136 ENGINEERING FOR LAND DRAINAGE. 



or by some surface relief. Such conditions give rise to 

 the complaint that the drains are not sufficiently large. 

 Additional drainage should be provided at such depres- 

 sions either by the use of more than the usual number 

 of underdrains or by some surface overflow. It is 

 always wise to place the drains closer together in the 

 depressions than on the surrounding land, not that 

 more water is precipitated there, but they are the nat- 

 ural receptacles for surface overflow. Every means 

 possible should be taken to intercept overflow water 

 from the higher lands. The actual head under which a 

 drain works when discharging its maximum quantity is 

 the difference in elevation of its outlet and head end, to 

 which should be added at least one half of the depth of 

 the drain at the upper end. A porous soil filled with 

 water nearly to the surface easily gives this additional 

 head and should be used in the formula when comput- 

 ing the full capacity of the drain. It is the impression 

 among many engineers that a city pipe sewer forms a 

 more perfect channel for the flow of water than a tile 

 drain. Observations upon this point indicate that this 

 opinion is unfounded. A well-laid tile drain is graded 

 as accurately as the best of sewers ; there is less resist- 

 ance at the joints, for the cemented joints in a pipe 

 sewer are rarely smooth and perfect; the flow of a 

 sewer must carry more or less solid matter, and the 

 surface of the pipe is often coated with a slime which 

 retards flow, while on the other hand the water in a tile 

 drain is clear and the walls of the tile are clean. 



Keeping in mind the cautions and conditions here- 

 tofore enjoined, formula 7 may be applied for determin- 

 ing the size of mains and submains. Assume a size 



