The world's largest saddle dam at risk: Multisensor geohazard analysis and downstream impacts

The Grand Ethiopian Renaissance Dam (GERD) Saddle Dam, which holds approximately 89% of the main reservoir’s live storage, is one of the largest and most critical auxiliary dams globally; its construction on Ethiopia's Blue Nile has consequently raised significant regional and international concerns regarding potential environmental impacts and geohazard risks. This study presents a comprehensive risk assessment of the GERD Saddle Dam by integrating high-resolution satellite data (GRACE, Sentinel-1, Sentinel-2, WorldView-3), hydrological modeling (SWAT), Persistent Scatterer Interferometry (PSI), geospatial analysis, and advanced statistical techniques. The results highlight critical structural vulnerabilities, including groundwater infiltration estimated at approximately 41 ± 6.2 billion cubic meters during reservoir filling, differential settlement of up to 40 mm, and emerging seepage and leakage pathways. Moreover, anomalous seismicity spatially aligned with pre-existing fault systems has been observed, with Poisson regression analysis indicating increased regional seismicity potentially linked to volcanic activity and the reservoir impoundment process, underscoring the dam’s transboundary geohazard risks. A dam-breach simulation reveals catastrophic downstream flood risks extending to Sudan and Egypt, with potential impacts on millions. These findings underscore the urgent need for international risk monitoring frameworks and contribute to advancing global dam safety protocols and Sustainable Development Goals (SDGs 9, 11, and 13). • GERD reservoir seeped ∼41 ± 6.2 BCM into groundwater, revealed by GRACE and hydrological modeling. • Satellite imagery has detected unexpected water zones adjacent to the Saddle Dam, indicating possible seepage and leakage concerns that warrant urgent investigation into structural integrity and safety. • PSI and imagery detected 40 mm dam deformation and leakage, indicating Saddle Dam instability. • Dam breach modeling shows floods up to 34.7 m depth, threatening millions in Sudan and Egypt.