Boscastle flood - 16th August 2004 Flashcards
causes
–Bostcastle dominated geographically by Millstone Grit (variety of sandstone)
->
–SATURATION of the bedrock from previous rainfall caused it to act as if it were impermeable, encouraging overland flow and shortening lag times.
Previous 2 weeks had seen above average rainfall (wet antecedent conditions)
–Boscstle is the confluence of the Valency, The Jordan and the Paradise
->
large quantity of water all arriving at once
- -River Valency basin around Boscastle = small and confined (c. 30km^2)
- > upper basin rounded in shape with a high drainage density
–steep sided v shaped valley with narrow floodplain meaning water was funelled into Boscastle
– vegetation- upper valley dominated by grassland
–Storm downpours - 89mm of rainfall (typically a months worth) fall in an hour. In total 185 mm of rainfall
The Flood
v. short lag time (1-2 hours for flood waters to be fully present
2. 15 rise in river levels in 1 hour (3m wave surged down the main road as water pooling round debris caught in the bridge was released when bridge collapsed
water speed over 4m/s
estimated 20 million cubic metres of water flowed through Boscastle that day
Impacts
75 cars, 5 caravans, 6 buildings and several boats washed into the sea
approximately 100 homes + businesses destroyed -> more had to be demolished
trees uprooted and debris scattered over a large area
operation lasting from mid-afternoon until 2:30 AM
-7 helicopters recsued 150 people, no major injuries or loss of life
estimated cost of damage was £15 million
about 1000 residents and tourists affected in total
Prevention measures post flood
£4.5 million flood defence scheme
lowered river bed by an average of 0.75m between sewer and downstream end of car park
new flood defence wall
wide, braided river formed upstream of the car park to create an area of slower flow for deposition - existing channel realigned away from head of car park
car park raised so that cars are above designed flood level
lower bridge removed and replaced with a wider span bridge
new drains allow water to run into the lowe section of the river quickly
new storm culvert joining the Jordan to the Valency
How effective have they been
21st June 2007 - after an afternoon of intense localised rainfall, Boscastle flooded again. Roads flooding cutting Boscastle off
- but most of their schemes did their job - the new storm culvert joining the river Jordan to the river Valency was at full capacity but did not flood
