DRAINAGE OF lEEIGATED 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 water. 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, 



Fig. 1. 





-Ideal cross section of open canal in medium soils, sliowing relation between sectional areas of 

 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 veiy stiff, homo- 

 geneous clay, where it may be 3 feet. 



COVERED DRAINS. 



LUMBER BOX DRAINS. 



Lumber box drains are chiefly employed hi 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 scftions, which affonl m, more stable ])caring and make for 

 a more uniform channel. The life of such a conduit is reasonably long 

 if tlio lumber is always wet, but where alternate wetting and drying 

 take })lace the material miiyfiiil in m, fevvyeiirs. TlHq)r('sence of jiikiili 

 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 constnicl Uic boxes that their integrity of fol-m 



