Electrical Resistivity and River Flow Velocity Studies of Ebonyi River Bridge Sites at Onicha-Oshiri and Idembia Areas, Southeastern Nigeria

Abstract

Pre-design Geoelectrical and River Velocity studies have been carried out at two bridge locations: Ebonyi River at Onicha-Oshiri road and Ebonyi River at Idembia Ezza. The objectives of the studies are to determine the geoelectrical parameters with respect to structure and rock type at these locations in combination with geology and carry out the analysis of the river velocity at various points. Four Vertical Electrical Sounding (VES) stations and six Velocity profiles were established (two VES stations and three velocity profiles per bridge site). Three geoelectric layers were proposed for Onicha-Oshiri and four to five geoelectric layers for Idembia. These layers comprise of the Top soil with resistivity range of 290-950Ωm, Silty lateritic soil with resistivity range of 140-181Ωm, Hard lateritic soil with resistivity range of 724-860Ωm, Silty sand with resistivity range of 163-494Ωm and Mudstone with resistivity range of 52-100Ωm. Results of Onicha-Oshiri lot show that the velocity values range from 1.28ms-1 to 1.95ms-1. Along profile D, velocity values rise from point 1 to point 3. Along the present bridge profile, the values are nearly the same for points 1 and 2 and rise to 1.37ms-1 at point 3. However along profile F, the velocity value is higher at point 2 (middle of the river). At Idembia site velocity values range from 0.634m s-1 to 0.99m s- 1. Apart from profile B where the value is highest at point 3 (edge of the river) values at profiles A and C have higher amounts at the edges and lower amounts at the central portion indicating increased scouring at the edges. The river at Onicha-Oshiri site has higher velocity possibly on the account of this site being at the youthful stage of the river. At Onicha end, the top lateritic matter is interbedded with fresh blue shale which may be offensive to any structure. This must be taken care of during construction by aiming at the hard sandstone below.