Analysis of the impacts of hydrology, soil properties, and geotechnics on gully propagation on the Edda-Afikpo Mesas of the Lower Cross River watershed (southeastern Nigeria)

Abstract

The effects of hydrology, soil structure, and geotechnics on soil loss and gully propagation on the Edda-Afikpo Mesas (southeastern Nigeria) were assessed by field studies, laboratory soil analyses, and GIS modeling.The study was carried out to attribute causality and recommend mitigation measures in the fast degrading settlement. Results of field studies indicate that two areas in the study region were of exceptionally negative pore pressure values (−654.03 kPa and −676.89 kPa, respectively) at ground surface level and significantly negative at 50 m below ground level (−163.53 kPa and −186.39 kPa, respectively); both areas aligned in the northeast-southwest direction. These two areas were further characterized by comparatively higher hydraulic heads (158–206 m), seasonal fluctuations of water level, and rapid drainage enhanced by springs flow. Developed models revealed that a slight water level increase in these areas, combined with high surface infiltration as a result of significant permeability values (ranged between 1.522 × 10−3 cm/s and 1.744 × 10−3 cm/s) would translate into positive pore pressure development and seepage flow due to hydraulic gradient towards the slope toes. Thus, the lower part of the slopes on the mesas experiences higher positive pore pressure build-up and particle uplift. This scenario most likely plays out seasonally and could be responsible for the toe failures that are predominant in the area. Results of laboratory analyses further revealed that the soils have geotechnical properties (significant natural moisture content, low to moderate plasticity, marginally low bulk density, and specific gravity and shear strength) that increased their soil loss and gully propagation potentials, exacerbated by hydrology, soil structure, and human factors. This study, therefore, advocates that the solution to erosion and gully menace in the area, and other places with similar geological, topographic, and climatic situations would require an integrated approach.