Flood Inundation Analysis of Gresik Regency, Indonesia Using Sentinel-1 InSAR Coherence Ratio and Backscatter

Indonesia is one of the most susceptible countries to natural disasters, with floods being the most significant threat. In 2024, floods contributed to 55% of the 1,478 disaster occurrences recorded by the National Disaster Management Agency. Gresik Regency is highly susceptible, particularly because of the Lamong River overflows. This study is aimed at determining the spatial extent of flood inundation in Gresik Regency on February 7th, 2024 using Sentinel-1 Synthetic Aperture Radar imagery. Sentinel-1 was selected due to the ability of obtaining data regardless of cloud cover or weather conditions. Two analytical methodologies have been employed: backscatter and InSAR coherence ratio. Backscatter method discovered that Benjeng, Ujungpangkah, and Manyar among the most affected sub-districts. Validation against historical flood data yielded an overall accuracy of 86% and a kappa accuracy of 81%, while comparison with the flood susceptibility map produced 86% overall accuracy and 80% kappa accuracy. InSAR Coherence method identified Benjeng, Dukun, and Balongpanggang as the sub-districts most affected by flooding. The severe flooding in Benjeng fits with its vulnerability to recurring inundation due to the excessive flow of the Lamong River, a major watercourse in southern Gresik. Validation using historical data produced an overall accuracy of 72% and a kappa accuracy of 64%, while comparison with the susceptibility map yielded 75% overall accuracy and 69% kappa accuracy. Based on the processing, it has been determined that the backscatter approach provides the highest accuracy in flood mapping in Gresik Regency, Indonesia.