I40 ENGINEERING FOR LAND DRAINAGE. 



receive added head from this source. While these 

 forces are variable and difficult to formulate, it is safe 

 to assume a small additional head made up from these 

 sources. 



To find the total flood head, find how much higher 

 the upper terminal of each lateral or sub-main is than 

 the upper terminal of the main drain, take the mean of 

 the differences and add it to the head of the main. To 

 this result add one half of the depth of the upper end 

 of the main, for open soils only, for the value of/ in 

 the formula. 



By Formula, 



f->^^+—+^C (9) 



k =: head of main. 



b = sum of excess of elevation of upper terminals 

 above upper terminal of main. 



n = number of laterals having additional head. 

 C = depth of main at upper end. 



Areas Drained by a Main, — The number of acres 

 that a main is to actually drain is not the only element 

 to be considered in this connection, but the manner in 

 which the water is to be brought to the main, together 

 with the general outline and contour of the surface, 

 should also be carefully investigated. The formula is 

 based upon the supposition that the surface of the tract 

 is reasonably even, and that the drainage is brought to 

 the main by lateral tile drains. Should a main be laid 

 through a watercourse like a slough, into which there 

 is considerable lateral surface drainage with no lines 

 of tile, the main is at the disadvantage of receiving all 

 of its water along one narrow line, instead of taking it 



