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Introduction
On 20 August 2014, Hiroshima Prefecture in Japan was struck by a series of landslides following heavy rain. The rain triggered 166 slope failures which included 107 debris flows and 59 shallow slides. The landslides hit residental areas including Kabe, Asakita Ward, Yagi, Yamamoto, Midorii, and Asaminami Ward. Of these areas, the Asakita and Asaminami Wards in Northern Hiroshima were hit the hardest.

Following the rainfall, Hiroshima issued an evacuation advisory at 4:15 am. Mayor Kazumi Matsui confessed regret over the advisory's late issuance and recommended that the city study the incident to improve the process. Under the law which took effect in 2001, prefectural governments and other authorities must carry out on-site research to designate areas for which evacuation plans and hazard maps will be created. In Hiroshima there are about 32,000 possibly dangerous areas, the highest of all 47 prefectures. There were 44 injuries and 74 deaths. According to the police, the deaths included three boys aged two, three and eleven.

Landslide Characteristics
The debris flows covered a 10 km by 2 km region that ran from NE to SW. The most impacted area was Yagi 3-Chrome, located near the Midorigaoka prefecture. Within this area, landslides and debris flows occurred behind the Abu-no-Sato housing complex, Mitsuhiro Shine, and Midorigaoka prefectural aparments. The charactersitics of some of these flows are described below.

Midorigaoka Debris Flow
The Midorigaoka debris flow, located in the Asaminami Ward, entered the residential area and spread out across a large area while continuing for a considerable distance. It consisted of mainly coarse granite. This material was transported out of the terminal area of the channel, and deposited into the residential area. Within these deposits, fine granite soil particles were found. After 10 days from the event, large amounts of water was found near the source of this flow, resulting in muddy conditions. Overall, around 33,000m3 of soil mass was generated by the debris flow.

Abu-no-Sato Debris Flow
The Abu-no-Sato debris flow, located in the Asaminami Ward, consisted of two separate channels. They came to a stop before the residential area, and was about 5m wide and 30m long. It consisted of metamorphic and sedimentary rocks (e.g., hard hornfels rock), with very little granite. The flows went through a narrow gully, then dropped down almost vertically for approximately 5m. Through its travel, the flow picked up various debris which resulted in the deposits consisting of fractured hornfels and colluvium.

Rainfall
The main triggering cause for the event was the highly intense, short duration rain that occurred in the area. Due to the warm, moist air from the Akisame front which passed over the western Shugoku Mountains, cumulonimbus clouds began to develop in the northwest of Hiroshima City. These rainy conditions were unusual, highly extreme, and difficult to predict. Following the torrential rain, in which over 200mm of cumulative rainfall fell, several landslides were triggered. Asakita-ku was the hardest-hit ward. It received 217.5 millimetres (8.56 in) of rain from 1:30 am to 4:30 am, causing two landslides which occurred between 4 am and 6 am. This significant volume of rain strikes the surface, resulting in constrained groundwater in the water-bearing layer and a development of a head. The layer then collapses as a result of the confined water pressure's uplift force.

Geological Conditions
As there were faults, fracture zones, and joints, there were several locations with significant permeability. Due to the heavy upstream rainfall, the groundwater in these places turned into artesian water. As a result, the slopes experienced destabilization.

The Midorigaoka debris flow was composed of coarse granite, which easily develops sheeting joints and micro-sheeting joints. These joints in weathered granite make the area more easy to erode away. As a result, the volume of the flow increased in size and energy as it flowed down the gully. The deposits were also rich in fine granite soil particles which can decrease the permeability of the flow and cause greater mobility. Due to these characteristics, the flow traveled far and had a great spread. Yagi 3-Chrome, which had the most damage reported, lies between an area composed of granite.

The Abu-no-Sato debris flow was composed of hard horfels rock, which is large in size. Thus, the permeability was quite high, resulting in less transportation. The flow deposits, then, did not reach the residential area, but were found at the bottom of the wide and gently-sloped gully. However, deaths still occurred in the area due to a collapse of an old debris flow deposit terrace that mobilized boulders and trees into the complex area.