Eleven people have been confirmed dead and a further fourteen are missing following a series of landslides and flash floods in Fukuoka and Oita prefectures, on Kyūshū Island, Japan, this week. The events occurred after very high rainfall in the area, with 600 mm of rain falling in parts of Fukuoka between Wednesday 5 and Friday 7 July 2017, in the wake of Tropical Storm Nanmadol which swept across Japan earlier. 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.
The aftermath of a flash flood in Asakura, Fukuoka Prefecture, on Thursday 6 July 2017.
Tropical Storm Nanmadol developed over the West Philippine Sea as a tropical depression on 1 July, sweeping northeast and growing in strength until it hit the Japanese coast near Nagasaki on 4 July, when it has wind speeds slightly below the threshold at which it could be considered a typhoon (sustained winds in excess of 119 kilometres per hour).
Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the in rushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.
The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are 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.
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