448 



Irrigation and Drainage 



fectly uniform fall for the whole main or the whole 

 of any given lateral, and the greatest care should be 

 exercised to lay the tiles perfectly true to the grade 

 when that has been determined. When this is done, 

 there is the least tendency for sediment to lodge and 

 clog the drain. 



It will not be possible in all cases to maintain a 

 constant gradient, and when this is true it is best 

 always to change from a less fall to one which is 

 greater, because then any sediment which should be 



carried in the upper part 

 of the drain will also be 

 carried when the fall is 

 increased ; but with the 

 reverse conditions the 

 lower fall must have a 

 tendency to cause the 

 drain to become clogged. 

 Where a change from 

 a larger fall to one less 

 must be made, and the 

 latter gradient is 3 inches 

 per 100 feet or less, it 

 will usually be prudent 

 to place a silt basin where 

 the change of grade oc- 

 curs, as represented in 

 Fig. 140. The silt basin, if the line of tiles is short 

 and small, may be made by sinking an 8-, 10- or 12- 

 inch tile below the level of the bottom of the ditch, 

 and then notching another section of the same size, 



^^^2^^ 



Fig. 140. Silt basin. 



