Analysis of Slope Instability Mechanisms and Rainfall Triggering Thresholds in the Anambra Highland Zone

The Anambra Highland Zone, encompassing parts of Anambra, Imo, and Ebonyi States in South East Nigeria, experiences recurrent rainfall-triggered landslides that cause fatalities, infrastructure damage, and agricultural losses. The rainfall thresholds and failure mechanisms specific to this zone have not been empirically established in the literature. This study analyses slope instability mechanisms and determines empirical rainfall triggering thresholds for landslides in the Anambra Highland Zone. Geotechnical investigation at six recent landslide sites characterised failure surfaces, mobilised shear strength parameters, and failure geometry. A rainfall event database was constructed from available rain gauge records spanning 2010 to 2023, linked to a documented landslide inventory. Rainfall threshold analysis was performed using intensity-duration-frequency analysis and cumulative event rainfall relationships. Stability back-analysis using the Morgenstern-Price method determined mobilised friction angles and cohesion values at failure. Results indicate that the critical cumulative rainfall threshold for shallow translational slides in colluvial soils is 120 to 180 mm over 72 hours. Residual shear strength parameters of the dominant clay-rich colluvial soils (phi = 12 to 18 degrees, c = 8 to 14 kPa) indicate that saturated slopes above 24 degrees inclination are chronically unstable. Antecedent moisture condition significantly influences initiation threshold. The study provides the first empirical rainfall threshold analysis for the Anambra Highland Zone, informing early warning system development and land use zoning recommendations.

Keywords: slope instability, landslide, rainfall threshold, Anambra Highland, South East Nigeria