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function of time, together with an infiltration equation to separate rainfall rate data
(intensity) into the amount entering the soil (infiltration), and the amount which
moves over the soil surface (runoff as overland flow).
The second method which is used to compute runoff on small upland areas is
based on rainfall depth alone or on rainfall depth and statistics representing rain-
fall intensity to compute runoff volume. Given runoff volume, other procedures
are used to estimate peak rate of runoff or the runoff hydrograph. The USDA Soil
Conservation Service runoff curve number procedure is the best known and widely
used model of this type.
6.5.3.1 The Rational Method
Peak Runoff from Single Storm Event
The rational method of estimating peak flow on small watershed (< 12 km
2
) is based
on the assumption that for storms of uniform intensity, distributed evenly over the
basin, the maximum rate of runoff occurs when the entire basin area is contributing
at the outlet, and this rate of runoff is equal to a percentage of the rainfall intensity.
Runoff from an area from a storm event can be calculated using Rational method
as
CIA
360
Q
=
(6.7)
where
runoff (m
3
/s)
Q
=
C
runoff coefficient (dimensionless), representing the ratio of peak runoff
rate to average rainfall intensity
=
I
=
rainfall intensity (mm/h)
A
=
area contributing runoff to the point of consideration (ha)
360
=
factor from unit conversion
The runoff coefficient varies with the surface condition. For given storm events of
same rainfall intensity, nonconstant coefficients of runoff are obtained. The vari-
ation is due to the different antecedent moisture conditions. For loose soil (high
infiltration) its value is low (0.3-0.6) whereas for hard surface its value may be up
to 0.8.
Design Peak Flow
This method is used to estimate peak flow for designing small structures like cul-
verts. The peak flow rate resulting from a storm with an average recurrence interval
(ARI) of Y years is calculated using the following formula:
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