CHAPTER XVI 

 SEWER CROSS-SECTIONS 



As has already been stated, in this country sewer-pipe are 

 invariably made circular, though attempts have been made 

 in England to make pipe of oval form. Experience shows 

 that the difficulties of burning (any eccentricity or deformation, 

 which in a circular pipe can be avoided by turning axially, spoils 

 the oval pipe), will probably prevent any similar attempts in 

 this country. The general advantage of circular pipe lies in 

 the fact that when half full or full there is a greater velocity 

 for the same grade than in any other pipe-section, and for the 

 amount of material in the pipe the circular section has the 

 greatest area; or, geometrically, for the same perimeter the 

 circle, of all polygons, has the greatest area. If, therefore, 

 sewers were always to flow full, they should, whether of pipe 

 or brick or concrete, be built of circular form, in order to econo- 

 mize material. But with a variable flow the circular section 

 loses its value, and the less the flow the poorer the section for 

 its purpose. According to the equation of flow, v = c V RS, 

 the velocity varies with the square root of the hydraulic radius, 

 and in a sewer where the depth of flow changes from hour to 

 hour the velocity decreases as the depth decreases, since the 

 ratio of A : p continually decreases. This applies especially 

 to the combined system, where the sewers are large to accom- 

 modate the rainfall, so that the house-sewage flows in a wide 

 shallow stream with a velocity much less than that at the half- 

 full point. To avoid this difficulty the cross-section of the 

 brick sewer has been changed in an attempt to make the ratio 

 of A : p as nearly constant as possible for every depth, that is, 

 to make the area of flow nearly semicircular for every depth. 

 The gain over the circular sewer in increased velocity for low 



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