1 62 ENGINEERING FOR LAND DRAINAGE. 



filled eight tenths full. Find the length of channel 

 which is wet by this depth of water, called the wet 

 perimeter. Find the fall in feet per mile and substi- 

 tute the quantities thus determined in the formula and 

 find the value of v. 



To find the number of acres which the proposed 

 ditch will drain, proceed as directed for finding the 

 number of acres which a tile will drain. Find the 

 value of Q and divide it by the decimal which represents 

 the quantity of w^ater that must be discharged per sec- 

 ond in order to relieve the land of the desired depth of 

 water per acre. By formula the expression would be 



A = number of acres, 7t = number taken from table 

 No. 5. 



Example: Given a ditch with bottom 8 feet, fall 3 

 feet per mile, side slopes i to i, water to run 5 feet 

 deep in flood time, how many acres will it give an 

 outlet for using the ^-inch standard? 



^ = 13 X S == 65, 



p zzi o — j— 2 T 25 ""I— 25 ^^^^^ 22.14. 



i|/r=: 4.5 Substituting in the formula 



4/^ 



22.14 ^ ^'^ " ^'^^' 

 g = 65 X 3-63 = 235.9 = c^- ft* disch. per sec, 



.0210 "^"^ 



With a fall of 2 feet per mile A = 9183. 



u u ii ** I fQot *' '' A = 6914. 



