Coastal Subsidence and Inundation Risk in the Gulf of Khambhat, India: A Geospatial Perspective

The Gulf of Khambhat, a 200 km stretch of coastline in Gujarat, India, is increasingly vulnerable to the impacts of rising sea levels, inundation, and erosion. The region is home to densely populated districts such as Bhavnagar, Surat, Bharuch, and Khambhat, as well as vital ports like Dahej, crucial for global trade and economic growth. However, urbanization, industrialization, and a growing population have placed additional pressure on the region's underground resources, making the soft sediments more prone to subsidence. This, coupled with the environmental pressures from climate change, significantly amplifies the area's vulnerability to coastal hazards. The land use and land cover (LULC) changes between 2017 and 2023 have shown an increase in built-up areas and a decline in vital ecosystems like mangroves. Between 2014 and 2017, approximately 28.66 square kilometers of high tidal mudflats were lost, which not only destroyed critical habitats but also exposed populated areas to tidal flooding. This accelerated erosion further threatens the stability of the coastline. According to the IPCC AR6, the sea level along the Gulf is projected to rise by 0.95 meters by 2100.  Tropical cyclones like Tauktae and Biparjoy, which caused significant damage in the region, may further intensify the risks of storm surges and flooding in the future. The combined effects of sea level rise (SLR), tropical cyclones, and vertical land motion (VLM) may further threaten the region’s biodiversity, health, and food security. In this context, this study aims to examine the combined effects of coastal subsidence and sea level rise on the coastal cities along the Gulf of Khambhat. Given the increasing frequency of cyclones in India, the study also assesses the risks of inundation and flooding due to SLR, storm surges, and land subsidence in the 21st century. The approach integrates scenario-based SLR projections from the IPCC AR6 (ranging from SSP1-1.9 to SSP5-8.5), vertical land motion rates, high-resolution Digital Elevation Models (DEMs), and historical storm surge data. The study uses C-band Sentinel-1 satellite data (92 SAR images) from March 2020 to June 2023, processed through the GMTSAR software with an advanced Small Baseline Subset (SBAS)-based Multi-Temporal Interferometric Synthetic Aperture Radar (MT-InSAR) technique. The analysis reveals a subsidence rate of over 5 mm/year in various areas of the Gulf, particularly in locations like Palsana, Volvad, Navetha, and Bhadbhut. Furthermore, the results suggest that if sea level rise continues as projected by the IPCC and if the subsidence rate persists, the inundated area will increase by approximately 1.57% by 2030, 4.70% by 2050, and 18.20% by 2100 under the worst-case scenario (SSP5-8.5). Additionally, a cyclone similar to Tauktae, with the worst 4-meter storm surge height, could further impact over 1,000 square kilometers of the Gulf region under the same scenario. Given these alarming projections, it is essential to develop comprehensive emergency response plans for flood-related disasters to mitigate the growing risks and protect both the environment and local communities.