Multi-Event Flood Inundation Detection Using Sentinel-1 SAR Ratio Image on Alluvial Plain: A Case Study of Klaten Regency, Indonesia

Abstract

Periodic river flooding occurs in Klaten Regency, consistently striking within close time intervals; however, spatial documentation for this area is currently unavailable. This study applies the Ratio Image method to Sentinel-1 SAR imagery processed using Google Earth Engine, to delineate floodwater distribution during two flood events that occurred in Klaten Regency in 2026. The First event, on January 12–13, was caused by the overflow of the Jabang Bayi and Gawe Rivers, and the second, on March 3– 4 event caused by the breach of the Dengkeng River embankment. An empirical threshold of 1.5 was established based on the bimodal distribution observed in the histogram, after which post-processing steps were applied, comprising the masking of permanent water bodies, slope, HAND, land cover, and a connectivity filter. Detection results indicated a total inundation area of of 1,150.46 ha across 24 subdistricts for the January event and 666.24 ha across 23 subdistricts for the March event. All subdistricts reported by the Klaten Regency Disaster Management Agency (BPBD) for each respective event were successfully identified within these results, a level of agreement that points on the realiable performance of the ratio image method on alluvial plains irrespective of flood source.