Characterization of subsurface sediment as a reconnaissance tool towards restoring the Nigerian Transgressive mud coast

Abstract

The unabated flooding and erosion of mudflats at the low-lying Nigerian Transgressive Mahin coast require prompt measures to forestall further damage/attacks on shorelines and adjacent communities. However, a proper understanding of the mudflat subsurface materials is key for the successful design of a sustainable protection measure in this section of the Nigerian coast. While conventional geotechnical techniques for subsurface investigations appear to be demanding on the delicate and less accessible mudflats, the use of geophysical methods seems feasible in delineating the near-surface sediment characteristics. Hence, in this study, we integrated electrical resistivity (ER) geophysical and geotechnical methods to investigate subsurface materials at a section of this muddy coast. Correlations were made between the sediment’s electrical resistivity and their geotechnical variables. Overall, soft, consistent, and highly compressible sediments occasioned by low resistivity are encountered near-surface. Four resistivity zones were delineated: the upper, intermediate, and lower muds, and the underlying sand. The muds contain inorganic clays/silts of high plasticity, with relatively low point resistance (qc) ranging from 4–20 kg/cm2. The sediment index properties obtained from laboratory test results show little or no correlations with field resistivity. Though hard shoreline protective structures (such as seawalls) requiring a deep-seated pile foundation is most ideal in the study area, the relatively stiffer materials of the near-surface upper mud layer can only support some magnitude of load from light retaining structures. The application of nature-based techniques is thus recommended for the long-term restoration and management of the Mahin mudflat.