Flood Dynamics and Frequency Mapping in the Lower Ganges Floodplain in India Using Multi-Temporal Sentinel-1 SAR Observations (2016–2024)

The Ganges floodplains are among the most flood-prone regions in India, where recurrent inundations cause significant socio-economic and ecological impacts. Understanding the spatial distribution, frequency, and dynamics of flooding is essential for effective floodplain management and enhancing climate resilience. This study examines the flood frequency and spatial extent across a section of the Ganga River floodplains in Bihar, utilising multi-temporal Sentinel-1 Synthetic Aperture Radar (SAR) data spanning the period from 2016 to 2024. Flooded areas were delineated through an optimal threshold-based classification of VH-polarised backscatter images, with threshold values ranging from -19.5 dB to -22.3 dB. Annual flood extents were mapped, and an inundation frequency composite was generated to identify zones experiencing recurrent flooding. The spatial analysis revealed substantial variability in flood occurrence, with extensive inundation observed in low-lying regions. Several areas were inundated in more than 60% of the study years, indicating chronic flood exposure. The decadal analysis revealed that August and September were the peak months for flooding, with some areas remaining inundated for more than one month, which had an adverse impact on both human settlements and agricultural lands. Validation using optical satellite imagery from Sentinel-2 confirmed a 98% accuracy in the SAR-derived flood extent, reinforcing the reliability of the classification method. The temporal flood frequency analysis provides crucial insights into long-term flood dynamics and helps identify hydrologically sensitive zones. Overall, this study highlights the effectiveness of SAR-based monitoring in understanding floodplain behaviour under changing climatic and hydrological conditions, and supports improved flood hazard mapping, hydrodynamic model calibration, and sustainable flood risk management in the Ganges Basin and other monsoon-affected regions.

Keywords: Flood Inundation, Multi-Temporal, Time-Series, Flood Frequency, Sentinel-1 SAR, Ganges River