GATE STRUCTURES FOR IRRIGATION CANALS. 31 



flow is divided as nearly proportional as the expense will justify. 

 Greater refinements of division mean greater cost to the device in 

 length of channel, baffle boards, etc. The weir board need not be 

 made sharp for this kind of a box, as the discharge over the square 

 edge is quite proportional to the length of the crest. Even though 

 partially submerged, the two discharges will hold the true propor- 

 tion quite closely. 



When it is desired that no water be turned to the delivery box, 

 then the gate is closed on its hinges and the water passes through the 

 box >and back into the channel below the box, keeping both sides of 

 the division wall clear of silt. The crack under the gate, left so 

 that the gate may swing freely, is closed by a stop board nailed 

 across the channel of the delivery box, as shown. The box contains 

 about 650 feet b. m. of lumber. 



LATERAL HEADGATES. 



It is a difficult matter to draw the line between a lateral headgate 

 on one system and a delivery gate on another. In this publication 

 structures will be classed for the most part in the way they were 

 classed by the companies using the plans in question, but the reader 

 should understand that most of the comments on the conditions of 

 divergence for a lateral gate are applicable also to a delivery gate, 

 turnout, or whatever this class of structures may be called in the 

 particular part of the country in which they are made. 



Lateral headgates divide themselves naturally into two distinct 

 classes — those having essentially a tube of some form through the 

 bank, and those which take an open-box culvert form. The first 

 type preserves the continuity of the surface of the levee for road or 

 other purposes, and the second breaks the levee surface and must be 

 bridged if the levee is to be used for a continuous road. Small lat- 

 erals may be served by either type, but as a rule very large laterals 

 receive their water through the open-box type. 



Another important factor entering the decision as to which type 

 to use, is the relationship between the top of the bank and the canal 

 water. Where the bank crest is more than 5 or 6 feet above the 

 water to be diverted it is better to use the tube type for compara- 

 tively small laterals, as the height of the side walls, with the added 

 detrimental feature of the break in the canal bank, causes greater 

 expense than would be required of a tube delivery. 



The tube form is desirable, especially in cases where the canal 

 occupies a supported position along a hillside and a drop of some 

 form is necessary between the canal and the general level of the 

 land which must support the lateral after it leaves the canal. 



