182 



SEWER DESIGN 



..99, 



In using Kutter's formula, or tables or diagrams pre- 

 pared from it, it must be [remembered that 

 the resulting value of v depends, even with 

 known values for 5 and R, upon the judgment 

 of the engineer in selecting the proper value 

 for n. Kutter gives a value of .on for 

 cement and terra-cotta pipe in good condi- 

 tion, and .013 for stoneware pipe in good 

 condition but not new, and for cement and 

 terra-cotta pipe not well jointed. These 

 values, from experiments made by the author 

 in pipe sewers, seem to be true only for per- 

 fectly clean pipes; and whenever accumula- 

 tions of silt occur, or in pipes with any 

 projecting cement, these values are too small. 

 Probably .013 for pipes and .015 for brick- 

 work would agree more closely with actual 

 sewer-gagings than the values given above. 



It may, however, be well to note that 

 there is some evidence tending to show that 

 the values of n as just given are not constant, 

 but change with the depth of flow. In a 

 thesis by Glenn D. Holmes written in 1897 

 under the direction of the author, are given 

 some values of n found experimentally for 

 clean sewer-pipe on different grades. The 

 values found varied from .007 to .02 1 for the 

 differing grades (.56 to 2.51 per cent) and for 

 the varying depths. As the depths increased 

 in the experiments, reaching the half-full 

 point as a limit, the values of n were in- 

 creased for the higher grades and decreased 

 for the lower, with the evident meeting-point 

 at n = .013, agreeing with the common assump- 

 tion. Further experiment in this direction would seem desirable. 



As a convenient and ingenious method of rinding values 



