182 



PRACTICAL DRAINAGE 



not located at the foot of steeper slopes or benches, nor 

 in pockets or depressions, nor in flat river bottoms where 

 it will receive surface run-off from higher land, nor where 

 it will receive water from deep sources by pressure. The 

 assumed slope of the tile is 2 feet per thousand feet. 



"If the soil be compact clay, a given size of tile will 

 drain larger areas than indicated. If the subsoil be joined 

 clay, .the 'sand' table should be used. If the drain be 

 located at the foot of a bench or in a gravel pocket, none 

 of the above bases will apply. A better basis for design 

 in such cases is the length of a given size of tile which it 

 is safe to use. A slope of 2 feet per 1000 feet is assumed, 

 as before. The following table will give a rough idea: 



TABLE XXI. SIZES OF TILE REQUIRED FOR DRAINS OF DIFFERENT 



LENGTHS 



u For greater slopes smaller tile is required, and for 

 flatter slopes larger tile is necessary, the variation in 

 capacity being as the square root of the slope. If lumber 

 boxes are used, the openings should be about the square 

 of the tile diameter. 



"For open ditches the bottom width should be 4 feet 

 and the side slopes should be at least i to i. Thus for a 

 depth of 6 feet the top width would be 16 feet or more, 



