56 SEWER DESIGN 



inch the same sewer discharged 74 per cent; and in the same 

 year the Irongate sewer, from a district entirely paved and 

 built up, discharged 94 per cent of a rainfall of 0.54 inch in 

 five hours, and in August the same sewer discharged 78 per 

 cent of a rain of 0.48 inch in 1.67 hours. Kuichling, in citing 

 these records, notes the absence of details as to the character 

 of the rain, manner of observation, location of gages; and 

 suggests possible inaccuracies in the recorded percentages. 

 He quotes another gaging by John Rae, C.E., engineer of the 

 Holborn and Finsbury sewers, who states that during the con- 

 tinuance of a rain of i inch per hour 41 to 54 per cent of the 

 precipitation will reach the sewer, according to the amount 

 of garden or lawn surface upon the drainage area. Kuichling 

 adds: 



" Upon the foregoing indefinite data, which may be found 

 quoted more or less extensively in nearly every treatise on 

 sewerage, and in most of the elaborate reports, engineers have 

 hitherto been content to rely, and thus it has come to be in some 

 measure traditional that about 50 per cent of the rainfall will 

 run off from urban surfaces during the progress of the storm, 

 while the remainder may follow at leisure." Until the recent 

 (1889) work of Mr. Kuichling, this has been undoubtedly true, 

 and in Providence, Brooklyn, St. Louis, and other cities the 

 old sewers, often gorged and overflowing, have proved that 

 the old assumptions in regard to rainfall are not accurate, but 

 require modification. Of late a German formula has been much 

 used, in which the coefficients may be modified for different 

 kinds of surface, and the amount of run-off considered in the 

 design has thereby been much increased. The discussion of 

 this formula is reserved for the next chapter. 



Kuichling proved by his experiments at Rochester that 

 these inconsistencies and failures were due to the unit period 

 of time used both for the rainfall and for the gagings. He 

 observed that the volume of water discharged at different 

 stages at the mouth of an outfall sewer increased and diminished 

 directly with the intensity of the rain, and that a certain time 



