Sea-level rise drives changes in salinisation patterns in low-lying Bangladesh

• Sea-level rise accelerates salinisation in the low-lying Bengal delta. • Salinity gradient intensifies from east to west across the delta. • Cyclonic surges and sea-level rise-induced changes amplify and persist the tidal wedge inland. • Findings guide adaptive management for climate-vulnerable deltas. In the Bengal delta, a region exemplifying low-lying deltas vulnerable to climate change, rising sea levels are significantly influencing salinisation patterns, posing risk to water security and agriculture. This study leverages a nearly two-decade-long dataset from 54 monitoring stations across the coastal Bangladesh to examine salinisation dynamics using advanced statistical analysis. Results reveal a stark intra-regional salinity gradient, accentuated from east to west, with levels spiking from 20 to 90 µS/cm yr −1 in areas of increased tidal influence and diminished freshwater discharge. Salinity persistence sharply increased after 2007, a period marked by cyclonic activity and storm surges, extending the inland reach of the tidal wedge, likely driven by sea-level rise and morphological changes constraining freshwater flow. We introduce the Offshore Controlled Estuarine and Aquifer Nexus (OCEAN) Salinisation framework, which conceptualises how the steep bathymetry of the western Bengal Delta modulates sediment deposition, tidal energy dissipation, and salinity retention. The steeper offshore gradient reduces tidal amplification, fostering dense tidal creek networks with slower flushing rates. These offshore controls, coupled with declining freshwater inflows, contribute to salinity entrapment in estuarine systems and persistent groundwater salinisation. The findings advance our understanding of environmental shifts in Bengal delta, highlighting the need to integrate offshore-terrestrial interactions into future salinity management strategies. Insights from this study offer broader implications for managing salinisation in other low-lying, tide-influenced deltas facing similar climate-induced challenges.