Sixty one people have been confirmed dead and twenty eight more are missing in Vietnam, after Typhoon Damrey made landfall on the country's east coast on Saturday 4 November 2017, triggering a series of floods and landslides. 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. Around 30 000 people have been evacuated from the path of the storm, which has destroyed around 2000 homes and damaged about 80 000 more. Authorities are raising concerns about stress on several dams in the country due to high rainfall levels, which may cause them to release water to prevent the catastrophic failure of these facilities, leading to worse flooding downstream - a dilemma that has been faced by several other South and Southeast Asian countries this year.
Typhoon Damrey battering the coastal city of Nha Trang in Khánh Hòa Province, Vietnam. Reuters.
Tropical storms (called Typhoons in the West Pacific and Indian Oceans) 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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