Arba Minch sheet

The Arba Minch Sheet is located in the southern main Ethiopian rift floor straddling the area between Lake Abaya and Lake Chamo shores. 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 Arba Minch map sheet is found covering an area that straddles the area between the western escarpment and the floor of Main Ethiopian Rift in southern Ethiopia. Geologically it is comprised of lower Eocene to Pleistocene volcanics of episodic eruptions which have a bimodal composition with alternating basic volcanic rocks and acidic pyroclastic rock intercalations. The geochemical signatures suggest a setting of intra-plate origin as expected with possible fractionation in the case of the acidic pyroclasts. Primary structures in the volcanic rocks have subhorizontal to gently E or W dipping flow foliation. Main tectonic overprints are of brittle origin in the form of extension joints and normal faults which are quite consistent with the marginal older faults and fault escarps of the main Ethiopian Rift that trend in the NNE–SSW with steep to moderately dipping slickensides towards ESE to NE. The geological map representing all identified lithological associations and structural relations are produced at a scale of 1:50,000 which served as the base for the subsequent maps and the investigations in the current study.

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 280 points in the area. Surface hydrological characteristics and groundwater dynamics have been addressed to develop water resource assessment in the region. The total water resources of the area were assessed to correspond to 172 Million m³/year. The quality of water resources available is also investigated showing transitional calcium bicarbonate and basic sodium-bicarbonate hydro-chemical composition. 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 Arba Minch map sheet in this study.

Engineering geological assessment of the area indicate regions of rock mass ranging in strength classes as high (>4 MPa), medium (2-4 MPa) and low (<2 MPa). 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. Prominent and sharp morphological regions standout as generally resistant and stronger rock mass units contrary to rock mass units in subdued and smoothly undulating morphology. Genetic associations of the soil in the area are also distinguished as residual lateritic soil, colluvium slope wash deposits, alluvial cones, thin lacustrine deposits on the Lake Abaya shore and alluvial plains.

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 aggradation 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.