IS 8 ENGINEERING FOR LAND DRAINAGE. 



ably well. The slope that is preferable in ordinary- 

 soils is that of I to I . After a ditch made in this form 

 has been in use a year or two, the action of the water 

 will change its form, the upper part becoming more 

 nearly perpendicular and the lower more nearly hori- 

 zontal. 



In the laying out of ditches for the drainage of large 

 areas, a depth of from 5 to 7 feet should be aimed at 

 for two reasons. First, this depth is needed for the 

 purpose of giving good lateral drainage, and second, 

 to get the velocity necessary to scour the bottoms and 

 make them self-cleaning as far as possible. Broad flat 

 bottom shallo"w ditches are adapted to carry floods in 

 freshet times, but not to give outlets for the 'thorough 

 drainage of level tracts. In this connection it may be 

 said that the bottom width of any ditch with a grade of 

 I to 2 feet per mile should not be less than 3 feet. 



Ditches with a grade of 10 feet per mile may be 

 made with any bottom width which will furnish the 

 necessary capacity. Such ditches will be self-cleaning, 

 and often precautions must be taken against injury of 

 the ditch by erosion. The side slopes must be gov- 

 erned by the kind of soil and clay through which they 

 are made, and also by the means that are to be used in 

 excavation. If the work is to be done with teams, a 

 slope of about 2 to i must be given to the sides. If 

 done with the steam dredge it can be made with slope 

 of I to I or less. 



Capacity Required for Open Ditches, 



How large a drainage-channel should be made to 

 afford an outlet for a given area is a question which 



