162 SEWER DESIGN 



apparently carrying along the solid matter and requiring no 

 more flushing than is usual. It would seem, then, that the stated 

 velocities are not the actual flow velocities, but are those required 

 at half-depth in order to get the needed velocities with the usual 

 flow, the actual velocities needed being from i foot to i| feet 

 per second. 



The velocity required being known, it can only be secured 

 by sufficient grade, and the minimum grades are those just 

 sufficient to produce the velocities given above. By the 

 rule of Latham, the larger the sewer the less need be the 

 velocity and grade, but it assumes that the amount of flow 

 is sufficient to keep the sewer flowing half full. 



Thus it is possible to carry away a definite amount of 

 sewage either by a large pipe and a small velocity or by a 

 small pipe and a correspondingly high velocity. According to 

 Latham, the following sewers laid at the grades given will all 

 have the same velocity flowing half full, but the amounts 

 carried must be in the ratios of 100, 25, 4, and i: 



A sewer of 10 ft. diameter, grade .038 per cent 

 " " 5 " ll .076 



" " 2 " il .IQO 



tt 



The velocity required is therefore a function of the quan- 

 tity as well as of the grade. 



The fact that large sewers may be laid on a light grade and 

 yet maintain the necessary velocity is sometimes responsible 

 for an attempt to reduce the necessary grade for a small pipe 

 by the substitution of a larger one. Thus one of the author's 

 students in a Thesis design planned 24-inch pipes for a number 

 of laterals, because his grades were very light and he mis- 

 takenly thought that by using a larger pipe, the grades for 

 that pipe if half full, could be used. The error is at once appar- 

 ent, a small flow in a large pipe requiring a greater slope for the 

 pipe than if the flow was confined in a smaller cross -section. 



