Variability in vertical land motion along SW Indian coast: implications for inundation hazards due to sea-level rise

The combined effect of land subsidence and sea-level rise (SLR) leading to coastal inundation is a major threat for global coastal cities. The densely populated tropical Asian cities are believed to be the most affected regions by the rise of sea level. While the SLR is a global phenomenon, the magnitude and extent of the vertical land motion (VLM) varies locally and are often not considered for the estimation of coastal inundation hazard. In this study, we analyse the spatio-temporal variations in the vertical land motion and its impact on the coastal inundation for two densely populated tropical coastal cities and surrounding regions (Kochi and Trivandrum) situated along the south-western coast of India using Interferometric Synthetic Aperture Radar measurements from ALOS-1 (2007-2011) and Sentinel-1 (2015–2022) satellites in conjunction with piezometric level, tide-gauge and satellite altimetry measurements. Our findings indicate that VLM in Kochi is mainly subsidence with a rate of -5 to -25 mm/yr and correlates well with the groundwater depletion trend. In contrast, the Trivandrum city situated ~ 170 km south, experienced land uplift with a rate of 0–10 mm/yr. We model future inundation scenarios for Kochi and Trivandrum regions using the vertical land motion estimations and a LIDAR based high resolution elevation model. We show that by 2100, the projected SLR and the VLM will increase the inundation risk for an area of ~ 24 km2 and 1.8 km2 for the coastal parts of the Kochi and Trivandrum, respectively. While the land subsidence at Kochi would cause an increase in inundation by ~ 21%, the land uplift at Trivandrum would cause a reduction in inundation by ~ 17%. The present study indicates the importance of local variability in vertical land motions for inundation hazard maps based on global projections of future sea-level rise.