Uncovering the Extent of Flood Damage using Sentinel-1 SAR Imagery: A Case Study of the July 2020 Flood in Assam

In India, Assam is known for being highly susceptible to floods, with the state being hit by multiple flooding events each year. The floods in Assam in July 2020 were mainly due to the heavy monsoon rainfall in the region. The Brahmaputra River and its tributaries, which pass through the state, received substantial rainfall, leading to a sudden increase in water levels and flooding in multiple districts. The state's topography, hills, and valleys also exacerbated the situation as the mountain water flowed into the plains and inundated low-lying areas. Identifying areas that floods have impacted is of utmost importance in facilitating an efficient response to the flood. The current work focuses on the case study of the 2020 Assam flood and aims to develop an operational methodology for potential flood-damaged area mapping. Sentinel-1 Synthetic Aperture Radar (SAR) images from April 2020 (before the flood) and July 2020 (after the flood) were used to create water masks based on global thresholding. This was followed by an RGB composite image highlighting the inundation extent. The resulting flood map was compared to a ground truth RGB composite generated by stacking the pre- and post-disaster images. The study’s methodology can be replicated to map floods in other areas around the world.