178 PRACTICAL DRAINAGE 



information, and it is a good idea to preserve the record 

 for the use of some one besides the original drainer of the 

 land. 



In laying out the system the depth of the drains, the 

 size of tile, the slope of the drain, and the distance apart 

 must be given careful consideration and will vary with 

 each set of conditions. These factors depend somewhat 

 upon each other. For example, the steeper the grade 

 the smaller the tile may be, and the deeper the drain the 

 farther apart they may be placed. In general, tile should 

 be placed from 5 to 7 feet deep and the space between 

 tile lines will usually vary from 200 to 1000 feet. 



Size of Drains. A number of formulas have been 

 worked out to help in deciding the size of tiles that will be 

 efficient and economical. Poncelet's formula for de- 

 termining the velocity of flow in drains, which has found 

 considerable use, is as follows: 





+ S4D 

 in which 



V = Velocity in feet per second, 

 D = Diameter of tile in feet, 

 F = Total fall of drain in feet, 

 L = Length of drain in feet. 



Knowing the velocity of flow in a tile of given diameter 

 the discharge may be determined by using the general 

 formula : 



Q = AV 

 in which 



Q = Discharge in cubic feet per second, and 

 A = Cross-section area of tile in square feet. 



