49 people are known to have died and a further 41 are still missing following a series of landslides that hit the Japanese city of Hiroshima on Wednesday 20 August 2014. The landslides occurred following a period of exceptional rainfall, with 215.7 mm of rain falling in just three hours, between 1.30 and 4.30 am local time; more than falls in the city in most months. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.
One of a series of landslides that hit Hiroshima on Wednesday 20 August 2014. Kyodo News/AP.
The worst affected areas were in the north of the city, in the wards of Asakita-ku and Asaminami-ku. These areas lie on a bedrock of decomposed granite, which can be particularly hazardous when it begins to erode. While granite is generally thought of as being extremely hard, it is made up of large grains of several different minerals, typically plagioclase feldspar, orthoclase feldspar, quartz and mica, and under some circumstances one or more of these minerals can become chemically weathered, breaking up the structure of the rock and allowing the other minerals to crumble as free grains. The most common form of chemical weathering involved is the breakdown of the feldspar minerals in granite to kaolinite clay (the white clay used to make porcelain ceramics), something that happens readily in warm wet climates, such as that of southern Japan.
Rescue workers searching for survivors in Asaminami-ku Ward, Hiroshima, following the 20 AUgust 2014 landslides. Reuters.
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