DRAINAGE OF IRRIGATED LAND. 5 
The size of drain required depends upon the amount of water to 
be carried, the slope of the canal, the condition of the channel, and 
the shape of the cross section of the flowing w a ter. All of these factors 
influence the velocity of flow, which should be low enough to pre- 
vent erosion and yet high enough to prevent silting and the growth of 
vegetation. The desired velocity controls, to a large extent, the 
values that should be given the foregoing factors. Average soils will 
stand a velocity of 3 feet per second, and a velocity of 2 feet per second 
will prevent the growth of vegetation and the deposition of silt. The 
slopes required to give these velocities vary from one-half foot per 
mile in very large canals to a number of feet per mile in the case of 
small laterals. In general, if the ratio of the depth of flow to the 
cross section of flow be small, greater slope will be necessary or per- 
missible. 
Laterals or farm drains of just the right capacity to care for the 
water would be too small for economical construction. Furthermore, 
28 
Canal 8 deep 
Water /'deep 
5 '*.-. 
Water /'deep ^^J^^^ W^^^ 
Fig. 1.— Ideal cross section of open canal in medium soils, showing relation between sectional 
stream and canal. 
a small amount of material falling into such drains would seriously 
obstruct them. It is therefore considered good practice to give open 
ditches a minimum bottom width of 4 feet, except in very stiff, homo- 
geneous clay, where it may be 3 feet. 
COVERED DRAINS. 
LUMBER BOX DRAINS. 
Lumber box drains are chiefly employed in isolated places where 
transportation rates are so high as to make the cost of tile prohibitive; 
their greatest advantage is, perhaps, their cheapness. This advan- 
tage disappears, however, in localities near tile factories. Another 
advantage of such conduits is that the boxes may be laid in compara- 
tively long sections, which afford a more stable bearing and make for 
a more uniform channel. The life of such a conduit is reasonably long 
if the lumber is always wet, but where alternate wetting and drying 
take place the material may fail in a few years. The presence of alkali 
salts seems to be beneficial rather than injurious to the wood, but 
sodium sulphate destroys the nails in a very short time. It is neces- 
ary, therefore, so to construct the boxes that their integrity of form 
