Download The catastrophic geomorphological processes in humid tropical

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This country is
characterised by the
large diversity of its
natural setting: relief,
climate, vegetation,
soils and geology etc.
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Frequency and trend of the
phenomenon
Losses in human Lives
Material losses
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Frequency and trend
the of
several of
hundreds
phenomenon
slides could be counted,

stretching over a few
squareofkilometres
An analysis of the frequency and magnitude
landslides
and mudflows in Cameroon makes it obvious that the
phenomenon is on a net increase.
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Losses in human Lives
A total of seventeen out of
twenty-six landslides
recorded during the period
led to losses in human lives.
Their distribution over time is
an eloquent proof of the
tendency for the number of
people affected to grow
exponentially
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Material losses
Two major groups of causes and factors can
be distinguished:
 Remote causes(遠因或天生因素) responsible for
the vulnerability of the physical milieu(天生地
貌上的脆弱度)

Immediate causes(直接因素) or opportunistic
factors(隨機因素)
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Topography
• 地形
Geology
• 地質
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Topography
In Cameroon, the relief(地形起伏)
of many regions is susceptible to mass
movement(土砂運動) (Fig. 4).
The highlands of the Cameroon volcanic line,
a chain of mountains which is more or less
deeply dissected, often with more or less
steep slopes.
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崩塌地發生的位置
幾乎都在火山鍊上
Fig. 4. Distribution of sites of
catastrophic landslides in
Cameroon (in the course of the
last three decades).
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Topography
•The isolated massifs(斷層) rising above
plains(平原) and plateaux(高原)
•The South Cameroon plateau region with
the half-orange topographic model
characterised by convex–concave(凹凸的)
slope profiles amenable to landslides
especially in urban areas.(都市地區)
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Ref. :
GRIMALDI Catherine (1), CURMI Pierre (1,2),
DOSSO Mireille (3), JOUVE Philippe (3) and
SIMOES Aquiles (4)：
Sustainability of agrarian systems in relation to
soils on Amazonian forest pioneer fronts
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Geology
The wide variation(變化) in relief is the result of a
complex geological basement(地層), characterised,
according to the location by:
The intense tectonic activity
The pre and post tectonic intrusions
The volcanic eruptions
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These involve rainfall, seismic activity and human action
excess rainfall
Rainfall deficit
impact of
rainfall
deficits
Seismic
activities
Mechanical
erosion by
mudflows
Human action
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
The abundance(次數), intensity(強度),
duration(延時) and frequency(頻率) of rainfall
determine the critical thresholds(臨界值) for
the saturation of soils and the volume of
runoff responsible for the rapid and intense
erosion that affects slopes.
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The total annual rainfall
• for the six stations
The maximum daily rainfall
• in two key stations
Frequency in terms of the number of days
of rainfall
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The maximum daily rainfall
The maximum daily rainfall are classified into
five categories:
(1)>100 mm
(2)50~100 mm
(3)25~50 mm
(4)10~25 mm
(5)<10 mm
以兩個雨量站分析
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Frequency in terms of the number of days of
rainfall

Statistical analysis of the number of days of
rainfall portrays the general tendency towards
a decrease, with an average deficit of close to
25 days a year in Douala and 16 days in
Yaounde.
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
Hydrologically, these consequences are
manifested by a fall of water input into the soil,
resulting to the depletion(消耗) of ground water
reserves, leading to a pronounced drop in
stream or river discharge.
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
These low discharges are vivid evidences
of the profound decline in ground water,
whose consequences are the development
of a network of crevasses that destabilise
the cohesion of the soils, exposing them
to mass movements, in case of heavy
rainfall.
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
Seismic manifestations destabilise the soil
and often trigger mass movements.

micro-earth tremors provoke vibrations and
liquefaction of masses of soil, and
consequently lead to their pulling out and
flow .
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

In fact, because of the huge volume of
material transported, the rocky composition of
debris and the power involved, considerable
abrasion is exerted on the flanks of the valley.
This results in instantaneous mechanical
erosion that cuts away and destabilises the
flanks of the valley, leading to subsidence and
a series of rock falls and landslides, which
increase the amount of material flowing.
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HUMAN ACTION

The destruction of the vegetation cover which
increases the runoff and consequently the erosive
capacity of water (torrents) which removes and
destabilises entire soil sheets, exposing them to
slides and flows.
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

The destruction of the cohesion of the soil
by weeding(除草), which weakens the
surface layers, making them more
permeable(被滲透) than the deeper ones.
The practice of ridging(田壟) across the
slope. Unfortunately, in times of prolonged
and/or heavy rainfall, the retentive capacity
of the ridges may become insufficient
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Ridging (田壟)
好處：消能、儲水
壞處：遇大雨，便會破壞
Ref. : W B Russell
Conservation of Farmland in KwaZulu-Natal
GENERAL RIDGE TILLAGE PRACTICES(1997)
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HUMAN ACTION
The consequences of this pressure are twofold,
involving the destabilisation of the slopes and
risky settlements.
*Mechanical slope destabilisation
*Indirect destabilisation, by gullying
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1.the permanent factors
2.direct or triggering factors.
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1.The relief
2.The geology
1.The climatic factors
2.The geophysical factors
3.The morphodynamic factors
4.The anthropogenic factors
– The destruction of the vegetation cover
– The destruction of the cohesion and
stability of the soil
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