A number of homes have been evacuated following a landslide in the town of St Leonards in the county of East Sussex on the southern coast of England, on Wednesday 15 November 2023. The landslide occurred on a sandstone cliff between two rows of Victorian houses, and is thought to have been triggered by high rainfall associated with Storm Ciarán, which battered Western Europe at the beginning of November. 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 area was affected by a smaller landslide on 5 November, prompting local authorities to evacuate the residents of eleven properties, most of whom were later allowed to return. However, rainfall levels have remained high across southern England since the storm, and on 15 November a second, larger, landslide occurred, demolishing a conservatory at the rear of one of the properties, and provoking new evacuations.
Storm Ciarán was detected as a low pressure system over the Celtic Sea by the UK Meteorological Office on 29 October, at which time it was hoped it would bring high winds on both sides of the English Channel before dissipating. However, the storm underwent explosive cyclogenesis, a process in which a low pressure system deepens suddenly creating a much larger storm, the causes of which are not fully understood. This led to the air pressure falling to 953.3 mb in Plymouth, the lowest pressure ever recorded in the UK in November, a tornado forming on the island of Jersey, and high winds battering Europe as far east as the Czech Republic and Albania. Twenty one people died as a result of the storm; eleven in Italy, four in France, two in Belgium, and one each in Germany, Spain, the Netherlands, and Albania.
Ocean storms form due to heating of air over the sea, usually in tropical zones. As the air is heated the air pressure drops and the air rises, causing new air to rush in from outside the forming storm zone. If this zone is sufficiently large, then it will be influenced by the Coriolis Effect, which loosely speaking means the winds closer to the equator will be faster than those further away, causing the storm to rotate, clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
Whilst the high winds associated these storms is extremely dangerous, the real danger from such storms is often the flooding. Each millibar drop in air pressure can lead to a 1 cm rise in sea level, and large storms can be accompanied by storm surges several meters high. This tends to be accompanied by high levels of rainfall, caused by water picked up by the storm while still at sea, which can lead to flooding, swollen rivers and landslides; which occur when waterlogged soils on hill slopes lose their cohesion and slump downwards, over whatever happens to be in their path.
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