Friday 22 November 2013

Cyclone Cleopatra kills at least eighteen people on Sardinia.

Eighteen people including four children are known to have died, and several more are missing on the Mediterranean island of Sardinia after Cyclone Cleopatra hit the island on the morning of 19 November 2013, brining with it over 440 mm of rain in 90 minutes as well as high winds and heavy seas. Rivers across the island were badly swollen, leading to flooding in many areas, with the water in parts of the city of Olbia exceeding 3 m in depth. Parts of mainland Italy also received exceptional rainfall.

Flooding in San Gavino Monreale, Sardinia, following the passage of Cyclone Cleopatra. Reuters.

A family of four including two children drowned in a basement flat in Arzachena, from which they were apparently unable to escape. The family are described as being of Brazilian origin. Three people were killed when a bridge collapsed on their can near Olbia, and a police officer escorting an ambulance was killed by another collapsing bridge in a separate incident, this time as he was attempting to cross. A mother and her child were killed when their car was swept off the road by floodwaters in Olbia. In Uras a 64-year old woman drowned in her own home.

A flooded road near Nuoro. EPA.

Cyclones 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.

Damage to houses in Olbia. Alessandra Chegria/AFP.

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.

Flooded streets and a swollen river in Uras. AFP.

However such events are unusual in the Mediterranean, and Sardinia was poorly prepared for this event, with roads and buildings not designed with severe flooding in mind. The Italian Government has declared a state of emergency on the island, and pledged €20 million (US$ 27 million) in government funds to help with repair efforts.

Clean-up operations in Terralba. AFP.

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