SIZE OF lATERAL DRAINS. 145 



where the soils are open and permit the use of drains 

 lOO or more feet apart should be 4- and 5 -inch tile, 

 which in some cases may be diminished to 3 inches at 

 the upper end. For drains at a less distance apart 

 3 -inch tile may be used for laterals. They are usually 

 required to carry only a small part of their full capacity 

 in order to relieve the soil of its surplus water. To do 

 this well, however, they should not be quite full at any 

 time unless it be when there is more than ordinary rain- 

 fall. They do not work under a pressure head, hence 

 their velocity of flow is as great when running half full 

 as when running full. Water in a drain attains great- 

 est velocity when three fourths lull and its greatest 

 rate of discharge when nine tenths full. 



Should the system of laterals have a heavy grade 

 compared with the mains into which they discharge, 

 the mains will often work under pressure and run full 

 when the rainfall is not very large . They will of course 

 run no more than full under a greater head, but the 

 velocity and discharge will be greater. It is often 

 thought that since a main drain is running full, it is 

 doing all that it is capable of doing. From what has 

 been said above, it will be seen that this is not the case. 



Provision for Unusual Rainfall. 



The rules thus far given for the size of tile do not 

 provide for the carrying off, through the tile system 

 alone, unusually heavy rainfalls of 2 to 3 inches in 

 twenty-four hours. These occur sometimes two or three 

 times a year. Again, two or three years may pass with- 

 out witnessing such a downpour. It is still more rare 



