Impact Assessment of Hydrologic Attributes and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 109

that the main stream of all the individual watersheds (sub-basins) is experiencing
large no. of stream union where the valley deepening as well as valley lengthening
is quite common. Such processes steepen the valley side slope and make the slope
material to move downward under the influence of gravity.

3.3.4 Drainage Network Orientation

Drainage orientation is the key to understanding the way in which a drainage
network fills the space that is available to it (Jarvis 1976 ). Drainage orientation is

related to the strike and dip slopes of the underlying rocks and is very sensitive to
fold axes and rock jointing. Basically, the regional slope surface and the underlying
rock structure play a dominant role in changing the drainage network orientation in
the Shivkhola Watershed. The interior and exterior links, length, and frequency of
streams are related with the azimuth of flow direction. The main drainage line of the
Shivkhola is oriented towards the south-east up to lower Paglajhora, and then it has
got its course in easterly direction with sinuosity within the sub-watershed-1. The
left hand two main tributaries are oriented in south-east direction through sub-
watershed-4 and 5 respectively. Sub-watershed 2, 3 and 6 consist of the tributaries
which are oriented in north-east direction. It is to be assumed that large part of the
sub-watershed-1; 2, 3 and 6; 4 and 5 are characterized by south-east and east; north-
east and south-east slope facets according to orientation of main drainage lines
where the landslide potentiality is high. The study depicts that the drainage ori-
entation plays an important role for promoting landslip as the availability of
moisture content depends on it which helps to reduce cohesion and increase pore-
water pressure in the soil and induces landslip.

3.3.5 Drainage Density and Landslide Potentiality

According to Horton ( 1945 ) Drainage density in an important indicator of the

linear scale landform elements in a drainage basin

'

. The views of Horton ( 1945 ) and
Strahler ( 1964 ) and others on drainage density are almost identical. The latter states,
“

'

find out drainage density the length of the streams of all hierarchical
orders should be measured, totaled up and divided by the geographical area of the
drainage basin

in order to

in order
to express the texture of dissection, we must use not the water course but the valley
bottom (the thalweg), whether wet or dry

” . Dury ( 1969 ) has used ' the expression of texture ' and says '

'

. The thalweg length divided by area is
equivalent to drainage density as suggested by Horton and others.
This may be taken as the index of dissection of a drainage basin. Two areas may
have identical drainage density but while one of them may have higher dissection
index because of the presence of a large number of streams, the other may have
lower dissection index because of the presence of small number of streams.