THE GEOLOGY AND GEOMORPHOLOGY OF GULLY DEVELOPMENT IN AGULU-NANKA AND ITS ENVIRONS, SOUTHEASTERN NIGERIA

ABSTRACT Gully erosion is one of the most damaging environmental problems in southeastern Nigeria, and it is especially severe in the Agulu–Nanka area of Anambra State. Agulu-Nanka lies between 6°066’ - 6°0912’N and 7°056’ - 7°105E of the equator. This study investigates the local reasons why gullies start and grow, and how geology, land shape (topography), climate, drainage, and human activities work together to worsen the problem, with the goal of guiding practical control measures. We combined background research, field mapping, and laboratory testing. Fieldwork covered Agulu, Nanka, Oko, and Amaokpala through a reconnaissance visit and a two-day detailed survey using GPS, base maps, and standard geologic tools to log outcrops, slope conditions, drainage paths, and land use. Soil and sand samples were collected and tested for particle size distribution, Atterberg limits (plastic and liquid limits), specific gravity, and shear strength test to understand how the materials behave when wet and under load. Results show that the area is dominated by the Eocene Nanka Formation (“Nanka Sands”), which consists of loose, weakly cemented sands that easily collapse into steep gully walls. Annual rainfall is high (about 1,500 2,200 mm) and comes in intense storms, producing strong runoff; the dendritic drainage network and steep slopes further focus flow into narrow lines of erosion. Human actions—blocked or poorly designed drains, road cuts, bare soil from farming and vegetation clearing, and building on unstable slopes—greatly accelerate gully initiation and headcut retreat. Laboratory data confirm the materials are very erosion-prone. Particle size results classify the soils mainly as poorly graded sands (SP) with very low fines (≈0.5–2.2%), meaning they drain quickly but have little cohesion to resist flow. Specific gravity values range from about 2.23 (Agulu) to 2.60 (Umuokpala) and 2.69 (Amaokpala), consistent with sandy, relatively porous materials. Atterberg limits indicate low to medium plasticity (plasticity index roughly 5–11%), showing limited binding when wet. Shear strength test shows low to moderate (about 900–1600 kN/m²), explaining frequent gully wall failure and rapid widening after heavy rains. Mapping also links active gully segments with slope breaks, drainage confluences, and areas where runoff is concentrated by infrastructure. The study concludes that gully erosion in Agulu–Nanka is driven by a fragile sandy formation, steep and varied topography, intense seasonal rainfall, mismanaged drainage, and land-use pressure. Effective control should therefore combine: (i) well-designed and maintained stormwater and roadside drainage; (ii) protection and quick re-vegetation of bare soils, especially on slopes and at gully heads; (iii) setbacks and land-use rules to keep buildings and roads off unstable ground; and (iv) early, small-scale interventions before gullies deepen. We also recommend a GIS-based susceptibility map that integrates geology, slope, drainage, and land use to prioritize sites for prevention and remediation. These findings provide practical, location-specific guidance for government agencies, communities, and NGOs working to reduce erosion risk and protect lives, land, and infrastructure in southeastern Nigeria.