Geophysical Assessment of Coastal Erosion Vulnerability in Bayelsa State

This study applies ground geophysical methods to assess the subsurface conditions contributing to coastal erosion vulnerability in Bayelsa State, South South Nigeria. The coastal communities of Bayelsa State face severe and accelerating erosion from a combination of sea level rise, wave action, and subsurface instability created by unconsolidated young deltaic sediments. Understanding the near-surface geotechnical conditions is important for coastal engineering design and community risk assessment. This study deploys electrical resistivity tomography along six shore-parallel and shore-perpendicular profiles totalling 2.4 kilometres in Brass, Southern Ijaw, and Ekeremor local government areas. A Syscal Pro 48-electrode system with 5-metre electrode spacing and Wenner-Schlumberger array is used. GPR with 200-megahertz antenna supplements the ERT along selected profiles. ERT data are inverted using RES2DINV and interpreted for subsurface stratigraphy and water content. Findings reveal near-surface resistivity below 3 ohm-metres to depths of 1.5 to 4 metres in the most erosion-active zones, attributed to saturated loose sands and organic muds with minimal bearing capacity. Below this, resistivities of 10 to 30 ohm-metres correspond to denser but still compressible deltaic sands. GPR images show a chaotic reflector pattern in the upper 2 metres indicating disturbed and poorly consolidated fill material in areas affected by storm events. The juxtaposition of very low resistivity near-surface material with seasonal wave loading creates conditions conducive to rapid undercutting and slope failure. The study recommends shore protection using gabion revetments in high-vulnerability zones identified from the ERT mapping.

Keywords: coastal erosion, resistivity tomography, GPR, Bayelsa State, deltaic sediments.