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sections across the rivers and along the rivers in both
downstream and upstream sides are made. Width of the river,
water flow areas, wetted perimeter and hydraulic mean depth
are calculated from the cross-sections. The longitudinal slope
is assumed to be the same as the hydraulic slope in the
past, and the mean velocity can be calculated from suitable
hydraulic formula like Chezy's formula. Past Flood discharge
can now be calculated by multiplying the velocity with the
probable water section the past.
b) Flood discharge formulae :
There is a number of mathematical formula used to
estimate the flood discharge depending upon the areas,
availability of data, etc. The basic form of such formulae is :
(Source : S.K. Garg, 1988)
Q = CAn, where
Q = flood discharge
A = catchment area
n = flood index
C = flood co-efficient.
Both C and n depend upon various factors such as (i) size, shape and location
of the catchment (2) topography of the catchment and (3) intensity and
duration of the rainfall, and distribution pattern over the basin.
Many of such formulae are derived by the study of drainage areas. For example,
Dicken's formula states :
Qp
= C A3/4
Where Qp = High flood or peak discharge in cumecs.
A
= A constant depending upon the various factors
affecting flood discharge.
