Hydrogeological And Geotechnical Investigations Of Gully Erosion Sites In Calabar Area, Southeastern Nigeria

Abstract

For many years, gully erosion and landslides are posing a serious threat to human existence, agricultural land, infrastructure and socio-economic activities in Calabar and its environs. Consequently, hydrogeological and geotechnical studies of gully erosion sites were carried out in order to provide information on the genesis and continual expansion of gullies in the area. The results indicate that gullies are located in the upper aquifer of the Benin Formation (Coastal Plain Sands). The estimated hydraulic parameters of the sand obtained from granulometric analysis give hydraulic conductivity K ranging from 3.6 X 10-5 to 2,25 X 10-2 cms-1, with a mean of 2.4 X 10-3 cms-1. specific discharge Vd varies from 1.8 X 10-7 to 1.13 X 10-4 cms-1 and average linear groundwater velocity Va ranging from 6.0 X 10-7 to 3.75 X 10-4 cms-1. The geotechnical results further indicate that the liquid limit LL ranges from 24.0 to 54.0%. The plasticity index PI varies from 10.1 to 29.0% with a mean of 16.9%. The optimum moisture content OMC ranges from 8,3 to 14.3% while the silt- clay fraction ranges from 4.0 t0 34.0 with mean of 26.2%. The maximum dry density MDD of the soil ranges from 1810kgm-3 to 2050kgm-3 while the shear strength test show values of cohesion C in the range 13kgm-2 to 32kgm-2 in an angle of internal friction, Φ ranging from 120 to 170. The implication of these data is that gully erosion is partly enhanced by the low shear strength and plasticity index of the soils. In addition, the values of hydraulic parameters suggest high seepage forces and pore pressure. These will tend to reduce the shear strength of the soil and enhance its erodibility. Recommendations were also made on how to control erosion in the area.

Keywords: Gully erosion, upper aquifer, hydraulic parameters, geotechnical, Calabar.

Global Journal or Pure and Applied Sciences Vol. 14 (4) 2008: pp. 423-432