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1 This country is characterised by the large diversity of its natural setting: relief, climate, vegetation, soils and geology etc. 2 Frequency and trend of the phenomenon Losses in human Lives Material losses 3 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. 4 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 5 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) 6 Topography • 地形 Geology • 地質 7 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. 8 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). 9 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) 10 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 11 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 12 These involve rainfall, seismic activity and human action excess rainfall Rainfall deficit impact of rainfall deficits Seismic activities Mechanical erosion by mudflows Human action 13 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. 14 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 15 16 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 17 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. 18 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. 19 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. 20 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 . 21 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. 22 23 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. 24 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 25 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) 26 27 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 28 1.the permanent factors 2.direct or triggering factors. 29 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 30 31 32