556
Climatic Geomorphology
or by long-lasting precipitation (Thomas, 1994). In March 1974 it rained 742 mm in 16
days in Tubarau in southeast Brazil, of which 240 mm were recorded on the last day.
Catastrophic consequences occurred, with numerous landslides and flooding, with 25
people died in one single slide (Bigarella and Becker, 1975).
Brand et al. (1985), in their studies on the relationship between precipitation and slope
failure in Hong Kong, indicated that if rain intensity exceeds 70 mm/h important failures
are triggered, whereas if it does not reach 100 mm/day failures are very scarce. These
authors also emphasised the role played by preceeding rains, because they condition the
development of surficial or deep slope failures.
Research on the influence of vegetation on slope stability was begun in the decade of
the 1960s. Until then many authors had considered it as a factor with a low incidence, but
at present it has been demonstrated under several circumstances that vegetation has an
important influence (Greenway, 1987). Nevertheless, tree and shrub afforestation has been
used for more than a century for controlling erosion and trying to stabilise slopes. So
(1971) studied 700 failures triggered by an intense rainstorm in Hong Kong and suggested
that such mass movements are more commonly located on the more vegetated slopes.
When studying the role of vegetation on slope stability, Greenway (1987) established a
distinction between hydrological and mechanical mechanisms. Among the former, the
beneficial role of raindrop interception can be mentioned, which produces water loss by
absorption and evaporation and hence reduces infiltration. On the other hand, roots and
stems increase infiltration capacity and subsurface flow. Roots also take water from the
soil and diminish pore water pressure. An adverse factor is the loss of soil moisture, which
leads to soil cracking and to a higher infiltration capacity. Physical mechanisms include
reinforcement by roots, which increase soil shear strength. Factors that also must be taken
into account include root-type, penetration depth, lateral extension, and root strength.
Tsukamoto and Kusakabe (1984, in Greenway, 1987), in their study of the influence of
vegetation on slopes in Japan, developed a simple slope classification in four types
(Figure 21.24). Type A slopes have a thin soil thickness where roots do not penetrate into
the bedrock; the soil-rock interface can act as sliding plane. Type B slopes are similar to
type A except that roots penetrate into the bedrock. Type C slopes have a thicker regolith
and contain a transition layer in which soil density and shear strength increase with depth.
Roots penetrating this layer provide a stabilizing force to the slope. Finally, type D slopes
have a very thick regolith and roots are "floating" within the soil, with little mechanical
influence of stability. Below the root zone deep slip planes can be produced.
Roots also reduce susceptibility to erosion. Tree weight depends on the species,
diameter, height, and separation. Overload on a slope increases shear stress and pore water
pressure. In a broad sense, if the slope angle is small then overload increases stability.
Finally, forces of instability derived from wind action can be transmitted from the
vegetation to the soil. Strong winds can pull out trees, especially those with superficial
roots.
In slopes with low inclination very few failures occur. In the African humid tropics
several authors (Thomas, 1994) have indicated that slope instability appears with gradients
higher than 26 to 28 ~ although these values can substantially differ in other regions of the
world. According to different authors, most slides develop between 28 and 45 ~ . When
slopes are very steep failures are very numerous, especially in relation to heavy rainfall
(Figure 21.20 and Figure 21.25). Slopes can modify their inclination by fluvial or marine
Climatic Geomorphology
or by long-lasting precipitation (Thomas, 1994). In March 1974 it rained 742 mm in 16
days in Tubarau in southeast Brazil, of which 240 mm were recorded on the last day.
Catastrophic consequences occurred, with numerous landslides and flooding, with 25
people died in one single slide (Bigarella and Becker, 1975).
Brand et al. (1985), in their studies on the relationship between precipitation and slope
failure in Hong Kong, indicated that if rain intensity exceeds 70 mm/h important failures
are triggered, whereas if it does not reach 100 mm/day failures are very scarce. These
authors also emphasised the role played by preceeding rains, because they condition the
development of surficial or deep slope failures.
Research on the influence of vegetation on slope stability was begun in the decade of
the 1960s. Until then many authors had considered it as a factor with a low incidence, but
at present it has been demonstrated under several circumstances that vegetation has an
important influence (Greenway, 1987). Nevertheless, tree and shrub afforestation has been
used for more than a century for controlling erosion and trying to stabilise slopes. So
(1971) studied 700 failures triggered by an intense rainstorm in Hong Kong and suggested
that such mass movements are more commonly located on the more vegetated slopes.
When studying the role of vegetation on slope stability, Greenway (1987) established a
distinction between hydrological and mechanical mechanisms. Among the former, the
beneficial role of raindrop interception can be mentioned, which produces water loss by
absorption and evaporation and hence reduces infiltration. On the other hand, roots and
stems increase infiltration capacity and subsurface flow. Roots also take water from the
soil and diminish pore water pressure. An adverse factor is the loss of soil moisture, which
leads to soil cracking and to a higher infiltration capacity. Physical mechanisms include
reinforcement by roots, which increase soil shear strength. Factors that also must be taken
into account include root-type, penetration depth, lateral extension, and root strength.
Tsukamoto and Kusakabe (1984, in Greenway, 1987), in their study of the influence of
vegetation on slopes in Japan, developed a simple slope classification in four types
(Figure 21.24). Type A slopes have a thin soil thickness where roots do not penetrate into
the bedrock; the soil-rock interface can act as sliding plane. Type B slopes are similar to
type A except that roots penetrate into the bedrock. Type C slopes have a thicker regolith
and contain a transition layer in which soil density and shear strength increase with depth.
Roots penetrating this layer provide a stabilizing force to the slope. Finally, type D slopes
have a very thick regolith and roots are "floating" within the soil, with little mechanical
influence of stability. Below the root zone deep slip planes can be produced.
Roots also reduce susceptibility to erosion. Tree weight depends on the species,
diameter, height, and separation. Overload on a slope increases shear stress and pore water
pressure. In a broad sense, if the slope angle is small then overload increases stability.
Finally, forces of instability derived from wind action can be transmitted from the
vegetation to the soil. Strong winds can pull out trees, especially those with superficial
roots.
In slopes with low inclination very few failures occur. In the African humid tropics
several authors (Thomas, 1994) have indicated that slope instability appears with gradients
higher than 26 to 28 ~ although these values can substantially differ in other regions of the
world. According to different authors, most slides develop between 28 and 45 ~ . When
slopes are very steep failures are very numerous, especially in relation to heavy rainfall
(Figure 21.20 and Figure 21.25). Slopes can modify their inclination by fluvial or marine