flash flood case study : Boscastle

Question 1

introduction to Boscastle

Answer 1

• small village in Cornwall
• 90% of jobs in the village are linked to tourism
• permanent population of under 1000
• experienced heavy flash-floods back in 2004

Question 2

natural causes to boscastle flooding - 1. heavy rainfall

Answer 2

• violent downpour to the river valency
• 89mm fell in an hour
• led to 2m rise in river levels in one hours

Question 3

natural causes to boscastle flooding - 2. antecedent conditions

Answer 3

• ground was already saturated from previous rainfall events

Question 4

natural causes to boscastle flooding - 3. topography of the land

Answer 4

• landscape upstream of Boscastle is a steep sided valley
• acted as a funnel directing vast volumes to water into the village

Question 5

natural causes to boscastle flooding - 4. channel width

Answer 5

• river has a narrow river channel

Question 6

natural causes to boscastle flooding - 5. confluence

Answer 6

• village of Boscastle is located at the confluence of River Valency and River Jordan
• means it’s receiving the discharge of TWO rivers

Question 7

natural causes to boscastle flooding - 6. high tide

Answer 7

• storm happened at the same time there was high tide
• means water flowing down river, couldn’t flow efficiently down the sea

Question 8

human causes to Boscastle flooding - 1) deforestation

Answer 8

1) - changes in farming practice caused a reduction of trees and hedges higher up the valley
- encouraged surface runoff and caused water to flow through more quickly than before

2) increased soil erosion - less roots to bind the soil on the valley
- means more soil flows into river and HEIGHTENS river bed,

Question 9

human causes to Boscastle flooding - 2) drainage system

Answer 9

• town had a very old drainage system
• not designed to hold a lot of water

Question 10

human causes to Boscastle flooding - 3) impermeable surfaces

Answer 10

• car park had been built in village for tourists
• not permeable which stopped water infiltrating the ground
  = surface runoff

Question 11

human causes to Boscastle flooding - 4) low bridges

Answer 11

• foot bridges in the town were built TOO LOW
• blocked by trees and debris — worsen flood as water couldn’t pass under the bridge
• when bridges finally broke (pressure of water) ,
  — a 3 metre high wave swept through the village at a speed of 4 metres per second.

Question 12

human causes to Boscastle flooding - 5) flood prediction

Answer 12

town had no way of predicting floods

Question 13

human causes to Boscastle flooding - 6) lack of investment in flood prevention

Answer 13

• never been a major flood in village