196 



SEWER DESIGN 



charge when the amount of dilution has reached that point 

 previously decided to be allowable. It is advisable, and these 

 relief-outlets make it practicable, to discharge the storm-water 

 at as many points as possible, keeping the size of the storm- 

 sewers small, carrying the water on the streets as long as pos- 

 sible and avoiding the pouring of a large mass of water and 

 sediment into the river at one point. 



The outfall being located, it is possible to arrange the mains 

 and laterals according to one of five systems;* and although 



FIG. 48. 



there may be combinations of these, so that it is sometimes 

 difficult to recognize the system adopted, yet in the first study 

 of the topography it is advisable to keep these separate arrange- 

 ments in mind. 



(A) Perpendicular system. When the city lies on the bank 

 of a large river or bay, as New York, Philadelphia, or Portland, 

 Me., where the volume of flow in the stream or the change of 

 water at each tide is sufficient to keep the sewage diluted so 

 as to be inoffensive, the only aim is to get the sewage into the 

 water by the shortest path. In this we have what is called 



* Baumeister. 



