A Geotechnical and Environmental Risk Assessment of Two Gully Erosion Sites in Nkwoegwu Autonomous Community, Umuahia North LGA of Abia State, South-Eastern Nigeria

Abstract

Gully erosion sites are increasingly becoming a threat to environmental sustainability, leading to severe environmental hazards. This study investigates the geotechnical and environmental risk assessment of two gully erosion sites in Nkwoegwu, Abia state. Four (4) Soil samples were collected at different depths ranging from 21.4 m - 20.3 m. The geotechnical properties of the soil samples were analyzed. Results of geotechnical investigations revealed that soils in Site A were composed of  (Clay & Silt – 29.48%, Sand – 69.01%, Gravel – 1.51%); Soils in Site B composed of (Clay and  Silt – 41.71%, Sand – 57.54%, Gravel – 0.75%) suggesting that they are nonplastic. The liquid limit values are 26% and %, plastic limit 19% while the plasticity index values are 7.0% and 6.0% for site A and site B respectively indicating that they were noncohesive. The maximum dry density (MDD) values are 1.99 mg/m3 and 2.02 mg/m for site A and site B showing their loose nature. The optimum moisture content (OMC) values are 12.1% and 13.3%; CBR values being 32% and 4% respectively. The moisture content of the erosive soil samples were 8.5% and 11.2%. The result for the bulk density gives 1.38 g/cm3 and 1.28 g/cm3   for site A and site B respectively, while the shear strength values comprising cohesion, C, and angle of internal friction, Ф, gave their C to be 34 N/m2 for both sites A and site B and Ф of 14° (site A) and 16° (site B) respectively. This revealed that the soils lack resistance to runoff. The soil materials were mostly loosed sands with a low percentage of silt/clay fractions and high porosity. Noncohesive properties of the soil materials make them vulnerable to agents of erosion such as rainfall, the slope of land-surface, and human activities. The topographical survey showed that gully sites are the lowest points in the area which is also the direction of drainage. The total surface runoff from the area of study is estimated to be 384000 m3/day. Environmental hazards such as loss of arable land, and the threat to lives and properties were identified as well. To mitigate against the expansion of the two gullies, terraces are recommended to be constructed. This will minimize the impact of surface runoff.