A comparative study of landslides and geohazard mitigation in Northern and Central Malawi

[摘要] English: In 2003, a number of landslides occurred in the Ntchenachena and the Chiweta Areas of the Rumphi District in Northern Malawi, and in the Livilivi/Mvai Catchments of Ntcheu District in Central Malawi. The landslide events caused significant damage to crops, farmland, livestock and infrastructure. Worse still, they caused the death of four people. The high density of landslides occurrences suggested instability of the slopes of these areas. In light of these landslides, this study set out to assess the slope stability status of the areas. The study addressed landslide mapping and classification of observed events; assessment of the causes and contributing factors; assessment of the socio-economic and environmental impacts of the events; exploration of traditional knowledge, beliefs and peoples perceptions surrounding landslides; determination of the coping strategies; and development of mitigations to landslides as geo-hazards. This study involved a landslide inventory of all observed This study involved a landslide inventory of all observed events. The physical characteristics of the terrain influencing slope instability were measured. The characteristics recorded included slope length, angles, aspect and altitude, and channel dimensions. Landslides were classified based on the type of movement, degree of stabilisation, and age, and materials involved in the movement. Soil samples were collected, using core and clod sampling methods and were tested for plastic limit, liquid limit, plasticity index, bulk density, hydraulic conductivity, aggregate stability, and particle sizes. Structural rock weaknesses were also measured. Vegetation data was collected, using the quadrant method and was analysed for average diameters at stump and breast height, canopy cover, and height. Questionnaires/surveys were used to assess local knowledge and perceptions towards landslides. A SPSS statistical package was used to analyse both social and physical data. It was found that 131 landslides had occurred of which 98 were in the Rumphi District, Northern Malawi and 33 occurred in the Ntcheu District, Central Malawi. The variations were observed to be due to the degree of disturbance of the physical environment. The Ntchenachena Area, with the highest density (88), was under cultivation and the afro-montane vegetation had been completely destroyed. The deepest channels were observed in the Ntchenachena Area, partly because of the deep chemical weathering of the basement. In contrast, the rest of the areas had thin soils. Slope aspect and type were found to be of little significance in the occurrence and spatial distribution of the events. The analysis of data suggested that the events were caused by liquefaction of sand and silt fractions due to high and prolonged precipitation. The evidence from the Chiweta and the Mvai Areas suggests that high cleft water pressure between rock and soil masses might have caused some failures. However, destruction of vegetation, cultivation on marginal lands, high slope angle, weathering of the basement, and slope cutting contributed to the instability. The study also noted that the Ntchenachena, the Mvai and the Livilivi Areas largely require soft solutions to the landslide problem. These include afforestation, proper siting of houses, and restricting settlement activities in danger-prone areas. Income generating activities to reduce poverty, community participation in natural resources management and public awareness and outreach programmes are highly recommended. The Chiweta Area requires urgent major engineering works such as construction of embankments, cable nets, wire meshes, improving drainage and plugging. Stabilisation and rehabilitation of river banks is also recommended to minimize bank collapse and flooding. Integration of traditional knowledge into the existing scientific body knowledge is critical to a better understanding of the mechanisms that generate landslides Further work needs to be carried out in areas of willingness to relocate to safer ground; change in production system; geological analysis of the Chiweta beds; hydro-geological assessment of the areas; development of landslides predictive models for Malawi; and the development of a landslide early warning system.

[发布机构] University of the Free State