DRAINAGE OF IRRIGATED LAND. 15 
CUNETTES. 
If the soil be semifluid, so that the banks will not stand, it is neces- 
sary to install a dinette. This is done by driving timber piles at 
intervals of a few feet along both edges of the proposed bottom of the 
canal, and building in a plank waterway between the piles. Such a 
cunette supports the canal banks admirably. Figure 7 shows a 
canal in a semifluid soil, before and after cunetting. 
ENTRANCE OF LATERAL DRAINS. 
Lateral drains should be so located that at their outlets the water 
they discharge will be flowing in nearly the same direction as that 
in the main ditch. Grades of laterals should be adjusted to that of 
the main so as to prevent erosion. If the fall is so great that suf- 
ficiently flat grades can not be secured, drops should be installed. 
At points where waste water from irrigation enters the ditch, flumes 
or drops should be constructed to prevent the water from damaging 
the sides of the ditch and filling the channel with eroded material. 
FLUMES AND BRIDGES. 
Irrigation canals and ditches should be carried across drains in 
carefully constructed flumes or pipe lines of ample capacity, or, if 
their elevations are about the same, one stream must be siphoned 
under the other. Properly constructed bridges should be provided 
wherever crossings are necessary, as culverts are usually not satis- 
factory. At least 2 feet of clearance should be left between the surface 
of the water and the stringers of a bridge, so that floating weeds or 
other debris will not be caught and cause obstruction. No diversion 
dams or similar obstructions should be permitted in the channel. 
PROTECTIVE DEVICES FOR COVERED DRAINS. 
MANHOLES. 
A change in direction of a line of tile should be made gradually by 
a smooth curve, or a manhole should be installed at the point of 
change. If the soil contains much fine sand, a combination manhole 
and sand trap should be located at such a point, as well as at every 
change from a steep to a lighter grade. Such a device serves as an 
observation well in which the flow may be seen and the general con- 
ditions of the system watched. It also serves as a settling basin for 
any sand or silt that may be carried by the drain, and if the trap is 
made to extend a foot or two below the drain, a chamber is formed in 
which a considerable amount of sediment is held until an opportunity 
is afforded for its removal. The manhole may also be provided with 
a surface inlet to enable the drain to take care of surface water, and, 
if desired, to provide for flushing the drain. As a manhole proper 
it provides a means for the operation of a root-cutting or drain- 
