Tuesday, 17 September 2013

Landslide kills twelve bus passengers as Hurricane Ingrid makes landfall in Veracruz State, Mexico.

Twelve people have died when a mudslide buried a bus in Altotonga in Veracruz State, Mexico, on Monday 16 September 2013. The event happened as Hurricane Ingrid made landfall in the state, bringing with it up to 63 cm of rain, leading to widespread flooding and numerous landslips. 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.

Mexican police and soldiers attempt to dig out a bus buried by a landslide in Altotonga, Veracruz State on Monday 16 September 2013. Oscar Martinez/Reuters.

Hurricane Ingrid arrived in Veracruz just a day after Tropical Storm Manuel hit the state of Guerro on Mexico's Pacific coast, devastating the resort of Acapulco. Between them they have led to flooding events across about two thirds of Mexico, killing at least 41 people, as well as contributing to flooding in the US states of New Mexico and Colorado, where so far nine people are known to have died. It is highly likely that the number of deaths will increase in both nations as the waters recede and the extent of the damage becomes more clear. The floods are also known to have caused extensive damage to infrastructure across the affected areas, washing away roads, homes and other buildings. The large number of landslips caused by the flooding will also affect agriculture in many areas, as valuable topsoil is washed away exposing rock beneath, something which is far harder to replace than buildings.

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.

The approximate location of the 16 September 2013 Altotongo landslip. Google Maps.

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