Boscastle August 2004

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Boscastle August 2004

• Case Study of a flooding incident in the U.K.
• What are we learning today Dino?
• The location of Boscastle.
• The cause of the flood
• The effects of the flooding
• How can it be stopped from
• happening again Boss.

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Where is Boscastle?
A small town on the north coast of Cornwall. 30
kms north west of Plymouth.
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CAUSE 1 VERY HEAVY RAIN Most of the rain fell in
a five hour periodPeak intensities were in
excess of 30mm/hr (0.5mm per minute) A months
rain fell in just 2 hours
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Studies of extreme rainfall patterns have
concluded that freak floods are more likely to
occur in June, July and August than at any other
month of the year. This is when atmospheric
conditions, such as a warm ground surface, lead
to the uplift of air masses which subsequently
cool, producing cloud and rainfall
formations. At midday, on the 16th August 2004,
heavy, thundery showers had developed across the
South West, these were the remnants of Hurricane
Alex which had crossed the Atlantic. 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.
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CAUSE 2 THE STEEP SIDED VALLEYS
It has been estimated that the Boscastle valleys
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 rainfall.
River Valency
River Jordan
Bodmin Moor
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CAUSE 3 SLATEY IMPERMEABLE ROCKS with CLAY SOILS

• The cause was the combination of
• intense convectional rain,
• local topography and
• the geology
• resulting in a flash flood no one could have
  predicted.

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Cause 4 The site of Boscastle?
The harbour area is on low ground beside the sea
and on the flood plain of two rivers.
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What happened?
12.15 Rain gauge at nearby Lesnewth some 4km (2½
miles) up the valley, shows no rainfall and it is
dry in Boscastles harbour area, yet there are
torrential showers at Camelford and at the top of
Boscastle.
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15.30 River Valency begins to break its banks
Cars swept away Cars were swept out to sea,
bridges were washed away and people clung to
rooftops and trees for safety as torrential rain
hit the area. Emergency workers mounted a huge
operation to rescue residents and holidaymakers
along a 32-km (20-mile) stretch of the north
Cornwall coast around Boscastle.
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16.00 a 3 metre wall of water runs through
Boscastle car park at 40 mph.
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17.00 Rescue Helicopters. In an operation lasting
from mid-afternoon until 230 AM, a fleet of
seven helicopters rescued about 150 people
clinging to trees and the roofs of buildings and
cars. Amazingly, no major injuries or loss of
life were reported. 55 residents were airlifted
out by the Royal Air Force after the flooding, 35
BBC staff were flown in by other means.
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Aftermath short term rescue and clear up
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Medium Term Effect on tourism About 90 of
Boscastles economy is dependent on tourism.
After the flood, more than 20 accommodation
providers were forced to shut, many of them
individually owned bed and breakfasts. As about
two thirds of the business is done during the six
week school holiday, the effects were even more
devastating with half the three weeks remaining.
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The timing of disasters such as that in
Boscastle is important. As this happened during
the day people were awake and could be rescued by
the emergency services. If it had happened during
the night most people would have been asleep and
there would have been a higher likelihood of
injuries or even deaths.
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What can be done to stop it happening again?
Freak event only once in 400 years but Control
developments on flood plain Bigger culverts on
both rivers What was done in Lynmouth up the
coast? Boscastle was 52 years to the day after
Lynmouth where over 50 live were lost!