Rapid and all-weather detection of flood areas is needed to monitor and mitigate flood disasters. This study addressed flood area detection using ALOS-2 PALSAR-2 data acquired during the 2015 heavy rainfall disaster in the Kanto and Tohoku area of Japan. We propose an approach to flood area detection by thresholding amplitude and interferometric coherence images for non-urban and urban areas, respectively. PALSAR-2-derived flood areas were validated using the inundation map provided by the Geospatial Information Authority of Japan (GSI) and showed 75% accuracy and a 0.51 kappa coefficient in flood/non-flood discrimination. The effectiveness of a lower incidence angle (less than 40 degrees) and a high-sensitivity observation mode (6-m resolution mode) for detecting non-urban flooding was also demonstrated by a comparative study. Interferometric phase variation was revealed to be more effective in detecting urban flooding than conventional interferometric coherence. Our results demonstrate the feasibility of PALSAR-2 for rapid flood monitoring and can be used as a reference for possible future flood disasters.
