STRUCTURES USED IX DRAINING AGRICULTURAL LAND 
25 
up of short sections is used, a good foundation must be provided and 
the joints surrounded with mortar. (See fig. 16 and fig. 7.) End 
walls should be similar to those shown in Figure 15. \Yater must 
be prevented from finding its way along the sides and bottom of the 
structure, and cut-off walls must be provided as shown in Figure 7. 
In some soils it will be necessary to pave the section of the canal 
passing over the culvert with concrete. 
FLUMES AND INVERTED SIPHONS 
Drains frequently cross irrigation or other canals, necessitating 
the construction of flumes. These are commonly used in irrigation 
to carry water across natural depressions, but because the cross sec- 
SECTION ON CENTER LINE 
Fig. 16. — Culvert under canal and connection for flushing open drain, drainage dis- 
trict No. 2, Yakima County, Wash. 
tion of a drainage ditch is very apt to change, greater care is neces- 
sary in the design of substructure, and intake, and outlet structures 
than is common for irrigation requirements. A design for small 
metal flumes is shown in Figure 17 and Plate 2, H and I. Intakes 
and outlets can usually be made similar in form. Figure 17 shows 
a design suitable for sizes to and including Xo. 72. If desirable the 
wing or transition walls for large flumes may be warped. The slope 
of the intake and outlet floors will depend upon available slope and 
change in cross section of the ditch. Usually the fall available in 
such cases is not great and flumes with large cross section will be 
required. For small flumes such structures should be made as sim- 
ple as is consistent with correct hydraulic design. Many failures 
are caused by washing and undermining due to seepage. To provide 
against this and erosion in very loose sandy soils, it may be necessary 
