Using Remote Sensing in Evaluating Spatial and Temporal Characteristics of Geological Hazards Triggered by Climate Change Events in Malawi

Climate change can enhance extreme weather conditions thereby significantly influencing change in precipitation patterns. This results in change to both rainfall intensity and distribution in many parts of the world. The increase in rainfall intensity triger hazards such as flooding and geohazards such as landslides. The impact and scale of flooding hazards are well evaluated and documented. However, geohazards from extreme weather events are not well documented in many developing countries making it difficult to quantify trends, frequency and spatial distribution. This knowledge gap is particularly evident in countries like Malawi, which have been significantly impacted by climate-related extremes, including cyclones. For instance, Tropical Cyclone Freddy which made a landfall in Southern Africa, induced above normal torrential rains in Malawi in March 2023. These rains triggered landslides, mudslides, debris flow and floods which affected 14 districts in the southern region of Malawi (GoM,2023).The size, magnitude and effect of flood hazard from the Tropical Cyclone Freddy was well studied. However, quantifying and mapping spatial patterns of geohazards was not well documented and less prioritized.

In this study, we used sentinel 2, rainfall and field data to a) characterise geohazard from extreme weather events, b) analyse extent of landslides c) develope inventory for geohazards d) quantify the relationship between extreme rainfall events and occurrence of geohazard in southern parts of Malawi. Field survey was conducted in the landslide scarps in Southern Malawi (Blantyre, Thyolo, Phalombe, Chiradzulu and Zomba districts) for ground truthing, mapping landslides, improving accuracy of mapping and validating data from remote sensing analysis. A total of 21 landslides were identified and mapped through field surveys. These ranged in size from localized to more than 50m along the slope. Through remote sensing analysis, it was observed that most of the hazards occurred on different times even if they were exposed to same extreme weather, but all occured within a window period of three days from onset of high intensity rains.  This correlates to the peak rainfall intensity observed across many areas. The characteristics of the material from the landslide scarps varied from loam clay, boulders to debri. However, most of the areas were charecterised with steep slopes of above 60^o slope angle. Additionally, in some areas geological influence of landlside occurrence was evident, this was inform of dolerite dyke intrustion and some faults. Thus, other than extreme rainfall occurence, landslides were highly influenced by the slope angle and geological factors. Type of material on the slope had minor influence and this was in agreement with the results from regression analysis. These study results will act as guide to predict the occurrence of future geohazards and understand their patterns which is key in predicting future occurrences of the hazards.

 

Keywords: Remote Sensing, climate change, Cyclone Freddy, landslides, Malawi

References

• Government of Malawi. 2023. Malawi 2023 Tropical Cyclone Freddy Post-Disaster Needs Assessment. Lilongwe
• Malawi: Tropical Cyclone Freddy Department of Disaster Management Affairs (DoDMA) Situation Report No3 (2023)