50
Values of 'C' adopted by Dicken's is given in the following table (Table 5.1):
Table 5.1 : Zonal values of 'C' for Dicken's; Formula in Indian conditions
(After S.K. Garg, 1988)
Region
Value of 'C' in Dicken's formula
Northern India
11.5
Central India Provinces
14 to 19.5
Western Ghats
22 to 26
There are other formulas like Ryve's formula, Inglish Formula, Nawab Jung
Bahadur formula, Boston Society of Civil Engineers formula, modified Myer
formula, Jarvis formula etc. - all derived based on 'C' and 'A'. Amongst them
Nawab Jung Bahadur formula are used extensively for small catchment areas
of Maharashtra.
Nawab Jung Bahadur formula = This formula has been derived for Hyderabad
Deccan catchment and states that [S.K. Garg, 1988].
Qp = C. A
1
[0.93 - (1/14) log A
1
]
in cumecs, where
C = 48 to 60
A
1
= Area in square km.
The rational formulae uses both rainfall intensity and drainage area and states
that :
Qp = 1/36 (K.pc.A), where
K = The percentage of rainfall that becomes surface run off,
called run off co-efficient.
PC = Critical rainfall intensity in cm/hr. generally taken as the
average rate during the time of concentration Te and can be
determined from the Intensity - duration curve for the given
frequency flood.
A = Catchment Area in ha.
