ANTHROPOGENIC CONTRIBUTION TO GULLY INITIATION AND PROPAGATION IN SE NIGERIA

Gullies are steep-sided ravines, cut into susceptible, often shallow slope materials by the surface water from heavy rainfalls. Once initiated, they offer avenues for easy down-slope movement of water from later storms. The flowing water erodes soil from the sides and floor of each gully, making it wider and deeper. Landslides, slumps, and related processes on the gully sides also contribute to the removal of slope materials. The head of the gully advances upslope, enlarging the gully system. Unchecked progress of the gullies results in ‘badland’ topography, and destroys the ecology and economy of the affected areas.

Several large gully systems are currently active in Abia, Anambra, Enugu, and Imo States of SE Nigeria. Poor design and construction of roadside drainage is a major cause of gully erosion. Improper termination of drains and blockage of drains by silt and debris causes the water to overflow. This erodes the sides and ends of drains, undercuts them, and causes their collapse. The resulting unregulated water flow causes rapid development and advance of gullies. Footpaths and trails with foot and wheeled traffic disrupt vegetation cover and are sites of increased compaction of surface soils. The compacted paths are less permeable, and serve as channels for surface water, giving rise to localized erosion and initiation of gullies. Annual advances of gully heads by up to 60 meters were documented.

The project, Gully Erosion, Nigeria, involved University of Windsor Earth Sciences, Windsor, Ontario, Canada, and two Nigerian universities, the Nnamdi Azikiwe University, Awka, and the University of Port Harcourt, Port Harcourt. The International Development Research Centre, Ottawa, Canada, funded the project.