OPEN DRAINS. 15! 



to the available grade. The fall that is usually found 

 in districts where it is necessary to construct large out- 

 let channels is from i foot to 5 feet per mile. These 

 are the main channels or outlets for the drainage of 

 large areas. Lateral open ditches for small areas and 

 for farms have usually a higher rate of fall, but not 

 always. It is desirable that ditches have sufficient fall 

 to be self-cleaning. This in soil and clay which is not 

 easily displaced is about 4 feet per mile, which for 

 ditches of ordinary size will give a mean velocity of 

 2| miles per hour when running full. This is given 

 as an approximate grade for ditches which may be re- 

 lied upon as self-cleaning, and applies to those with 

 bottoms not less than 3 feet wide and a depth not 

 less than 4 feet, and so adjusted in size that they will 

 run three fourths full at flood height. This is by no 

 means the minimum grade upon which effective ditches 

 may be constructed. As a matter of fact there are many 

 large tracts of land drained by outlet ditches having a 

 fall of from 6 to 24 inches per mile. These are not, how- 

 ever, self-cleaning except when made deep and large 

 with a final free discharge, and even then a liberal 

 annual allowance should be made for cleaning. As 

 can be readily understood, there can be but little or 

 no scouring action of the water upon the bottom of the 

 channel unless the head is furnished by the depth of 

 water in the ditch. Hence light-grade channels must 

 be deep — 6 to 8 feet if possible — or else provision must 

 be made for frequent artificial cleaning. This continual 

 expense will in the course of ten years go far towards 

 paying the additional first cost of a deeper ditch. 

 Where tracts are level and grades light, depth of ditch 



