Electrical Resistivity Imaging of Gully Erosion Features and Subsurface Trigger Mechanisms in Nanka, Anambra State

Nanka in Orumba North Local Government Area of Anambra State is severely affected by gully erosion, which has destroyed farmland, infrastructure, and entire villages, yet the subsurface mechanisms driving erosion initiation and propagation remain inadequately characterized. This study applies electrical resistivity imaging to investigate gully erosion features and their subsurface triggers in Nanka. Eight two-dimensional ERT profiles of lengths between 160 and 400 m were acquired across active and incipient gully systems using the Wenner and dipole-dipole arrays with 5 m electrode spacing. Data inversion using RES2DINV produced subsurface resistivity tomograms that were interpreted in the context of the local geological and hydrological setting. The tomograms reveal a shallow, high-resistivity sandy layer (200 to 800 ohm-m) overlying a low-resistivity clay horizon (5 to 25 ohm-m) at depths of 5 to 18 m, a configuration that promotes piping and subsurface erosion. Preferential subsurface flow paths are imaged as vertically elongated low-resistivity conduits within the sandy overburden that converge toward gully head positions. Zones of anomalously high resistivity within gully walls are interpreted as remnant structural sand pipes that serve as conduits for groundwater seepage and subsequent tunnel collapse. The role of the impermeable clay layer as a confining horizon generating positive pore pressure during rainfall events is confirmed by the tomographic evidence. Seven incipient gully nucleation sites are identified from resistivity anomaly patterns, enabling proactive erosion management intervention. These findings provide essential geophysical insight into the hydrogeological causes of gully erosion in Nanka.

Keywords: electrical resistivity, gully erosion, piping, Nanka, Anambra State