Geological Hazards and Anthropogenic Impacts on the Environment in Malawi: Lesson from a Case Study of Debris Flows in Zomba

Abstract

Natural geological hazards such as flash floods, landslides, debris flows and mudflows have caused, and will continue to cause, many problems in Malawi. Landslide risks are primarily a geological problem and it is Quaternary geology that concerns our environment most. It is a typical geological task to reconstruct the old events using geological, geomorphological and dating methods and based on these results, predict possible future disasters. Our studies of old and recent debris flow events in Southern Malawi have found out that these phenomena are mainly triggered by tropical cyclones that bring heavy rainfalls. They occur in mountain slopes with certain slope instability, soil cover and discontinuity patterns of the underlying rocks Human impacts on the environment such as large scale earthworks projects and replacement of natural forests by non endemic tree species have also played an important role in a rising risk of flash floods and debris flows. It is recommended that the most sustainable method of mitigation against these hazards is to incorporate the geological hazard zoning maps into town and regional planning schemes. Where this may not be possible, measures that can reduce or minimise the hazard impact should be adopted.

Zomba Mountain is made of a Mesozoic syenite pluton that intruded Precambrian high-grade metamorphic gneisses and granulites. Weathering erosion and uplift have left the syenite standing at an elevation of 2000 meters while the surrounding area is between 700 and 1000 meters above sea level. Zomba town is located at the foot of the mountain. Steep slopes with up to 50°, the highly jointed syenite, and the thick lateritic soil covers are the main factors likely to influence rock instability. Debris flows triggered by heavy rainfall have affected Zomba since memorial times. People of Zomba and the authorities are well aware of past landslides and the continued risk. As a result from our investigations the detailed risk zoning of Zomba contributes to the disaster preparedness plan of the town.