Modeling Salinity Intrusion in the Gorai-Pasur River Network Under IPCC AR6 Projected Sea-Level Rise Scenarios

The southwestern coastal region of Bangladesh, home to over 35 million people, is increasingly vulnerable to salinity intrusion driven by sea-level rise (SLR) and climate change. This study investigates how projected SLR, based on the IPCC’s AR6 under the SSP2-4.5 scenario, may affect salinity dynamics in the Gorai-Madhumati-Nabaganga-Rupsha-Pashur River system, a key freshwater source for the region. Freshwater flow in the Gorai River has declined over the years, largely due to reduced dry season flow in the Ganges River following the commissioning of the Farakka Barrage. As the Gorai depends on the Ganges, this reduction has intensified salinity intrusion during the dry season. To understand how future sea-level rise could further affect salinity intrusion, the HEC-RAS 1D hydrodynamic and water quality model was used to simulate salinity intrusion for the years 2050 and 2100. Sea-level boundary conditions were derived from the latest AR6 projections, while upstream discharge was held constant to reflect a conservative case in which freshwater availability remains unchanged. The hydrodynamic model was calibrated and validated using observed river stage data, and the salinity model was calibrated with observed salinity concentrations from key monitoring stations. Model results indicate a significant increase in salinity intrusion by the mid and late century, especially during the dry season. Saltwater is projected to penetrate farther inland, with concentrations exceeding safe limits in areas previously unaffected. This has serious implications for drinking water, agriculture, and ecosystem health. The Sundarbans, a UNESCO World Heritage Site, is particularly at risk, with rising salinity threatening its ecological integrity and biodiversity. The findings highlight the urgent need for adaptive water management, including improved freshwater distribution, salt-tolerant crops, and climate-resilient infrastructure.