GATE STRUCTURES FOR IRRIGATION CANALS. 39 



fled. The top of the gate guide rests against the top of a side post 

 and the brace is carried from the angle between the floor and the bot- 

 tom of the post up to the gate guide and at right angles to the latter. 



The sheeting of the wings and cut-offs usually is set vertical and 

 spiked or bolted through a waling strip. On small gates this sheet- 

 ing is made of a single thickness. On large ones it is made triple 

 thickness, each set of three boards being spiked together. The middle 

 board is offset so as to form a tongue-and-groove joint. This " Wake- 

 field " piling, as it is called, may then be driven with a maul or light 

 pile hammer. The lower end of each piling is slightly sharpened at 

 the exposed edge. This causes it to crowd closely to the preceding 

 pile and make a tight joint. 



The depth to which it was necessary to carry the cut-offs depends 

 a great deal on the height to which the water is to be held up by the 

 structure. If the water above and below the gate will be on approxi- 

 mately the same level, say within 1 or 2 feet, then it is not necessary 

 to go more than one-half the depth below grade that the side walls 

 extend above grade, but if more than that amount of pressure is de- 

 veloped the wings and cut-offs should extend as far below grade as the 

 side walls do above. 



High side walls that are separated too far to carry caps as struts 

 are braced diagonally from the floor at about the line of the first 

 division wall. High "A" frames have several diagonal braces, with 

 horizontal sashes to prevent buckling. 



The "A" frame catches trash easily and the water pounds vio- 

 lently if under much pressure, but both these defects may be much 

 remedied by sheeting all the "A" frames on both sides with light 

 boards. If used as a simple lateral headgate or check the upper and 

 lower wings and cut-offs are identical, but if there is a drop in the 

 grade line at the structure the lower posts are carried to the bottom 

 of the water cushion and sheeted horizontally on the earth side as 

 shown in figure 9. 



DELIVERY GATES. 



The same general discussion applies to delivery gates as to lateral 

 gates. In addition it is generally necessary to have some form of 

 locking device. Many companies place a device such as this on all 

 gates, but do not use it unless it is found that the consumer under the 

 delivery gate is abusing the confidence placed in him. In deciding 

 upon a locking device for a delivery gate the designer must know 

 the system of delivery of water. Some companies allow the con- 

 sumer to shut the gate after he has received what water he wishes 

 for that particular irrigation. For such a consumer a lock must be 

 used which permits the gate to be opened to a certain point, deter- 

 mined by the position of the lock, but permits the gate shutter to 



