Gullies are steep-sided ravines cut into susceptible, frequently shallow slope materials by the surface water from heavy rainfalls. Once initiated, they offer avenues for easy downslope 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 badlands 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 southeastern 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 cause the water to overflow. This erodes the sides and ends of drains, undercuts them, and causes their collapse. The resulting, unregulated water flow causes the rapid development and advance of gullies. Footpaths and trails with foot and wheeled traffic disrupt the 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 m were documented.