Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 178

Table 7.1 The role of vegetation in promoting slope (in)stability (after Bell, 1998 )

Mechanisms Influence

Hydrological
Foliage intercepts rainfall, causing absorptive and evaporative losses
that reduce rainfall available for infiltration

Beneficial

Roots extract moisture from the soil, which is lost to the
atmosphere via transpiration, leading to lower water pressure

Beneficial

Roots and stems increase the roughness of the ground surface and
the permeability of the soil, leading to increased infiltration
capacity

Adverse

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

Adverse

Mechanical
Roots reinforce the soil, increasing the shear strength

Beneficial

Tree roots may anchor into firm strata, providing support to the
up-slope soil mantle through buttressing and arching

Beneficial

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

Beneficial

Weight of trees surcharges the slope, increasing normal and
downhill force components

Adverse or beneficial

Vegetation exposed to the wind transmits dynamic forces into the
slope

Adverse

several thousand years) before a similar size extreme rainfall event could cause
afailure of the same magnitude. In other words, the long-term processes need
considerable time to render the slope marginally stable by once again building
up sufficient saprolite on the slope.
Changes in vegetation on a slope can increase or decrease that slope's suscep-
tibility to a landslide. However, it is often the case that removal of vegetation
usually leads to increased slope instability and higher erosion potential. Bell
(1998)summarises the role of vegetation on slope stability as shown in Table 7. 1 .
The processes causing slope failure are complex. As such, several classifica-
tions of slope failure have been presented in the scientific literature. Many of
these classifications are based on slope failure specific to one particular area
and are not applicable to other sites where the geological and environmental
variables differ. Due to the difficulties in defining the commonly used term
landslide, several authors (Crozier, 1986;Bell, 1998)have advocated using the
term slope movement for mass movements restricted to slopes. The term slope
movement encompasses three forms of mass movement in rocks and soils, these
being falls, slides and flows (Bell, 1998)(Fig.7. 1 ).