Leku map sheet

The Leku map sheet is located in the central main Ethiopian rift floor south of Lake Hawassa. Comprehensive thematic maps, including basic geological map, soil map, hydrogeological map and geo-hazards risk map are compiled based on field and remote sensing data. An accompanying detailed explanatory booklet on each of the thematic layer content and recommendation on preventive actions and strategy for geo-hazards risk identified are also prepared.

Abstract

The Leku map sheet is located within the middle part of the Main Ethiopian Rift System, on the border of its central and southern segment. Geologically it is comprised of upper Miocene to Pleistocene volcanics. The predominant rocks represent products of voluminous eruptions and pyroclastic deposits are associated with the Hawasa Caldera formation. The tectonic depressions and grabens located mainly in the northern part of map sheet are filled by re-sedimented volcanoclastics, alluvial and lacustrine deposits of Pleistocene to Holocene ages.

Soil type classification was compiled based on the geological map, field survey, satellite image interpretation and digital elevation models. Four soil catena units have been identified as reference groups: Cambisols, Nitisols-Acrisols, Regosols-Cambisols, and Fluvisols in respective order from the top of the escarpment to the rift floor. The soil classes have been identified based on chemical and physical properties.

Hydrogeological assessment has been made following the regional morphology, local geological and water point’s inventory from around 190 points in the area. Surface hydrological characteristics and groundwater dynamics have been addressed to develop water resource assessment in the region. The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 95 Million m³/year. The quality of water resources available is also investigated showing transitional calcium bicarbonate and basic sodium-bicarbonate hydro-chemical composition. Part of the groundwater has high fluoride content that causes a range of serious public health problems Distribution of the aquifers along with the hydro-chemical compositions are combined in the produced hydrogeological map at a scale of 1:50,000 of Leku map sheet in this study.

Engineering geological assessment of the area indicate regions of rock mass ranging in strength classes as moderate (3–4 MPa) and low (strength has been almost completely undermined). These correspond to variation in lithological units which on average show distinctly variable intact strength and variable susceptibility to weathering based on the field documentation and physiographic areal disposition. Genetic associations of the soil in the area are also distinguished as volcanoclastic deposits, residual lateritic soil, colluvium and eluvium deposits, and slope scree deposits.

Exogenous and endogenous hazards in the area have been defined based on existing studies and field documentation. In the case of endogenous hazards, seismicity has been found to be more critical as it has a higher probability of occurrence combined with low preparedness and low legislation to enforce precautionary measures such as conservative design codes and emergency response systems as well as early warning systems. On the other hand extensive exogenous processes which pose danger to everyday life are extensively documented. Due to the morphological set up of the area both erosion and suffusion related hydrological hazards appear to be overwhelming affecting farmlands as well as infrastructure. In the hazard maps, all the potential risk areas from such exogenous hazards are shown.