Thirty seven people have died and about forty more are still missing after Tropical Depression 23W made landfall in the central part of the country on Tuesday 10 October 2017, triggering a series of flash floods and landslides. The worst hit area is in Hoa Binh Province in northern Vietnam, where eleven people have died and twenty one more are missing, including the victims of a landslide on Thursday 12 October that is known to have killed six, with twenty on still missing.
A Tropical Depressiion is a Tropical Storm not powerful enough to be formally named. 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.
Flooding in Ho Chi Minh City this week. Vietnam News Agency.
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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