8S 



ENGINEERING FOR LAND DRAINAGE. 



inch .50. To compute this, add the drop to the ele- 

 vation of grade line at the point of junction, which will 

 give elevation of starting-point of branch. Example : 

 At station 4.50 of notes, Branch No. I is to have a 

 drop of .20. The grade line 98.35 + .20 = 98.55 

 = elevation of grade line at outlet branch. This 





% 



A 





Fig. 18. — Angle and Drop foi' Tile Dx-ain. 



should be transferred to the notes of Branch No. i 

 and used as the initial point for computing the grade 

 of that line. If short bends or curves are necessary a 

 little additional grade should be allowed, provided it 

 can be had. 



The depth at which it is desired to lay the drains 

 will often have much to do with the determination of 

 the grades. No inflexible rule can be given, but uni- 

 formity should as much as possible be secured. If the 

 flow in drains is alternately slow and rapid and then 

 stands still, the tile being full at one place and half 

 full at others, the efficiency of the drain is only part of 

 what it might be were the grades carefully arranged 

 and the size of the tile proportioned to them. A care- 

 ful survey and corresponding adjusted grade will often 



