Boscastle Floods; A Natural Disaster?

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Boscastle Floods; A Natural Disaster?

Heavy seasonal rainfall is a common feature of the climate of South West England and has been documented many times, sometimes it has little affect on the landscape and just drains away but very occasionally it has far more destructive effects. On the 16th of August this year, massive amounts of rainfall over the North Cornish coast caused a disaster on a scale that has not been seen since 1952. A flash flood devastated the small coastal town of Boscastle, this investigation will highlight the causes, effects and responses seen in the event.

Heavy Rainfall

The root cause of the flood is heavy rainfall; to understand all the effects of this we must examine the cause of the heavy rainfall.

Thunderstorms are common in Britain, they are usually small and short and while they may generate a lot of rainfall in a short time it is not normally a problem. The thunderstorm over Boscastle was slightly different, it is a phenomenon only seen roughly once a decade and very rarely with such devastating results.

Thunderstorms are caused by huge masses of warm, wet air rising high into the atmosphere to form cumulonimbus clouds. The updraft that is generated by the air being heated near the ground causes the air to rise taking the moisture with it, it then cools and condenses causing a downdraft that carries a lot of precipitation. Boscastle is an exceptional example of this however; the amount of precipitation involved is an indicator of how rare an event it was.

The first of the unique conditions was the wind direction, onshore winds from both the north and the south were bringing air into the low pressure zone that was located over Cornwall, this low pressure zone was due to the air over the land being heated and rising higher into the atmosphere throughout the day.

Here you can see the direction of the wind during the day.

As the day went on the evaporation rate in the areas of sea increased as the water was heated by the sun, this lead to an increase in the humidity of the air, especially in the south where the air was warmer. These masses of air then continued to be dragged inland to fill the air void over Cornwall. As the air was going in opposing directions it would meet at one point and rise. This diagram shows the pattern of the air movement over Cornwall.

North                                                                                                       South

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As the air from the North hits the coast it is forced to rise by the steep sloping coast, this causes relief rainfall as the air rises and cools, this air was then met by the wind from the south. The air blown by this wind was significantly warmer due to the south being warmer and it travelling across the land in the south and being heated. As the two met their movements cancelled each other out in the north to south direction. The air then started to rise, caused to do so by it being heated and by ...

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