Understanding two-dimensional floodplain dynamics using wide-swath altimetry from the surface water and ocean topography (SWOT) mission: a case study of the 2024 Ganga River floods in North Bihar, India

This study aims to evaluate the potential of the newly launched Surface Water and Ocean Topography (SWOT) mission for assessing flood dynamics during the 2024 Ganges floods in North Bihar, India. As one of the first studies to utilize SWOT data for flood analysis in the Indian subcontinent, this study focuses on high-flow conditions observed during the 2024 monsoon season, when the Ganga River experienced severe inundation. The SWOT L2_HR_PIXC product, derived from Ka-band radar interferometry (KaRIN), was pre-processed to extract the water surface elevation (WSE) and water surface spatial extent (WSSE). The SWOT-derived WSE was validated against in-situ gauge data from Hatidah and Sentinel-3A altimetry observations, yielding strong correlations with R² values of 0.97 and 0.90, respectively. Three major flood waves, inundating nearly 1660 km², were recorded between July 10 and October 22, 2024. The WSE rose from 34.23 m on June 19 to 43.56 m on September 19, reflecting an increase of 9.33 m, with adjacent low-lying areas remaining submerged for up to four months. The analysis also revealed stronger backscatter (σ⁰) values for water surfaces, ranging from 13 to 16 dB. This study demonstrates SWOT’s capacity to enhance traditional EO-based 2D flood mapping for more informed decision-making.