Adams : Q = const. X r\

Burkli-Ziegler: Q = const. X *'•
McMath : Q = const. X r.
N. Y. diagrams: Q = const. X f.
Hering and Gray say: " There is hardly a question
that, 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 absorp'
tion and evaporation can be neglected. Therefore, as
these assumptions can generally be made for city
work, the three latter formulae, which have 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 before this takes place, the slope
will have a decided influence upon the amount of
water accumulated. The greater the slope of the sur-
face, that is, the steeper the territory, the more rapidly
will the water run off and accumulate along the lowest



74 SEWER DESIGN.
lines. It is not practicable at this time to state ho«
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 rat'«
in the different formulae is shown as follows:

Hawksley: Q = const. X .r^.
