Impact Assessment of Hydrologic Attributes and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 113

Table 3.7 Frequency distribution and Landslide Potential Index (LPI) of upslope contributing
area
Upslope con-
tributing area
(Km 2 )

Number
of pixels
in %

Number of landslide
occurrences pixels
in %

Landslide
occurrence
ratio

Landslide potential
index (LPI) = F 2 /
F 1 × 100

<5.00 85.33 84.48 0.76 76.56

5.00 10.00 8.00 6.90 0.66 66.67

-

10.00 15.00 1.33 1.72 1.00 100

-

15.00 - 20.00 4.00 5.17 1.00 100

>20.00 1.33 1.72 1.00 100

3.4 Drainage Morphometry and Slope Instability

Geo-hydrologic and geometric parameters of the drainage play a signi cant role in

changing the quality of soil and water of a small hilly sub-watershed and make the
hill slope more vulnerable to slope instability. The morphometric study on river
basin was

first introduced by Horton ( 1932 ). Later on, this idea was developed by
Miller ( 1953 ), Schumm ( 1956 ), Coates ( 1958 ), Melton ( 1958 ), Maxell ( 1960 ),
Strahler ( 1957 ), Chorley ( 1969 ), and Mulder and Syvitsky ( 1996 ). During 1950s
and 1960s the morphometric proposals made by Horton again were developed in
the studies of drainage basin morphometry, which aimed to analyse regularities of
stream networks and subsequently led to the variation in network characteristics
and to investigation of statistical or topological properties of network. The hierar-
chical ranking of the stream in a river basin was studied by Strahler ( 1964 ). Sig-
ni cant contribution in various aspects of morphometric parameters have been

studied by host of scientists from time to time e.g. Morisawa ( 1962 ), Gregory and
Wallings ( 1968 , 1973 ), Singh ( 1978a , b , 1981 ), Singh et al. ( 1986 ), and Richards
( 1981 ). Recently Chandra et al. ( 1984 ) studied watershed with the help of remote
sensing method using simulation model. Quantitative geomorphology provides a
systematic approach to the analysis of a complex landscape of any size and origin
of the drainage basin. Some important parameters i.e. Basin Shape, Stream Length,
Length of Overland Flow, Form Factor, Circularity Ratio, Elongation Ratio,
Compactness Factor, Elipticity Index, Bifurcation Ratio, Relative Relief, Drainage
Density, Stream Frequency, Constant of Channel Maintenance, Ruggedness Index,
Dissection Index, Slope Inclination, and Upslope contributing area of the sub-
watersheds have been considered to develop the priority scale.
Basin shape is referred to the shape of the outline of the drainage basin that is
determined as shape of projected surface on the horizontal plane of the basin
map. Mulder and Syvitsky ( 1996 ) have indicated that a majority of the rivers have
elongated basins. Generally, the shape of the basin affects the flow pattern and
consequently affects the surface run-off and erosional mechanism within the
catchment. The basin shape may be expressed through Form Factor, proposed by
Horton in 1932. The cumulative length of channel segments increases with the