GATE STRUCTURES FOR IRRIGATION CANALS. 43 
raphy of the country permits, a grade of about 1-J inches to the rod, 
or about 9 inches to 100 feet, is desirable. As a rule, the pipe tubes 
are 20 to 30 1 feet long. 
The concrete or masonry face in which the gate is set should extend 
1 to 3 feet below the bottom of the tube, between 2 and 5 feet on 
either side, and at the top from 1 foot to above the surface of water 
in the canal when the latter is running to capacity. In new con- 
struction where there is little or no vegetation to prevent erosion, 
or in light soil with relatively high velocities in the canal, the larger 
face should be used, but where a gate is being installed in an old 
well-set canal, with the banks well sodded and the permanent side 
of the canal well established, the smaller dimensions may be used 
safely. Such questions as these must always be finally determined 
on the ground. 
The most difficult question and the one that must be settled satis- 
factorily in order to keep down maintenance charges is the general 
position of the face and iron gate. This is not so hard to determine 
on old ditches, but most construction is done at a time when the canal 
banks have not yet assumed their permanent form, and the tendency is 
to place the gate face snugly back in the bank so that a recess results 
when the banks become set. Here a quiet pool is formed where much 
silt is deposited, requiring a great deal of cleaning in addition to 
the annual cleaning of the system. This construction usually results 
in keeping down the initial cost to the extent of saving one or two 
joints of pipe, but the maintenance charges more than make up for 
the amount saved. 
When a new gate is installed in an old canal, the face and gate 
should be set at such a point that the bank of the canal is disturbed 
as little as possible, as any irregularities in the bank cause the deposi- 
tion of silt at some place near by. All things being considered, it 
probably is better to install the face too far into the canal rather 
than not far enough. 
The construction at the outlet end of the tube is governed by many 
of the same conditions that held for the other end. A delivery from 
an old canal into a well-sodded head ditch will require little or no 
concrete or other protection around the outlet end of the pipe, but 
a delivery into a raw earth channel should be protected by concrete 
or rock riprap. 
The picture shown in Plate VIII, figure 3, was taken at a time 
when practically all the water was out of the canal and the clean 
condition in front of the gates indicates that the position of the 
masonry face is about right with relation to the general bank of the 
canal. In this installation it is noticed that the face is set too far 
into the canal rather than not far enough, as is usually the case. 
