DRAINAGE 



105 



water in ditches. One of the most important of these is the 

 cleanness of the ditch. A very little rubbish, if allowed to 

 accumulate in an open ditch, will decrease its capacity materi- 

 ally. Grass and weeds may grow in an open ditch to such an 

 extent as to reduce the capacity of the ditch to less than half. 



Fii 



67. An excavator for digging open ditches, which is carried on 

 tracks laid at each side of the ditch. 



The following tables computed by Kutter's formula will 

 be useful in this connection.* These tables are taken from 



*Kutter's formula for the velocity of flow in open ditches is as follows- 



( 1.811 



V = 



+ 41.65 + 



.00281 



' + < 



1 + /41.65 + 



.00281 



J V 



X 



in which y = velocity of flow in feet per second. 



i = sine of the inclination of the slope, or the fall of the water surface in a 



given distance divided by that distance. 

 r — area of the cross section in square feet divided by the wet perimeter 



in lineal feet. 

 n = coefficient of friction for different sizes of canals and with different 



degrees of roughness. 



