The Mechanisms of Enlargement and Hydrogeological Characteristics of Nanka Gully, Southeast Nigeria

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Abstract
The mechanisms, failure processes and the hydrogeological factors leading to the enlargement of Nanka gully, one of the biggest landslide sites in Nigeria, were investigated. The gully is situated on unconsolidated, friable, collapsible sandstones capped in some places by lateritic overburden. Cross-section analysis shows that a NE-SW profile closely corresponds to the length of the gully which is over 2km while the NW-SE profile represents the width which is approximately 500m. The depth from the section is between 60 – 100m. Field investigation found gullies were abundant only in the eastern part of the study area. Digital elevation model (DEM) indicates that the abundance is related to the numerous surface runoff drainages in that part. The large Nanka gully (7° 5’E, 6° 2’20”N) forms the major tributary of the Mamu river. Expectedly, intense rainfall generates strong surface runoffs and remove large quantities of soil and bedrock. Shear strength tests show that overburden pressure can further weaken the materials and increase their vulnerability to the erosive impacts of runoffs and groundwater during the wet season. The gully’s progressive expansion to its present size is generally by headward extension of stream channels, head-cutting and landslides.

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