86 SEWER DESIGN 



all other factors being equal, the run-off from such small areas 

 as are considered for city drainage should vary directly with 

 the rainfall in all cases of heavy storms, and also for short periods 

 if absorption and evaporation can be neglected. Therefore, 

 as these assumptions can generally be made for city work, the 

 three latter formulae, which ha,ve a direct variation with the 

 rainfall, are preferred. 



Slope. " When the maximum rate of fall does not cease 

 before the run-off from the entire area has reached its lowest 

 point, then for this area the run-off will be independent of the 

 slope. But when the maximum rate ceases be/ore this takes 

 place, the slope will have a decided influence upon the amount 

 of water accumulated. The greater the slope of the surface, 

 that is, the steeper the territory, the more rapidly will the water 

 run off and accumulate along the lowest lines. It is not prac- 

 ticable at this time to state how large the area must be before 

 the variation of the slope should be considered. It depends 

 upon the maximum rate of rainfall, upon the steepness of the 

 area, and upon other local conditions. Assuming that the 

 run-off increases with the slope, what is the ratio between 

 these two quantities? If all factors except these two are assumed 

 to be constant, then the ratio in the different formulae is shown 

 as follows: 



Hawksley: Q = const. Xs' 25 . 



Adams: Q = const. Xs 083 . 



Biirkli-Ziegler: Q = const. Xs 25 . 



McMath: Q = const. Xs' 20 . 



N. Y. diagrams: Q = const. Xs 27 . 



" The exponent showing little variation indicates that there 

 is but slight difference in the formulae as to the weight attached 

 to the slope, but that the N. Y. diagrams with the largest 

 exponent give it the most importance. 



Area. " The larger the area the greater is the total run-off. 

 But the larger the area the smaller is the run-off per unit of 

 area. This variation is important and demonstrates that a 



