LAND DRAINAGE 651 



evaporation. Underdrains are designed with a capacity 

 to remove only part of the normally largest rainfall 

 in a period of twenty-four hours. The absorptive 

 power of the soil and its hindrance to the flow of 

 water through its pores permits the use of a tile-drain 

 system capable of removing from one-quarter to one- 

 half inch of water over the drainage basin in twenty- 

 four hours. This is termed the drainage coefficient of the 

 area. The drainage coefficient of the system, especially 

 if it is a large system, should be determined after careful 

 study of the amount and distribution of the rainfall 

 and the extent to which surface and subterranean water 

 is accumulated. 



554. Carrying capacity of a tile-drain system. — The 

 carrying capacity of a system of tile drains depends on 

 their respective sizes, their grade, or fall, their total length, 

 their depth in the ground, the straightness of their course, 

 and the smoothness of the interior of the tile. Some of 

 these factors affect the flow directly as they increase, 

 others inversely. The two most important elements 

 in determining the capacity of a drain are the diameter 

 and the grade. The capacity of a drain varies as the 

 square of the diameter. Doubling the grade increases 

 the capacity by approximately one-third. With cer- 

 tain additional corrections and modifications, all the 

 factors that affect the flow have been put together in a 

 formula to determine the necessary size of the outlet 

 tile for a given area. This formula, known as Ponce- 

 let's formula, as modified by Elliott ! for large systems, 

 is as follows : — 



1 Elliott, C. G. Engineering for Land Drainage, Chapters 

 VII, VIII, IX. New York. 1912. 



