Photogrammetry and GIS Analyses of Houses Destroyed Caused by the 2014 Hiroshima Debris Flow

Abstract

Severe debris flows caused by a localized torrential downpour, attacked residential areas in the outskirt of Hiroshima City before the dawn on August 20, 2014. Seventy-four people caught by the debris flows were killed. Houses of the victims were commonly located on gentle slopes formed by older debris flows at the foot of mountains. The area called as Yagi-sanchome was most heavily damaged by the debris flows, and forty-one people lost their lives. Many reports described that sprawling of residential area during the period of high economic growth (1955–1973) in the post-WWII period of Japan, could be one of the reasons for the severe damage.

We identified construction period of the houses in Yagi-sanchome based on photogrammetric analysis of aerial photographs taken six times since 1947, and counted the number of houses built during each period. For easier and more efficient identification of houses built between consequent periods, we carried out a residual analysis of digital surface model (DSM) processed by Structure-from-Motion (SfM) software using aerial photographs.

We also counted numbers of the houses in the debris flow area, and the demolished houses by debris flow, and the houses victims lived. The results showed that the ratio of demolished houses and houses victims lived were higher among the houses built after the economic growth period than those built during the period.

Photographs taken by unmanned aerial vehicle (UAV) taken before after the debris occurred in Yagi-sanchome by one of the authors, reveals one of the typical examples of human neglect of potential debris flow hazards that resulted in unnecessary loss lives.