Geotechnical and geomorphological blueprints of the gullied Ogwashi-Asaba Formation: examples of the Obomkpa and Nkwu-Nzu gullies, Southern Nigeria

Rapid land degradation caused by gullies has impacted roads, buildings, farms, and electricity development in Obomkpa and Nkwu-Nzu areas of the Niger Delta in southern Nigeria. As a result, a study was carried out to examine the soil behavior and erosion dynamics of the gullied terrains from both geotechnical and geomorphological viewpoints. Eighteen soil samples and the morphology of the two gully sites were analysed through walk-over surveys, geotechnical testing (including gradation analysis, consistency tests, triaxial, and shear box tests), and gully morphological assessments. Gradation and consistency tests revealed that the gullies are mainly composed of silty sands that exhibit cohesionless behavior in Obomkpa and cohesive properties in Nkwu-Nzu. Geomorphological studies identified the gullies as permanent, large gullies located on high-relief zones with steep slopes, featuring unstable U-shaped and stable V-shaped channels. Ultimately, integrated analyses indicated that the gullies originated from weak soil cohesion and intense runoff energy driven by steep slopes. This study provides a resource database for gully remediation efforts in southern Nigeria and tropical sedimentary terrains.