Geoscience Reference
In-Depth Information
The deviation of the values of circularity ratio from 1 shows the irregularity of
the catchment. Here in case of sub-basin-I, the values of the circularity index
deviates more and showing most irregularity and elongation within the whole
drainage basin. Besides, sub-basin-II and IV are also indicating the lower values of
circularity index and consequently revealing another two irregular segments of the
Siva-khola watershed. The sub-basin III, V and VI are more regular in respect of the
values of the index (values which are closer to 1). A circular basin is more likely to
have a shorter lag time and a higher peak flow than an elongated basin. Elongation
Ratio indicates how the shape of the basin deviates from a circle (Schumm
1956
). It
is an index to mark the shape of the drainage basin. The value of R varies from 0
(highly elongated shape to the unity (1.00) circular shape. So, the higher values of
R mean more circular shape of the basin and vice versa.
Elongation ratio is de
ned as the ratio of the diameter of a circle which has same
area as the basin to the maximum basin length.
R=
Dc
Lbm
ð
3
:
12
Þ
where,
Dc Diameter of the circle having same area as the given drainage basin
Lbm Maximum length (km).
Compactness factor of the basin is used to express the basin shape, which is
indicated by the deviation of the basin area from a circle of an equal area. Com-
pactness factor is expressed as the shape of the basin that was used by Horton and
was devised by Gravelius (Gupta
1999
). The compactness factor was obtained from
the ratio of the perimeter of the watershed to the circumference of a circle whose
area is equal to that of the drainage basin.
P
Cc =
p
2
ð
3
:
13
Þ
p
A
where,
P Perimeter of the basin (km)
A Area of basin (km
2
).
3.4.1 Determination of Composite Ranking Coef
cient Value
and Instability Rank
The form factor reveals that the sub-basin 1 and sub-basin 5 are more elongated
than other basins and that is why erosional activities become prominent within these
two sub-basins. The value of the circularity index for sub-watershed 1 deviates
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