GRADES AND SELF-CLEANSING VELOCITIES 161 



Rankin i to 4.5 ft. per sec. 



Adams 2.5 to 3 " " 



Philbrick 2.5 to 3 



Gebhard 2 to 3 



Baldwin Latham gives a little more detail, saying that in 

 his experience he has found that in order to prevent deposits 

 in small sewers or drains, such as those of 6 or 9 inches diam- 

 eter, a velocity of not less than 3 feet per second should be 

 secured. Sewers from 12 to 24 inches diameter should have 

 a velocity of not less than 2\ feet per second, and in sewers of 

 larger diameter in no case should the velocity be less than 

 2 feet per second. This statement would evidently imply an 

 expected or experienced increase of transporting or scouring 

 power in the current with an increase of depth. 



A fact still further contributing to the general uncertainty 

 of this subject is that the velocities given above are those for 

 the pipes flowing full or half full. Since a small pipe sewer 

 rarely flows half full, and since the velocity decreases rapidly 

 as the depth in the pipe decreases, it follows that the bottom 

 velocity on which the scouring power depends must be much 

 less than the 2\ or 3 feet per second which by the table seems 

 necessary. For example, an inch flow in an 8-inch sewer, laid 

 on such a grade that it has a velocity of 3 feet per second 

 when flowing half full, has with the less depth a velocity of but 

 1.6 feet per second, which, by the table on p. 156, is not sufficient 

 to move anything except the smallest gravel. 



When it is remembered that sewers are designed for a period 

 of years in advance and that the full capacity of the sewer is 

 to be reached only at that future time and then only at that 

 hour of that day throughout the entire year when the flow will 

 be the greatest, it is readily seen that in fact at all times the 

 flow is less than that estimated and that therefore the velocity 

 will always be less than that assumed to exist. 



In examining in the Ithaca sewers the velocities with small 

 depths, the author has found velocities of 0.98 foot per second 



