Impact Assessment of Hydrologic Attributes and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 95

encouraging and changing others sub-areal processes such as surface run-off,
seepage, and sub-surface flow. But, for the development of the drainage network
and for changing actual nature of the slope materials rainfall must plays an
important role. The history of evolution of a drainage basin and its morphology are
closely related to rainfall characteristics and geology, i.e. the type of rocks and their
structure. Depending upon the susceptibility to erosion and structure of the rocks
the fluvial processes operate and the landform evolution proceeds through
successive stages with drainage network branching and head ward extension.
Hydrological attributes mainly rainfall and drainage are the two important factors of
slide and soil erosion as the material moves down slope with the help of water and it
may be either surface or sub-surface flow. The water when flows on the surface
with huge amount can perform remarkable erosion as the potential energy it pos-
sesses is readily transfers to kinetic energy which takes part in the mass transfer.
The soil water saturates the soil and increases the pore water pressure and the
weight of the mass which then becomes more prone to slide or subsidence. The
occurrence of few days continuous rainfall and their presence within the subsoil

may act as the lubricating agent and which makes the down slope movement easier
by reducing the friction, cohesion and increasing shearing stress in the slope
material. Rainfall as a trigger has been extensively studied by number of authors i.e.
Larson ( 1995 ), Polloni et al. ( 1996 ), Glade ( 2000 ), Wieczorek and Guzzetti ( 2000 ),
Polemio and Petrucci ( 2000 ), Toll ( 2001 ) and Zezere ( 2000 ).
One of the most important geomorphic properties is the degree of dissection of
the topography, sometimes expressed in terms of drainage density. Gradual
extension of drainage network and their regular branching increase the slope
steepness by increasing slope concavity and convexity. Gilbert ( 1909 ) argued that
convex-concave forms reflect a gradual transition in process dominance from creep
to wash with increasing distance from a drainage divide. Gilbert

'

' s model was

quanti ed in terms of a linear stability analysis by Smith and Bretherton ( 1972 ).

Drainage density and landscape structure may alternatively be controlled, by
thresholds for run-off generation (Kirkby 1980 ; Ijjasz-Vasquez et al. 1992 )orby
thresholds of slope stability (Montgomery and Dietrich 1989 ). Drainage density in
particular may be controlled to varying degrees by any of these thresholds, and each
different threshold may produce a different functional relationship between drainage
density and factors related to climate geology and relief. Howard ( 1997 ) repre-
sented a detachment-limited model in which the relationship between drainage
density and mean erosion rates depends on (i) the dominant hill slope transport
process (creep/landsliding) and (ii) the presence or absence of a threshold for run-
off erosion.
The Shivkhola basin is under the humid climatic condition receiving excessive
orographic monsoon rainfall, which enhances the erosion and denudation of the
surface within the basin. The nature of terrain, evolution of landscape, amount of
soil erosion and their removal, in the Shivkhola watershed could be well acquainted
by studying various hydrological factors/attributes i.e. climatic attributes (rainfall
and evaporation), drainage confluence, drainage density, upslope contributing area
and others. To understand the probability or chances of landslide occurrences