204 



SEWER DESIGN 



for an almost indefinite length. In the case of storm-water, 

 the circuitous path has the further advantage that the gradual 

 accumulation of storm-water in the lower parts of the city will 

 not require such large sewers as if the sewage were brought down 

 in the short time required on the steepest streets. 

 Other points that may be noted are as follows: 

 Since manholes or flush-tanks are usually built at the ends 

 of laterals, it is often possible to run the ends of two laterals 

 into the same manhole, thereby saving the cost of one manhole 

 and flush-tank, though increasing the length of the sewer. 



\ 



FIG. 56. 



The comparative cost is here to be considered, though the 

 single manhole or flush-tank probably gives better ventilation. 

 It is generally recommended that, since the flow of air is 

 along the top of the pipes, wherever the sizes of the pipes are 

 changed the smaller pipe be raised enough to bring the crowns 

 of the pipes on a continuous line, in order to have a continuous 

 ventilation upward through the sewer. For example, a 1 2-inch 

 pipe emptying into a 1 5-inch pipe should be 3 inches higher on 

 account of the ventilation. On long lines laid on a minimum 

 grade this is a serious matter and requires that the lower end 

 of a main be a foot or more deeper than the grades themselves 

 would call for. Since the pipes are expected to run only half 

 full, leaving half of the pipe for ventilation, it seems to the 



