194 SEWER DESIGN 



for 44 cubic feet per minute, with a velocity of 2.0 feet per 

 second, a 1 2-inch pipe is required. This pipe will run from the 

 lower end to the first lateral, where the volume of flow will 

 be diminished by the amount there contributed. The smaller 

 pipe is continued until the next lateral is reached, and so on. 

 It is to be noted that while the size of pipe as just taken 

 is calculated so that the estimated flow will fill the pipe only 

 half full, this factor of safety is not always necessary, and in 

 the lower ends of large mains, where the flow is large and 

 comparatively constant, the size of pipe as determined for the 

 exact flow is taken, or the pipe increased by one or two sizes 

 only, as it is evidently bad engineering to require the expendi- 

 ture of a large amount of money unnecessarily. In studying 

 the grades and sizes for a city, various ways of tabulating 

 and simplifying the large amount of computation can be 

 devised. The relative effect of the grades of one line on another 

 is best seen by plotting the profiles one above another so that 

 the lower end of every lateral is in a straight line directly above 

 the starting-point. 



PROBLEM 



78. Design the size and grades for an outfall sewer along the route 

 shown on Plate VI. The following suggestions should be followed: 



Determine the volume of storm-water and domestic sewage that will 

 have to be taken care of at the point A from District I. Assume that the 

 sewer will be 10 feet below the surface at this point and that it will be 

 laid on a slope to give a velocity of 2.5 feet per second to B. Assume the 

 drainage from all of District II to enter at B. Assume a minimum grade 

 again to E, and that the drainage from three-quarters of District III will 

 enter at D. From E to F the grade will be greater, the elevation of the 

 invert at E being +5. From F to G, the grade is i per cent and the 

 drainage of one-quarter of District III will enter at G. From G to the 

 river the grade is that necessary to give a velocity of 2 feet per second. 

 Assume a ground water leakage of 8 cubic feet per day per square foot 

 of interior surface. 



