164 SEWER DESIGN 



grades adopted as the minimum, implying either that the 

 knowledge of the grade needed for the minimum velocity given 

 above is indefinite, or that ideas of what the minimum velocity 

 is, vary. At the lower end of a main where there are no 

 house-connections and where, should the sewer get blocked, 

 backing up of the sewage would do no harm and would prob- 

 ably accumulate a head which would force out the obstruc- 

 tion, a grade or velocity less than those given might be toler- 

 ated. And at the upper ends of laterals where, although the 

 amount of flow is probably small, a flush-tank can be placed to 

 wash out periodically whatever might form a stoppage, light 

 grades can be used, however undesirable. Between these two 

 extremities of the line, grades less than those given are unwise 

 and a source of continual trouble. Advantage is sometimes 

 taken of natural aids to get intermediate flushing, as proximity 

 to some stream, or to breweries or swimming-tanks, and on this 

 account the grades are lessened. The whole subject gives 

 an excellent opportunity for experimental work in sewers 

 actually in use, and is open to much more enlightenment. 



PROBLEMS 



52. If a i2-inch sewer flowing half-full has a velocity of 3 feet per 

 second, what would be the velocity of flow with a depth of i inch? (Use 

 tables from Traut wine's or Kent's Pocketbooks for values of area and 

 wetted perimeter.) 



53. If a combined sewer, 4 feet diameter, is so laid as to secure a velocity 

 of flow of 2.5 feet per second flowing full, what would be the velocity 

 of flow for the house-sewage with a depth of 4 inches? 



54. Using the Chezy formula (v=c\/R-S), find the value of S to give 

 a velocity of 3 feet per second in a 24-inch pipe, half-full (c = ioo). 



55. If the estimated sewage-flow is 2 cubic feet per second and the 

 available fall is only f inch in 100 feet, find the necessary diameter of 

 sewer, flowing full, to give a velocity of 2.5 feet per second, assuming 

 creek-water can be added. Use formula v=c\/R-S with c = 120. 



56. If a 6-inch lateral is laid on a T\ per cent grade, what is the theo- 

 retical velocity? (Use v=c\/'R'-S with c = 70.) 



