An Original Hydro-Mechanical Coupled Model for Predicting Gully Erosion Evolution in the Anambra-Imo Gully Belt of South East Nigeria

The Anambra-Imo gully belt in South East Nigeria encompasses one of the most severe gully erosion landscapes in sub-Saharan Africa, with gullies measuring hundreds of metres in length and tens of metres in depth destroying farmland, infrastructure, and communities. Predictive modelling of gully evolution to guide intervention design requires a hydro-mechanical coupled framework that captures the interaction between subsurface flow, pipeflow, and mass wasting mechanisms specific to this environment, which has not been developed. This study develops an original hydro-mechanical coupled model for gully erosion evolution in the Anambra-Imo gully belt. The theoretical framework couples three original model components: a variably saturated groundwater flow model representing the seasonal perched aquifer dynamics that drive gully initiation and headcut advance; a shear strength degradation model for the saturated highly leachable sandy clay soils; and a sediment detachment and transport model incorporating piping failure initiation criteria calibrated to the regional soil behaviour. The integrated model is implemented in COMSOL Multiphysics with a custom gully geometry updating algorithm. Calibration and validation use 15 years of gully survey data from three monitored gully systems in Anambra State, including LiDAR surveys at two-year intervals. The model achieves cross-validated Nash-Sutcliffe efficiency of 0.79 for annual headcut advance rate prediction. Scenario modelling evaluates the effectiveness of check dam installation, slope revegetation, and subsurface drainage as gully control measures. The framework constitutes an original theoretical contribution to gully erosion science.

Keywords: gully erosion, hydro-mechanical model, Anambra-Imo gully belt, piping erosion, South East Nigeria