Landslide Mitigation - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 241

Table 8.2 Effects of vegetation on slope stability (After Greenway 1987 )
Hydrological mechanisms Mechanical mechanisms

1. Foliage intercepts the rainfall, causing
absorptive and evaporative losses that reduce
rainfall available for in ltration

1. Roots reinforce the soil, increasing soil
shearing strength

2. Roots and stems increase the roughness of
the ground surface and the permeability of the
soil, leading to increased in ltration capacity

2. Tree roots may anchor into film strata,
providing support to the upslope soil mantle
through buttressing and arching

3. Root extracts soil moisture from the soil,
which is lost to the atmosphere via transpi-
ration, leading to lower pore water pressures

3. Vegetation exposed to the wind transmits
dynamic forces into the slope

4. Depletion of soil moisture may accentuate
cracking in the soil, resulting in higher
infiltration capacity

4. Roots bind soil particles at the ground
surface, reducing their susceptibility to soil
erosion

are bene cial, while those that yield higher soil water are adverse. Mechanical

mechanisms that increase shear resistance in the slope are bene cial, while those

that increase shear stress are adverse. Simply, vegetation covered surface generally
reduce the ability of rainfall to cause slope failure through the process of inter-
ception and evaporation and consequently, increases the shear strength in under-
ground soil by the network of roots. For the interception by trees, the structure of
the canopy is to be analysed by the free throughfall co-ef

cient, the stemflow

partitioning coef cient, the canopy storage capacity and the trunk storage capacity.

In the present study of Shivkhola watershed, suitable vegetation cover is of great
importance at Paglajhora up and down slope, Tindharia T.E., 14 Miles Bustee and
Nurbong Tea Garden area as it affects the slope hydrology and slope stability and
determines throughfall, stem flow, root water uptake, permeability, root rein-
forcement, vegetation surcharge and interception loss from input rainfall that is
described here in an integrated vegetation-slope model (Greenway 1987 ; Fig. 8.15 ).
The connection between forest cover and deep-seated landslide initiation is less
clear than with shallow landslide; however, any land use or management practice
that alters hydrological pathways and timing could potentially influence deep-seated
mass movement activity (Phillips et al. 1990 ).
Dense vegetation cover and the network of tree roots are considered to be a
major contributor to soil strength and slope stability in the area where roots are
present (Phillips and Watson 1994 ). Various studies have also used the tensile
strength of tree roots as a measure of the contribution of vegetation to increasing a
slope

'

s stability and thereby reducing the incidence of landsliding (Gray and Sotir
1996 ; Montgomery et al. 2000 ). As a result of increase rate of deforestation, the
Shivkhola Watershed is facing the problems of soil erosion due to direct falling of
rain drops and slope failure due to sub-surface soil saturation very easily. Con-
sidering the importance of vegetation cover and to avoid such problems of erosion
and slope failure, afforestation programme is to be introduced in the landslide
affeceted area such as Paglajhora (Fig. 8.16 ), Tindharia, Mahanadi, Sepoydhura,
Shiviter and Nurbong immediately where most of the slope surface area is exposed