Thursday 19 September 2013

58 feared dead following Mexican landslide.

58 people are feared to have died after a landslide hit the coffee-growing village of La Pintada in Guerrero State, southern Mexico, in the aftermath of Tropical Storm Manuel, on Monday 16 September 2013. The landslide passed through the middle of the village, which normally has a population of about 600 but which has now been largely evacuated, destroying a church and an unknown number of homes. 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 approximate location of the 16 September 2013 La Pintada landslide. Google Maps.

Much of Mexico has suffered from flooding this week, and the country has suffered a series of landslides, after two major storms converged on the nation at the same time; Tropical Storm Manuel from the west and Hurricane Ingrid from the west. These storms have also contributed to flooding across much of the southern US, most notably in Colorado. The death toll in Mexico currently stands at around 80, but is likely to rise as rescue and reconstruction efforts proceed.

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.

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.

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