Coastal Vulnerability Index (CVI) Assessment of a Data-Sparse Delta: Quantifying the Contribution of InSAR-Derived Land Subsidence in the Volta Delta, Ghana

Land subsidence amplifies the impacts of sea-level rise (SLR) in low-lying deltas, yet most Coastal Vulnerability Index (CVI) assessments omit it or rely on global estimates, drastically understating deltaic vulnerability. This study presents the first CVI assessment along Ghana’s coast to integrate validated, spatially resolved land subsidence derived from Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR). Nine geological, geomorphological, hydrodynamics and anthropogenic variables were quantified across 72 contiguous grid cells spanning ~150 km of the Volta Delta coastline and ranked on a 1–5 vulnerability scale. Two composite indices were computed using an unweighted square root of product mean formulation, one incorporating subsidence (CVI(+sub)) and one excluding it (CVI(-sub)), classified against a common quantile distribution for direct comparability. By incorporating measured subsidence, ranging from -2.44 to -4.23 mm/yr (grid cell means), 75% of grid cells indicated High to Very High vulnerability under CVI(+sub), compared with 28% under CVI(-sub); a Wilcoxon signed-rank test confirmed the increase as significant (p < .001). Vulnerability peaked along the Keta and Songor lagoonal margins. Omitting measured subsidence understates deltaic vulnerability, and this approach offers a transferable method for data-sparse deltas.