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Drainage Works : Computation of runoff is a complicated matter which depends on many factors like the ground permeability, rainfall duration,

What are the limitations of Rational Method in calculating runoff?

Computation of runoff is a
complicated matter which depends on many factors like the ground permeability,
rainfall duration, rainfall pattern, catchment area characteristics etc.
Basically, Rational Method is a means to find out the maximum discharge
suitable for design purpose. In this method, it is assumed that the rainfall
duration is the same as the time of concentration and the return period of
rainfall intensity is the same as the peak runoff. Time of concentration refers
to the time required for the most remote location of stormwater inside the
catchment to flow to the outlet. When the time of concentration is equal to the
rainfall period, the maximum discharge occurs and rainfall collected inside the
catchment comes to the same outlet point.

Rational Method provides the peak
discharge only and it cannot produce a hydrograph. If a more detailed pattern
of runoff is required, unit hydrograph or other methods have to be used. The
accuracy of rational method depends very much on our correct selection of
runoff coefficient and delineation of catchment area.

Rational Method is a rather
conservative method. One of the basic assumptions of the rational formula is
that the rainfall intensity must be constant for an interval at least equal to
the time of concentration. For long duration of rainfall, this assumption may
not hold true. Moreover, the runoff coefficient in Rational Method is difficult
to be determined accurately and it depends on many factors like moisture
condition of soils, rainfall intensity and duration, degree of soil compaction,
vegetation etc. In addition, In Rational Method the runoff coefficient is
independent of rainfall intensity and this does not reflect the actual
situation.

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