At least 32 people have died in a series of landslides in the Oromia Region of Ethiopia this weekend, with the worst incidents occurring in Gamo Gofa, where a landslide killed nine people and injured seventeen, and Sidama, where a landslide killed 23 people and injured six. The events happened amid heavy rains that have brought widespread flooding and related problems to East Africa, associated with cylones Sagar, which has been attributed with the deaths of 49 people in Somalia (where an ongoing civil conflict has hampered relief efforts) and two in Djibouti (where as much rain fell in 24 hours as usually falls in a year) and Mekunu, which is attributed with seven deaths in the Socotra Islands (Yemen) and six in Oman, with eight sailors still missing on two vessels in the region. Landslides 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.
The aftermath of a landslide at Sidama in the Oromia Region of Ethiopia, which killed 23 people on Saturday 27 May 2018. Atnaf Brhane/Twitter.
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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