The Effect of Groundwater and Rainfall on Landslide Triggering in the Central Highlands of Ethiopia: the case of Debre Sina area

Tesfay Mebrahtu1, Stefan Wohnlich1
1 Applied Geology, Ruhr-Universität Bochum

P 11.5 in Beiträge zur Hydrogeologie Südamerikas und Afrikas

Over the past 60 years, natural disasters within the central highlands of Ethiopian have increased in both frequency and intensity, and have severe social impacts. Between 1990 and 2005, landslides and landslide-generated hazards have claimed about 435 human lives. It damaged over 100km asphalt road, demolished more than 1500 dwelling houses, more than 6500 people have been displaced, and devastated about 2000 hectare of agricultural land. To understand the causes and mechanisms of failures, and to identify the most relevant influencing factors, detailed geological and topographical, geohydrogeological, and geophysical investigations were carried out. The analysis involved in-situ hydraulic conductivity tests of soils, seismic refraction survey, and laboratory analyses of water samples to determine the hydrochemical content of the water in relation to the cause of the landslide of the study area. A total of 50 water samples were collected inside and outside the landslide body and analyzed for pH, Electrical Conductivity, major-ion chemistry (Cl-, F-, HCO3-, NO3-, SO42-, Li+, Na+, K+, Ca2+ , Mg2+ and Fe), and stable isotopes (18O & 2H ).

The recently affected areas indicate that numerous springs emerged at Yizaba Wein, Shotel Amba and Nib Amba localities. Particularly, the springs in the Yizaba Wein locality has changed its course. This might show that there is still active mass movement in the area. Several springs, which appear at the interface where fractured rock overlays the weathered part and volcanic ash or where there is paleosol between the various lava flows, are common. Due to the nature of rocks and terrain conditions, groundwater occurrence, flow and distribution conditions are not the same in the area. It is manifested in the form of seepage at elevated sloppy grounds and water level conditions in the lower elevated grounds. The resulting refraction seismic revealed that the groundwater is expected within the highly weathered and fractured units and fractured zones associated with geological structures represented by 1500-2500m/sec velocities. The analysis of the water samples shows that the major water chemistry is Ca-Mg-HCO3 and Ca-HCO3 with low EC varying from 72 to 542µS/cm. The chemical composition of groundwater of the study area is weakly influenced by rock-water interaction and the mineral content of the water samples were generally low. Therefore, the groundwater chemistry indicate that there is a fast hydrogeological regime receiving a relatively high amount of precipitation with infiltrated water flowing in relatively lithologicaly homogeneous fissured and mixed aquifers developed in various volcanic rocks and intercalated Quaternary superficial deposits. The steep slope topography, the pyroclastic sediments which are susceptible to slaking erosion and the pressure formed during precipitation, the presence of geological structures and weathered material would be one of the triggering factors of landslides in this area.