Abstract
Triggered by two typhoons, heavy rainfall hit Kanto and Tohoku regions of Japan from September 9 to 11, 2015. Increased river water by the continuous rainfall overflowed and destroyed several river banks and caused damaging floods in wide areas. PALSAR-2 onboard ALOS-2 satellite carried out emergency observation for the impacted areas during and after the heavy rainfall. In this study, two pre-event and four co- and post-event PALSAR-2 images were used to extract the inundation area in Joso city, Ibaraki prefecture. First, using the pre-event SAR intensity image and a detailed topographic map, the backscattering coefficient of river water was investigated. Then the flooded areas were extracted by a common threshold value of backscatter for water bodies in the six temporal images. The colour composite of the sigma naught values was also made to visualize pixels that had been converted from ground to water. Finally, the extracted results were compared with those from the visual interpretation of aerial photographs and field survey reports. This comparison revealed that the accuracy of the flood extraction was fairly good for agricultural lands and non-urban land uses. But for built-up urban areas, it was not easy to extract water body since radar illumination did to reach the ground (water) surface.
Highlights
Satellite remote sensing is a useful tool for observing wide areas on the Earth's surface repeatedly
Remote sensing is classified by two categories based on the type of sensors: the passive remote sensing and the active remote sensing
The flooded areas were extracted by a common threshold value of backscatter for water bodies in the six temporal images
Summary
Satellite remote sensing is a useful tool for observing wide areas on the Earth's surface repeatedly. Remote sensing is classified by two categories based on the type of sensors: the passive remote sensing (mainly optical and thermal sensors) and the active remote sensing (mainly radar sensors). Optical satellite systems only work in the daytime and cannot observe objects under cloud-cover. Radar systems such as a Synthetic Aperture Radar (SAR) overcome this problem. It is widely used in various emergency situations due to natural hazards, such as earthquakes, tsunamis, volcanic eruptions, tropical cyclones and floods (International Charter, 2016). Extract inundation areas in Joso city, Ibaraki prefecture, Japan. The extracted results were compared with those from visual interpretation of aerial images and field survey reports
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