resulting erosion below 

 the structure is less. One advantage of the type shown in figure 8 

 (p. 37) is that the whole shutter may be lifted by the stem, giving the 

 advantage of more delicate adjustment and still allow some water to 

 pour over the upper flashboards. This water will fall on and tend to 

 break the force of whatever water is passing under the gate. 



The disadvantage of flashboards lies in the difficulty of operation 

 and the time this requires. The jamming of a board may be reduced 

 by rounding off the corners of each board as shown in Plate I, h. 



Where used in a check gate flashboards develop a very bad feature 

 of maintenance. If a delivery of water is not being made above the 

 check, then all of the vents should be " pulled " and the water allowed 

 to flow as nearly as possible in the same manner that it does when 

 there are no gates in the canal, thus scouring out any deposit which 

 has accumulated while the water was checked up. Unless the ditch 

 tender is exceptionally conscientious he will " pull " one or two vents, 

 causing an excessive scour through the openings pulled, and leaving 



