Geophysical Monitoring of Active Gully Erosion Processes Using Time-Lapse ERT in Ngor-Okpala, Anambra State

Active gully erosion in southeastern Nigeria, including areas bordering Ngor-Okpala in Anambra State, causes continuous land loss and threatens community safety, warranting systematic geophysical monitoring to understand erosion dynamics and subsurface hydrological drivers. This study employs time-lapse electrical resistivity tomography to monitor active gully erosion processes across two consecutive rainy and dry seasons in Ngor-Okpala. Permanent electrode arrays were installed along three monitoring profiles spanning active gully walls and head scarps. Resistivity data were acquired at monthly intervals using an automated multichannel resistivity system. Difference resistivity tomograms were computed by subtracting successive inversion models to isolate temporal changes in subsurface moisture distribution and structural alteration. During rainfall events, resistivity values in the sandy overburden decrease by 40 to 75 percent, reflecting rapid infiltration and saturation. Post-rainfall recovery of resistivity in the upper horizon follows an exponential trend with time constants of 3 to 10 days, suggesting moderate drainage rates. Progressive decrease in resistivity at the base of the active gully wall between dry and wet season surveys is interpreted as deepening of the saturated zone and advancement of the seepage face. Monitoring data confirm that subsurface seepage and piping, driven by perched water accumulation above a clay confining layer, are the primary mechanisms of gully head retreat. Retreat rates estimated from successive ERT difference sections are consistent with independently measured gully head advance of 3 to 8 m per year. These findings provide quantitative mechanistic insight for erosion control planning.

Keywords: time-lapse ERT, gully erosion monitoring, piping, seepage, Anambra State