Why did Boscastle flood

why did boscastle flood geography edexcel as

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What caused the Boscastle flood of August 2004?
Meteorology
The rainfall on the afternoon of 16 August 2004 was very heavy. 185
mm of rain fell over the high ground just inland of Boscastle. In
Boscastle itself, 89 mm of rain was recorded in 60 minutes. The rain
was very localized: four of the nearest 10 rain gauges, all within a few
miles of Boscastle, showed less than 3 mm of rain that day. The
torrential rain led to a 2 m rise in river levels in one hour. The low-
pressure weather system that caused it was in part caused by the
several hurricane remnants drifting over Britain via the Gulf
Stream, (notably hurricane Alex) which carried across a warm
moist tropical air mass.
Topography
The steep sided valleys that converge down to the sea, known
as "flashy catchments", acted as huge funnels and produced
flash floods after prolonged heavy rainfall.
The basin size is estimated to exceed 23 square km, which is a
small catchment size. However, because there are two rivers
and Bodmin receives lots of relief rainfall, it meant that the area was
overwhelmed.
We measured slope angels between 23° and 24° on the path on the upper
valley. This is extraordinarily steep, leading to fast streams of surface run
off water coming down the valley.
Rock, soils and vegetation
The area is predominantly a shale and slate area. Most of the local
buildings have slate roofs and along the river path, it can clearly be seen
(see right). This type of rock is impermeable which lead to the flash floods.
Above the rock is a thin layer (we measures soil depth of as little as 5cm)
of clay (see data sheets). Clay is also impermeable.
The vegetation either side of the river Valency consisted of thick native
forest, which would have kept the soil intact and prevented soil erosion. It
would have slowed down the rain as it passed through the woods and
banks. The surrounding area is farmland.
Unfortunately, several of the trees could not withstand the downpour, and
lots of debris was washed downstream causing damage and several
problems. The damming effect of debris and local bridges caused the floodwater to build up
and to surge suddenly down the valley.
Conclusion
Meteorologists have said that the flood was a freak event and had a 1 in 400 chance of
happening. The weather events that lead to the disaster were extreme and the natural
environment was prone to flash floods. The events were not human-induced. However, the
flood prevention improvements that the town have implemented will hopefully be helpful in the
future.
Sources
Data from the school field trip
http://en.wikipedia.org/wiki/Boscastle_flood_of_2004
http://www.geoprojects.co.uk/Keyfile/KeyBoscastle.htm
http://tsunamisfloods.suite101.com/article.cfm/flash_floods_in_the_cornish_village_of_boscastle
http://www.metoffice.gov.uk/education/teens/casestudy_boscastle.html

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