Impact Assessment of Hydrologic Attributes and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 110

Drainage density is an effective indicator of slope failure as the drainage ef ciency

from a rainfall is determined by the drainage length in an area. The length of
drainage per unit area is thus an indicator of surface flow and potential instability.
The source of the drainage channels are the slide prone areas and the valley sides
are also the zones of slope failure. So the potential slope failure zones are extended
along the drainage lines and are concentrated at the source points. The greater
drainage density is thus an indicator of potential instability. At mid-central areas,
near Paglajhora, numbers of drainage channels from the upper marginal watershed
region meet the main stream, and so drainage density is maximum indicating the
excess surface water (Fig. 3.6 ). This is the main cause of slope instability at
Paglajhora.

3.3.6 Upslope Contributing Area and Landslide Potentiality

Upslope Contributing Area (UCA) is an effective indicator of drainage concen-
tration over space. The surface water accumulates in a cumulative rate away from
the water divide as more Upslope Contributing Area helps in accumulation of more
water and so indicates more surplus moisture and instability of slope and soil. The
calculation of contributing area (Fig. 3.7 ) has to be made considering multiple flow
direction where the cumulative flow at a point (surrogated by the upslope area)
should be distributed among more than one neighboring down slope pixel (Borga
et al. 1998 ).The topographic irregularities at concave and convex slope are
responsible for the convergence and divergence of flow and thus necessitate the
implication of topographic index by Quinn et al. ( 1991 ). Another concept of
speci

c contributing area (total contributing area divided by the contour length) is
computed by distributing flow from a pixel among its entire lower elevation
neighbour pixel (Borga et al. 1998 ).
Quinn et al. ( 1991 ) proposed that Fraction of Flow (Fi) i ) allocated to each lower
neighbour is to be determined by:

S i L i
R

F i ¼ ð 3 : 9 Þ

S j L i

where the summation is for the entire lower neighbour; S is the directional slope,
and L is an effective contour length that acts as the weighting factor. The value of L
used here is 10 m of the pixel size of the cardinal neighbour and 14.14 m of the
pixel diagonal for diagonal neighbour.
The study on Landslide Potential Index (LPI) of Upslope Contributing Area
depict that the percentage of occurrence of landslides and its distribution among the
different groups of contributing area shows the increasing Landslide Potential Index
(LPI) with increasing value of upslope contributing area (Table 3.6 ). The regions of
contributing area of less than 5 Km 2 experiences the LPI of 76.56 and that of 10
Km 2 experiences the LPI of 100 (Table 3.7 ). Figure 3.7 shows the grid wise value