Tuesday 15 October 2013

At least five dead as Typhoon Nari hits Vietnam.

Five people are known to have died after Typhoon Nari made landfall in central Vietnam on the evening of Monday 14 October 2013, brining with it widespread flooding and winds of up to 133 km per hour. Over 122 000 people were evacuated from low-lying areas across several provinces ahead of the storm, which killed at least 13 people in the Philippines over the weekend. Nari is thought to have caused extensive damage to homes, roads and other infrastructure across the affected area.

Flooding and high winds in the port city of Hội An in Quảng Nam Province as it is hit by Typhoon Nari. Vietnam Bridge.

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 inrushing 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 Quảng Nam Province. Vietnam Bridge.

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.

Flooding on the Perfume River in Huế City, Thừa Thiên–Huế Province. Vietnam Bridge.

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