OPEN DRAINS. I 5/ 



only guides to the work. From such observations the 

 following empirical rules may be deduced : 



For ditches with minimum bottom width of 4 feet 

 and maximum grade of 2 feet per mile use 20-degree 

 curve = radius of 288 feet. For ditches with bottom 

 width 4 feet to 8 feet and grade of 3 feet to 6 feet per 

 mile use la-degree curve = 478 feet. 



For larger ditches and greater fall, or for the above- 

 named ditches which have a greater fall than indicated, 

 curves ranging between 6 degrees and 12 degrees may 

 be used with such latitude as conditions of earth and 

 fall may indicate to the careful designer. 



Form and Depth of Ditches. 



The foregoing facts have an important bearing in 

 determining the form that should be given to a ditch. 

 A channel with vertical sides offers the least resistance 

 to the current, so that if such a form could be main- 

 tained, it would carry a greater volume of water in pro- 

 portion to its cross-sectional area than any other form. 

 It will be also seen that the velocity is greater in a full 

 channel than in one partly full, and in a deep channel 

 running full than in a shallow one running full. 



It follows, then, that the form of a ditch where the 

 fall is light, and where it is desired to secure the best 

 results with the least excavation, should approach as 

 near as possible to that of a rectangle, and should be 

 as deep as practicable. Nothing but rocky material 

 will ^ stand perpendicular. Ordinary clays will stand 

 very well at a slope of 45 degrees, or i to i as it is 

 called. Loose, loamy, and sandy soils will sometimes 

 require a side slope of i^ to i in order to stand reason- 



