Influence of Localized Rainfall Patterns on Landslide Occurrence: A Case Study of Southern Hiroshima with XRAIN Data During the July 2018 Heavy Rain Disasters

The landslide problem in Japan was exemplified by the July 2018 landslides and flood disasters in Southwestern Japan induced by extremely heavy rainfall. In this study, GIS and other analytical platforms were utilized to analyze landslide distribution patterns in the July 2018 heavy rain disasters around the city of Kure (Southern Hiroshima) in conjunction with chronological XRAIN (eXtended Radar Information Network) radar-acquired localized rainfall data in order to achieve a better understanding of the relationships between rainfall characteristics and landslide probability and to investigate the potentials of radar-acquired rainfall data in landslide susceptibility study approaches. Analysis of the rainfall data demonstrates that mean annual precipitation in the whole study area ranges between 2,004 mm and 2,961 mm, with an average of about 2,152 mm for the whole extension of the area. Observation of the spatial distribution of rainfall volumes throughout the sampled years shows that rainfall is remarkably localized, with higher values concentrated on elevated areas. It is observed, however, that the peak rainfall volumes are not so closely related to landslide occurrence. Analysis of event rainfall from the July 2018 disasters determines that landslide-inducing rainfall started from 8:30 AM on July 5th and continued with a mean intensity of 7.8 mm/h for about 47 hours until 7:30 AM of July 7th, accumulating to up to 368 mm in total and that there were two intensity peaks, one around 7:30 PM on the night of July 6th, and another one around 4:30 AM on July 7th. These two events are associated with particularly high landslide activity, which indicates that landslide activation is related to peak intensity rainfall combined with accumulated continuous precipitation. This study demonstrates a clear association of landslide activity with heavy rainfall both in spatial and chronological aspects, weighing into the evidence that rainfall intensity, timing, and spatial distribution are true of vital relevance for landslide occurrence.