50 SEWER DESIGN 



which the saving in the size of sewers occasioned by assuming 

 storms of less than the absolute maximum is carried, must 

 be offset by occasional damage suits, including costs, and an 

 important factor in the latter is the use made of the buildings 

 occupying the area drained. If the watershed were covered with 

 business blocks, where extra stock was stored in the basements, 

 and were subject to water damage, it might easily happen that 

 one suit decided against the city would cost more than would 

 justify the construction of the smaller sewer. In residential 

 districts, on the other hand, an occasional flooding of the cellars, 

 that is, once in three or four years, might cause comparatively 

 little damage or even inconvenience. In Kansas City, Mo., 

 the practice according to Mr. J. B. Balcomb * is to design the 

 main sewers with the expectation of flooding every ten years, 

 branch sewers every five years, and laterals every two years. 

 The possible future development of a district must, however, 

 not be lost sight of, nor the fact that sewers known to be 

 inadequate and subject to overflowing may seriously affect 

 real estate values and manufacturing enterprises and so indi- 

 rectly be a source of loss to the city. 



When the curve has been finally fixed on the diagram show- 

 ing rates of fall varying directly with the duration of the storm, 

 what rate is to be taken as that by which the sewers are to be 

 designed? Following Kuichling, the time by which the intensity 

 is made determinate should be equal to that required for water, 

 starting from the point on the line of the sewer farthest from 

 the outfall to reach that outfall. It is plain that, considering 

 the outlet-pipe, a maximum flow will occur when all the laterals 

 are discharging their maximum at the same time. But as some 

 laterals are near and some far away, it is possible for one set to 

 have discharged its volume before the water from the more 

 distant pipes has reached the outfall; so that a rain must 

 continue at a definite rate for a definite time in order that 

 the outfall discharge may represent the maximum discharge 

 due to that rain-intensity. The time required must be that 

 * Jour. West. Soc. C. E., Vol. XV. p. 706. 



