10 BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 



LOCATION OF OPEN CANALS. 



In considering the question of location, the specific purpose of the 

 drain must be kept in mind. Outlet systems require treatment 

 different from that given small canals or ditches intended to accom- 

 plish farm drainage directly. The former must usually follow the 

 natural depressions and watercourses, while the latter must be 

 located with strict regard to the source of the damaging water, as 

 is the case with tile drains. For this reason the discussion of location 

 of covered drains (pp. 18-24) may be understood to refer to the 

 location of open farm drains as well. 



It is generally a feasible and satisfactory practice to have small, 

 open ditch outlet drains and laterals extend along the highways, as 

 the roadway is thus drained and less right of way is required, since 

 the spoil may be thrown into the roadway and crowned, making an 

 excellent thoroughfare where roads may ordinarily be impassable 

 during wet seasons. Figure 3 shows how this arrangement may be 



SS' Roadway • >■-■ 28' >--5^'-. 



— ee- Origina, Roac, 1 ^' ^L'r%fsL^^- 



99 i- 



Fig. 3. — Illustrating how a canal may be built along a 4-rod road by purchasing a 2-rod right of way and 

 placing aU the spoil on the road. 



effected, in the case of a 4-rod road, by the acquisition of a right of 

 way 33 feet in width. A walk 5 feet in width is provided on either 

 side, the drainage canal has the necessary depth and a desirable 

 cross section, a small surface drain is provided at the left side of the 

 road, and a roadway is afforded which is 58 feet in width and has a 

 crowned surface rising 3^ feet above the general ground surface. 



DEPTH AND LOCATION OF COVERED DRAINS. 



One of the most important questions in drainage practice in the 

 irrigated section is the proper depth at which to lay drains. Water 

 often rises in soils, by capillary attraction, to a height of several feet 

 above the free water level, and the presence of salts in solution in- 

 creases the height of the rise and the rapidity of the movement. 

 Evaporation takes place, and as a result the salt solution is concen- 

 trated at the upper limit of saturation. The height to which capil- 

 lary water will rise depends upon the type of the soil, the wetness, 

 the amount of foreign material in the soil, the amount of salts in the 

 water, and the temperature. The rise may vary from a few inches 



