DRAIiTAGE OF IRRIGATED LAND. 15 



CUNETTES. 



If the soil be semifluid, so that the banks will not stand, it is neces- 

 sary to install a cunette. 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 BRmGES. 



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 contaiiLs 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 wliich the flow may be seen and tlio general con- 

 ditions of the system watclied. 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 cxtond u foot or two Ix'low t}i(5 drain, a cJianibcr is formed in 

 which a coasid(!rablc amount of scidiment is Jield until an op])ortuniLy 

 is afforded for its removal. The manhole may also be provided with 

 a surfuco inl(!t to enabK^ tli(^ drain to take cure of surface^ water, and, 

 if desired, to provide for llu.siiing the drain. As a maiijioh; ]>ropcr 

 it provides a means for the operation of a root-cutting or drain- 



