Comparative Analysis of the Expansion Rate and Soil Erodibility Factor of Some Gullies in Nnewi and Nnobi, Southeastern Nigeria.

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Abstract The study involved an integration of field observations, geotechnical analysis, slope stability analysis, gully expansion rate analysis and determination of the soil erodibility factor (K). Fifteen representative gullies were studied extensively. The Grain size distribution analysis revealed that the soils are composed of gravel (5.77–17.67% and 7.01–13.65%), sand (79.90-91.01% and 82.47–88.67%), and fines (2.36–4.05% and 3.78–5.02%) for Nnewi and Nnobi respectively. The cohesion and internal friction angle values range from 1–5kPa and 2-5kPa and from 29–38° and 30–34° for Nnewi and Nnobi respectively, which suggests that the soils have low shear strength and are susceptible to shear failure. The plasticity index (PI) of the fines showed that they are nonplastic to low plastic soils and highly liquefiable with values ranging from 0–10% and 0–9% for Nnewi and Nnobi respectively. Slope stability analysis gave factor of safety (FoS) values in the range of 0.50–0.76 and 0.82–0.95 for saturated condition and 0.73–0.98 and 0.87–1.04 for unsaturated condition for both Nnewi and Nnobi respectively indicating that the slopes are generally unstable to critically stable. The erosion expansion rate analysis for a fifteen-year period (2005–2020) revealed an average longitudinal expansion rate of 36.05m/yr and 10.76m/yr for Nnewi and Nnobi gullies respectively. The soil erodibility factor (K) are 8.57x10− 2 and 1.62x10− 4 for Nnewi and Nnobi respectively indicating that the soils in Nnewi have higher erodibility potentials than those of Nnobi. Conclusively, both the Nnewi and Nnobi areas are highly prone to erosion.

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