Friday 23 August 2013

Widespread flooding as Typhoon Trami hits Fujian Province, China.

Typhoon Trami made landfall in close to  Fuqing Bay in Fujian Province, southeast China, at around 2.40 am on Thursday 22 August 2013, brining with it severe rains affecting much of Fujian and Zhejiang Provinces. Eleven counties experienced over 100 mm of rain overnight, with 338.2 mm reported on Pingtan Island. At least a thirty people were injured and one is still missing following a tidal bore on the river Qiantang (a tidal bore is not necessarily caused by a tide, but by a large volume of water suddenly entering an enclosed area such as a river mouth or bay).

People caught by the tidal bore on the Qiantang River at Hangzhou in Zhejiang Province. Sina.

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.

Tidal bore breaking over a waterfront at Hangzhou in Zhejiang Province. Sina.

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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