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Case study: The Boscastle Flood
Background information Where and when?
Monday, 16th of August, 2004
Boscastle is a small town on the north coast of Cornwall. 30kms North West of Plymouth.
12:30pm Heavy rain begins to fall
12:39pm Flood Warning Issue
17:00 Floods reach peak level, cars swept away etc.
17:10 Major incident declared
This flood occurred in two villages: Boscastle and Crackington Haven.
This flood was a flash flood this is an immediate response to short periods of intense rainfall. River
levels rise without warning, often, reaching a peak within minutes or hours. This type of flooding is
very destructive and can cause a significant danger to life. This was also a freak incident only
happens once every 400 years.
Causes of the flood:
600mm of rainfall in 2 hours heavy rain, a month's rain fell in 2 hours
The valley shape steep and narrow, these valleys funnel the water quickly downstream.
Cause 1: Heavy Rain
How flash floods occur: 1) Heavy rain falls onto waterlogged ground
2) Rainfall cannot soak in, so runs down into river.
3) River rises dramatically and bursts its flood valley floor.
Bands of showers aligned themselves with winds that had converged along the coastal high ground
around Boscastle, creating Cumulonimbus clouds 12192m (40,000ft) high and kept them stationary
for many hours.
Cause 2: The Steep Sided Valleys
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It has been estimated that the Boscastle valley's catchment area exceeds 23sq kms spanning inland
to Bodmin Moor where many small rivers spring.
The steep sided valleys that converge down to the sea, known in the trade as "flashy catchments",
act as huge funnels and can produce true flash floods after a sudden cloudburst or prolonged heavy
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Dredge and strengthen/straighten river channels.…read more