Thousands of people have been evacuated from homes in North Carolina, after debris from a landslide hit the Lake Tahoma Dam in McDowell County, North Carolina, compromising the integrity of the structure, on Wednesday 30 May 2018. The evacuation order was issued after engineers inspected the dam and found water spilling around its sides, prompting fears of an imminent catastrophic failure. The landslide was one of several such events triggered by heavy rains in the area associated with Tropical Storm Alberto, which has caused the worst flooding in the area since 2004, when the area was hit by the combined effects of hurricanes Frances and Ivan.
The Lake Tahoma Dam in McDowell County, North Carolina. Joshua Moore/Getty Images.
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, 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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