The hydrogeological attributes and mechanisms of a receding sedimentary terrain in the Anambra Basin, Southern Nigeria

Abstract

This research investigated the hydrogeological aspects associated with severe gulling and landsliding on a vulnerable sedimentary terrain in the context of the newly identified deformation patterns and large numbers of recently discovered scars clustering slopes on identical coordinates. Results of field survey, remote sensing, vertical electrical sounding and experimental analyses were presented to explain the causes and mechanisms of the mass movements ravaging the terrain. A digital elevation model accurately captured the diverse geomorphologic elements including landslide scars, hills, ridges, valleys, plains and rivers that matched field observations. The model aided the identification of previously unknown steep incised channels, active headcuts, unstable side walls and a seasonal river flowing near the slopes having large number of landslide scars. It was observed that areas that lay on higher elevations where the clusters occur were dominated by fresh scars while the moderate to low elevations were dominated by older ones. Majority of the slope failures initiated within the sandstone as shallow slides (<1 m to the sliding surface) covering <10⁴ m² with short runout distances. Bigger landslides, although few, occurred as isolated individual events initiated at the sandstone–shale boundary on distinct slopes. The steepness of the slopes seemed controlled by slope morphology (concavity) which probably influenced the number and distribution of landslide events. The steeper slopes were more concave and had the highest concentration of scars. Shallow water table (4–11 m) and increased instability (brittleness index) under increasing over-consolidation ratio are the critical failure factors.