130 ENGINEERING FOR LAND DRAINAGE. 



and the quantity discharged is cubic feet per second. 

 When the velocity is found the discharge is obtained 

 by multiplying the area of the column or jet of water 

 expressed in square feet by the velocity in feet per sec- 

 ond. The result will be cubic feet per second. By 

 formula the expression would be 



Qz=z av, (5) 



where Q = quantity in cubic feet ; 



a = area of column of flowing water; 

 V = velocity as determined by formula. 



Substituting the value of v in equation 5 we have 



Q = looa VRS . . . • (6) 



Flow of Water through Tile Drains. 



As previously stated, the object of draining land is 

 the removal of such soil water as is not needed for the 

 profitable growth of the plants we desire to produce. 

 The source of all water is the rainfall as it is distributed 

 over the surface of the soil at irregular times and in 

 varying quantities. The removal of the part not 

 wanted is accomplished by underground tile drains, 

 the water reaching them by percolation through the 

 soil. The problem in drainage hydraulics is not only 

 to determine the quantity of the water which a certain 

 drain will carry, but also to ascertain how much water 

 should be removed from the soil at certain times in 

 order to place the land in the desired condition. 



The very quick and rapid removal of soil water is 

 not desirable in the drainage of farm land. The object 



