Present-day coastal subsidence and inundation risk to socioeconomic exposure in Chennai City, India

Chennai, a coastal megacity on India's southeastern coast, has experienced frequent coastal hazards, notably the 2004 tsunami and 2015 floods. Land subsidence along urban coastlines, a critical but often overlooked component of vertical land motion (VLM), exacerbates relative sea-level rise (RSLR) and intensifies flood hazards, undermining long-term coastal resilience. This study quantifies Chennai's coastal vulnerability by integrating high-resolution interferometric synthetic aperture radar (InSAR) observations from Sentinel-1 (2016-2024), tide gauge (1993-2023), and satellite altimetry (1993-2024) data with elevation, population, and infrastructure datasets. Results reveal InSAR-derived subsidence rates up to -15 mm/yr within 30 km of the coastline, combined with a sea-level rise of ~ 2.8 mm/yr. Projected inundation for 2030, 2050, and 2100 under IPCC SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios incorporates RSLR, VLM, and 6 m storm surge effects. By 2100, inundation extent is expected to increase by 136.9 km2 (SSP2-4.5) to 142.9 km2 (SSP5-8.5), expanding the exposed area by over 300% relative to 2030. The affected population may rise from 2.45 million to 9.85 million, with asset exposure surpassing USD 100 million by 2100. These findings highlight the urgent need to integrate VLM into regional sea-level assessments and adaptation planning to enhance Chennai's coastal resilience.