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 iand 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 tof> 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. 
