Multi-Temporal Sentinel-1 SAR-Based Assessment of Flood Inundation Expansion in the Kosi–Ganga Confluence Zone, Bihar, India

Abstract
Flood inundation is a recurrent hazard in the alluvial plains of Bihar, particularly within the interconnected Kosi–Ganga river system. This study assesses the spatial expansion of flood inundation in the Kosi–Ganga confluence zone of eastern Bihar during August–September 2026 using multi-temporal Sentinel-1 Synthetic Aperture Radar (SAR) imagery. SAR observations acquired on 19 August 2026 and 12 September 2026 were used to derive water masks and identify changes in surface-water extent between the reference and flood-period conditions. The results reveal a substantial expansion of mapped water/flood area from 358.090 km² to 749.516 km², representing an increase of 391.425 km² (109.31%). The Ganga-associated area increased from 264.156 to 501.616 km² (89.89%), while the Kosi-associated area increased from 93.935 to 247.900 km² (163.88%). Digital Elevation Model based analysis indicates that extensive inundation occurred across relatively low-lying floodplain surfaces particularly in diara land. Subsequent Government of Bihar observations during 17–19 September 2026 recorded water levels above danger levels at selected Kosi and Ganga gauge stations, providing supporting hydrological context for continued elevated river conditions. The findings demonstrate the effectiveness of multi-temporal Sentinel-1 SAR for rapid flood-inundation assessment and provide a spatial baseline for subsequent flood exposure and risk studies in the Kosi–Ganga confluence zone.

Flood Inundation, Kosi–Ganga Confluence Zone, Flood Area, Topographic Setting, Digital Elevation Model

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