Review article: Detection of inundation areas due to the 2015 Kanto and Tohoku torrential rain in Japan based on multi-temporal ALOS-2 imagery

Abstract

Abstract. Torrential rain triggered by two typhoons hit the Kanto and Tohoku regions of Japan from 9 to 11 September 2015. Due to the record-breaking amount of rainfall, several riverbanks were overflowed and destroyed, causing floods over wide areas. The PALSAR-2 sensor on board the ALOS-2 satellite engaged in emergency observations of the affected areas during and after the heavy rain. Two pre-event and three co-event PALSAR-2 images were employed in this study to extract flooded areas in the city of Joso, Ibaraki Prefecture. The backscattering coefficient of the river water was investigated first using the PALSAR-2 intensity images and a land-cover map with a 10 m resolution. The inundation areas were then extracted by setting threshold values for backscattering from water surfaces in the three temporal synthetic aperture radar (SAR) images. The extracted results were modified by considering the land cover and a digital elevation model (DEM). Next, the inundated built-up urban areas were extracted from the changes in SAR backscattering. The results were finally compared with those from visual inspections of airborne imagery by the Geospatial Information Authority of Japan (GSI), and more than 85 % of the maximum inundation areas were extracted successfully.

Drawback in the Change Detection Approach: False Detection during the 2018 Western Japan Floods

“…The use of complex coherence is another approach that has been frequently used in recent years to map damaged areas [5,18]. Coherence is computed from a pair of complex SAR data using the following expression,…”

“…The architecture of the proposed network is independent of the satellite acquisition condition and normalization was not necessary. Liu and Yamazaki [5] performed a similar approach to map the waterbodies in open areas due to the 2015 Kanto and Tohoku torrential rain in Japan. It was pointed out, however, that a thresholding method applied over the backscattering intensity has a poor performance in urban areas.…”

“…The combined effect of the specular reflection and double-bounce backscattering mechanism produces high backscattering intensity. Thus, Liu and Yamazaki [5] applied the threshold over the difference of backscattering between a pair of SAR images taken before and after the referred flood. Boni et al [6] introduced a prototype system for flood monitoring in near-real-time.…”

“…Previous studies have stated that their results might contain changes produced by the flood and agricultural activities. Liu and Yamazaki [5] reported that the change of waterbodies included large paddy fields flooded for irrigation purposes, which could not be removed. Boni et al [6] pointed out it is necessary to include a pre-event image recorded a few days before the flood to successfully remove all the permanent waters, including those from the agricultural activities.…”

Delimitation of flood areas based on a calibrated a DEM and geoprocessing: case study on the Uruguay River, Itaqui, southern Brazil

“…However, creating flood maps is a complex process which is affected by the input data, flow design, and consistent topographic information (APFM, 2013). A major aid in the construction of flood maps is use of geoprocessing (Chen et al, 2009;Sarhadi et al, 2012;Demir and Kisi, 2016;Ovando et al, 2016;Liu and Yamazaki, 2018). Geoprocessing is a set of techniques based on the study of spatially distributed information in order to describe the characteristics of the phenomenon under investigation in the whole area of interest (Costa and Lourenço, 2011).…”