Seasonal reversals in groundwater flow direction and its role in the recurrent Agwagune landslide problem: a geophysical and geological appraisal

Abstract

The character of the shallow subsurface materials in the landslide and flood-prone Agwagune Community has been investigated using geophysical (electrical resistivity), geological and geotechnical techniques. Arising from the continuous suffering of the people in the affected areas, high occurrence rate and the desire to characterise the surficial materials, two dimensional resistivity tomography, vertical electrical sounding, soil sampling and hydrodynamic monitoring were conducted. Results show that the surficial materials are dominated by two main lithologic units: thin sandy materials with resistivity >30 Ωm and thick argillaceous materials with resistivity <30 Ωm. Particle size analyses confirm the dominance of argillaceous: silty-clay (56.2 %) and very fine (55.3 %) over arenaceous (9.2 %) materials. Bulk and particle densities of the surficial materials vary from 0.99 × 103 to 1.40 × 103 and 2.06 × 103 to 2.50 × 103 kg/m3, respectively; pH, EC and porosity range from 4.46 to 6.68, 8.0 to 23.0 mS/cm and 34.0 to 60.0 %, respectively. Atterberg Limits vary from 15.82 to 38.70 for liquid limit and 18.10–24.00 for plastic limit. Plasticity index varies from 11.92 to 15.70 %. Shallow groundwater in the coastal area is always in hydrodynamic equilibrium with water inside the Cross and Efefa Rivers and pond fields. To sustain this equilibrium condition, excess water inside the Cross River is usually discharged into Efefa River through both surface and underground flow processes. However, when water inside the Cross River dries up, a new hydraulic gradient, which distorts the existing equilibrium, is formed. This results in the reversal of groundwater flow direction. Continuous flow of groundwater into the Cross River channel (CRC) without any corresponding recharge results in the formation of voids, which gravitational force, forces to collapse. The collapsed materials usually roll toward the CRC, resulting in landslides. Models illustrating the main cause of the repeated landslide problem and suggested measures to minimise its occurrence rate have been included.