Dila map sheet

Dila map sheet is located on the eastern rift margin of the southern main Ethiopian rift valley. 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 Dila map sheet at a scale of 1:50,000 is located in southeastern Ethiopia in the eastern margin of the southern Main Ethiopian Rift escarpment. The volcanic, pyroclastic and volcanoclastic rocks exposed in the mapping area are the products of episodic volcanic activity from Eocene to Pleistocene. The pre-rift volcanic activity is represented by Nazret Group (Eocene to Pliocene), whereas syn-rift event by Dino Formation (Pliocene to Pleistocene). Rocks from both formations show intraplate volcanic signatures and chemical composition rocks ranging from rhyolite, through trachyte, to basalt. The Quaternary deposits are represented by alluvial fans, colluvial deposits and fluvial sediments preserved mainly in the river valleys. Main tectonic overprints are of brittle origin in the form of extension joints and normal faults dip steeply to ~W and ~WNW or less also to ~E and ~ESE.

Soil map was compiled based on the geological map, field survey and sampling, satellite image interpretation and digital elevation models. Six major Reference Soil Groups were identified. Luvisols are situated at all altitudes, accompanied by Nitisols in the residual landforms of the rolling to hilly plateaus. Leptosols, Regosols and Cambisols mostly occur in dissected relief with outcrops, river gorges and escarpments. Fluvisols are found on fluvial and alluvial deposits. Some soils in the highland plateaus and plains in the SE part of the area have gleyic properties. Highly eroded badlands are situated to the NW of Dila.

The hydrogeological system of the Dila sheet is characterised by dominant groundwater accumulations in porous aquifers developed in alluvial, fluvial and colluvial sediments and in aquifers with fissured and mixed fissured-porous permeability developed in volcanic and mixed sedimentary-volcanic rocks. During the field investigation, 105 water points were inventoried and 25 water samples were collected. The total water resources of the area have been assessed to be 217 Mm3/year. The water types in the study area are dominantly Ca–HCO₃ in the highlands and the escarpment area. The water is transformed through transient Ca–HCO₃ and Na–HCO₃ types into a basic Na–HCO3 type, which is characteristic for the rift floor. Local pollution of groundwater by nitrates occurs in rural as well as urban areas due to a lack of protection of water sources.

Engineering geological assessment of the area indicates regions of rock mass ranging in strength classes as high (welded ignimbrites, trachyte, and rhyolite), moderate (weathered basalts and some ignimbrites) and low (non-welded ignimbrites with values ranging between 20 and 30 MPa). Prominent and sharp morphological regions stand out as generally resistant and stronger rock mass units, contrary to rock mass units in subdued and smoothly undulating morphology.

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 (e. g. conservative design codes, emergency response systems, early warning systems). On the other hand, extensive exogenous processes, which pose danger to everyday life are extensively documented. Several small landslides and rock fall collapses were observed at road cuts, particularly in places where highly weathered rocks (regolith), and colluvial deposits occur; for example in the road from Dila to Bule and between Bedesa and Abera. 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.