GEOLOGICAL CONTROLS IN THE FORMATIONS AND EXPANSIONS OF GULLIES OVER HILLSLOPE HYDROLOGICAL PROCESSES IN THE HIGHLANDS OF ETHIOPIA, NORTHERN BLUE NILE REGION

The Northern Blue Nile River source region shallow depth highly weathered and fractured flood basalt with several local structures and intrusive dykes. Volcanic morphological features and erosion has formed smaller watersheds that exist within the Blue Nile basin. The control of the flow behavior of both surface and subsurface water form a central pool for the sediment transported to the Sudanese plain. The objective of this study is to define the geologic controls of hydrological processes aiding in the formation and expansion of gullies along hill slopes in a micro watershed in the Blue Nile River source region. Twenty-four piezometers were installed, and soil hydrological behavior was studied in 17 ha of land in the Debremawi watershed. Geologic features of the watershed include shallow depth, highly weathered and fractured basalt, an impermeable layer and a local dyke. Layers of clay soil deposits are defined in the middle down slope area of the watershed. The basalt is exposed in the upper slope area and underlies most of the watershed, forming a fractured media aquifer. The impermeable layer consists of weathered silt sand clay pyroclastic fall that has formed a compacted layer at the surface on the up hill. The local intrusive basaltic dyke, located at the middle of the watershed and perpendicular to the flow direction, has significant effect on the local ground water table distribution. Clay soil covering the middle area of the watershed and overlying on the basalt layer confines the water in the fractured media aquifer. Different sites with saturation excess runoff and infiltration excess runoff are identified to be controlled by the combined effect of the local geological material and land use type. Water head upstream of the dyke is near or above surface, but it is at a considerable depth below the surface when downstream of the dyke. Local saturation zones are subject to pore water pressure development and landslides. Saturated area soils have little strength and result in soil slumps. High piezometric head and small scale earth movement are identified in relation to the confining effect of the clay layer. Topographically controlled saturation zones are also vulnerable to landslides and extensional soil cracking failures. The ultimate impact of the local geology control is subsurface erosion features (soil pipes and tunnels) that develop into gullies. Hence, land management practices should consider detailed studies of the local geologic materials and structures. Incorporation of subsurface drainage mechanisms with the usual soil and water conservation practices are of paramount importance for a better achievement in resolving the existing erosion and sedimentation problems. BIBLIOGRAPHIC SKETCH Antenhe Zewdie Abiy is born in the 27, July-1980 to his lovely mother Tiruwork Eyasu and father Zewdie Abiy. He completed his first degree at Mekelle University in Applied geology during July 2003. Anteneh has worked on developmental activities in rural Amhara, Ethiopia, specifically in projects for planning and implementing intergrated water resource development. He is particularly interested in understanding the integrated system dynamics of environment and water resource development. He would like to combine science, politics and application within developmental policy planning and implementation for sustainable development in developing countries.