OPEN DRAINS. i6l 



Formula for Open Ditches. 



The force which impels water along the ditch is the 

 rate of fall. The forces acting against this are the re- 

 sistance offered to the water by the sides and bottom 

 of the ditch. The difference between these forces is 

 the velocity head and causes whatever current there 

 may be produced. When the water becomes low in a 

 ditch which has slight fall, the bottom being covered 

 with silt or overgrown with grass, the two forces balance 

 and there is no flow. 



A formula used for velocity of flow in ordinary drain- 

 age-canals with good results is the following : 



^= /^X li/. . . . (lo) 



Q = av. 

 Where v = mean velocity in feet per second. 

 a = area of waterway. 



p z= wet perimeter or border of the channel 

 which is wet. 



/ = fall in feet per mile. 



Q = discharge in cubic feet per second. 



This formula gives results sufficiently close for the 

 purpose. It must be remembered that in applying any 

 of the drainage formulas the data at hand are not always 

 of the most accurate kind, and there must be a margin 

 allowed for contingences that cannot be provided for 

 by means of definite quantities which may be inserted 

 in a formula. 



To Apply the Formula. — Find the area of the water- 

 way of the proposed ditch in square feet when it is 



