Probabilistic Retreat and Population Risk in Subsiding Bangkok: Multi-Scenario Sea-Level Rise and Flood Modeling

Global mean sea level has accelerated to record-high rates over the past decade, with 2024 exhibiting an anomalous rise linked to exceptional ocean heat content. Relative sea-level rise (RSLR) in deltaic megacities, such as Bangkok, is further amplified by rapid land subsidence caused by groundwater extraction and urban development. This study develops a novel integrated framework to assess future coastal risks in Bangkok by combining bias-corrected CMIP6 sea-level projections with a hierarchy of five flood models ranging from simple static bathtub approaches to advanced shallow-water equations. The framework also incorporates subsidence-adjusted probabilistic retreat modeling and machine-learning-based downscaling of population data to approximately 200-meter resolution, allowing detailed spatial analysis of exposure. Our results indicate that by 2100, retreat probabilities exceed 90% in coastal districts such as Samut Prakan and Samut Sakhon under moderate emissions scenarios (SSP2-4.5), escalating to near-universal land loss (>99%) under high emissions (SSP5-8.5). Population exposure peaks at over 30 million people in these scenarios. Validation using satellite-derived NDWI data demonstrates the highest predictive skill for the shallow-water model (R² = 0.92). Policy analysis uncovers an “urban resilience paradox” where investments in protective infrastructure encourage expansion into vulnerable zones, increasing long-term risks to build equitable and resilient futures in subsiding deltaic megacities like Bangkok.