A concise revision sheet with all the necessary detail for the Boscastle case study. An ideal study guide for Hydrology section B of the CIE AS Geography paper. I scored highly on this paper and achieved an A* in the subject using Boscastle as my case study.
Causes
• Boscastle area dominated geologically by Millstone Grit (a variety of sandstone)
o Saturation of the bedrock from previous rainfall caused it to act as if it were
impermeable, encouraging overland flow and shortening lag times
o Previous two weeks had had above-average rainfall
• Boscastle is the confluence of the Valency, the Jordan and the Paradise
o Large quantity of water all arriving at once
• River Valency basin around Boscastle is very small – c. 30 km2
o Upper basin is rounded in shape with a high drainage density
• Steep relief - slopes behind and to the sides of Boscastle have a considerable gradient
o Steep-sided valley funnelling the water into Boscastle
• Vegetation - Upper valley dominated by grassland
• Storm downpours
o 89mm of rain recorded in Boscastle in 1 hour
o In total, 185mm (7 inches) of rain fell on the ground above Boscastle
The Flood
• V short lag time
o Only took 1 to 2 hours for the flood waters to be fully present
• 2.15m rise in river levels in one hour
o 3m wave surged down the main road as water pooling behind debris caught in the
bridge was released when the bridge collapsed
• Water speed over 4m/s
• Estimated that 20,000,000 cubic metres of water flowed through Boscastle that day alone
Impacts
• 75 cars, 5 caravans, 6 buildings and several boasts washed into the sea
• Approximately 100 homes and businesses destroyed
o More had to be demolished
• Trees uprooted & debris scattered over a large area
• Operation lasting from mid-afternoon until 2.30AM
o Fleet of 7 helicopters rescued 150 people clinging to trees and roofs
o No major injuries or loss of life reported
• Estimated cost of damage was £15 million
• About 1,000 residents and tourists affected in total
o Residents all compulsorily evacuated on Monday 16th August (day of the flood)
Prevention Measures post-Boscastle
• £4.5m has been spent on a flood defence scheme
o Lowered river bed by an average of 0.75m between the sewer and the
downstream end of the car park
o New flood defence wall
o Wide, braided river formed upstream of the car park to create an area of slower
flow for deposition
▪ Existing channel realigned away from the head of the car park
o River channel adjacent to car park widened
o Car park raised so that cars are above a designed flood level
o Lower bridge removed and replaced with a wider span bridge
o River channel adjacent to Riverside Hotel widened
o New drains allow water to run into the lower section of the river quickly
o New storm culvert joining the Jordan to the Valency
How effective have they been?
• 21st June 2007
o After an afternoon of intense localised rainfall, following a week of heavy rainfall,
Boscastle flooded again
o Road sin the area flooded, cutting Boscastle off
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