Accelerated Settlement Expansion in High-Hazard Areas of the Ganges–Brahmaputra–Meghna Delta

The Ganges–Brahmaputra–Meghna (GBM) delta is one of the most densely populated and flood-prone regions in the world. Identifying the exposure patterns of settlement expansion under different flood hazard levels in the GBM delta is of significant importance for enhancing the delta’s regional resilience. This research regionally screens settlement flood exposure by overlaying the Global Urban Expansion Simulation Dataset and the Aqueduct Floods Hazard Map at a 1 km spatial resolution. To account for inter-model variability, this study utilized the ensemble mean of five global climate models for future projections in 2030 and 2050 under SSP2-4.5 and SSP5-8.5 scenarios. Flood hazards were categorized into four specific levels based on inundation depth, namely low-hazard (0–0.15 m), medium-hazard (0.15–0.5 m), high-hazard (0.5–1.5 m), and highest-hazard (≥1.5 m). The study employed spatial overlay analysis and excluded missing pixels to avoid statistical bias from incomplete data. The findings indicate that under historical and future socioeconomic scenarios, both high- and highest-hazard zones exhibit significant settlement expansion, and the expansion rate within highest-hazard zones (270.9–357.1%) is expected to increase substantially compared to the historical baseline, reaching 1.57–1.85 times the expansion rate of flood-safe zones. Within the high- and highest-hazard categories, the contribution rate of fluvial and coastal flood coincidence zones reaches 21% to 22%. Furthermore, approximately 87% of the settlements within these fluvial–coastal coincidence zones are exposed to high-hazard levels or above. This study characterizes the variation characteristics of settlement exposure within fluvial–coastal flood coincidence zones under future socioeconomic scenarios. These results provide a first-order regional screening and macro-scale support for identifying broad exposure trends and establishing a baseline for future high-resolution assessments in the GBM delta.