1. The Lynmouth Flood 1952

2. Lynmouth Floods 1952 A flash flood!
Was this natural disaster made worse by human activity?
What lessons can be learned from the disaster?

3. Where is Lynmouth? Location: Country: United Kingdom
Region: South West England
County: Devon
A small coastal town , Lynmouth is located in steep-side narrow valley at the confluence of two rivers – the East Lyn and the West Lyn.

4. What caused the flood?
Causes
Meteorology
Topography
Human Activity

5. Flash Floods

6. What caused the flood? Meteorology -The Weather weather on line
Combination of factors:
Dry summer up to early August
Period of heavy rain leading up to Aug 15th
Extreme rainfall event on August 15th
weather on line
Image from BBC Devon

7. The Weather
Low pressure system centred over the South Devon/Dorset coast
A zone of warm damp air produced heavy thunderstorms during afternoon of Aug 15th
Late evening – the Low moved east & caused a rapid movement of cold air to force its way up the northern slope of Exmoor.
This cold air shunted the already unstable air upwards, accelerating uplift -chilling of damp air and produced violent downpour!

8. What happens in a Low Pressure System?

9. Early Afternoon Thunderstorms – unstable warm air rising
Exmoor – warm ground

10. August 15th 1952 Late Afternoon
Wedge of Cold Air
forces warm air upwards
Rapid Chilling
Rapid Condensation
Massive downpour
Saturated ground
RUN-OFF

11. Rainfall in August 1952
Graph shows rainfall in the 2 days leading up to the disaster.
Peaks at 5pm on the 15th and 9pm on the 16th August
230mms of rain in 14 hrs!

12. Rainfall on 15 August 1952
The map opposite shows the area of maximum rainfall
Notice:
Over 204mm on Longstone Barrow & ‘The Chains’ – area of moorland we visited
Numerous tributaries feed East and West Lyn Rivers
Rivers meet (confluence) at Lynmouth
Figure from ‘Lyn in Flood’, Keene & Elsom, Oxford Brookes Univ.1990

13. What caused the flood? Relief /Topography – the shape of the land
The Lyn Valleys are both narrow and steep sided
The rivers drop steeply from the moors of Exmoor
Both rivers meet at the end of the ‘drop’ at Lynmouth
Image from Google Earth

14. What caused the flood? Relief /Soils
The black, peaty, soils on Exmoor are very thin.
The heavy rain in August meant that they became saturated.
The rain falling on Aug 15th could not be absorbed by the soil and simply ran off, carrying the soil with it.
The Moors (taken 4 Sept 2009)

15. The East Lyn Valley – our walk!
Technocentric bridge
Ecocentric bridge
Watersmeet
Notice the tight contours in the valley – evidence of steep slopes!
Drop off point
Extract from Lynton Lynmouth Walking Map.

16. The East Lyn Valley – Narrow and Steep

17. A Raging Flood in two confined valleys!
13.6 million cubic meters of water fell on the drainage basin of the East and West Lyn Rivers.
The water surged down the valleys picking up boulders weighing 50 tonnes.
Rivers carried huge volume of soil from moors
Trees swept into rivers
Bridges formed natural dams – when they gave way a 12 meter high wall of water thundered through the town of Lynmouth.
BBC report
Pathe Newsreel

18. How did Lynmouth Change?
Look the this map of Lynmouth as it was before the flood.

19. Both river beds widened and deepened.
This is Lynmouth today
Changes made to the town.
Both river beds widened and deepened.
Soak-away areas built into river channel
Banks strengthened and reinforced
Bridges designed to allow flow of much larger volumes of water.
Buildings in high risk areas not replaced or demolished

20. Bridges – Ecocentric or Technocentric?
Which bridge is designed to easily wash away and which is designed to resist a flood?

21. A very similar event! Boscastle Flood 2004
Video News Report d

22. Beware the unexpected!

23. What do Lynmouth & Boscastle have in common?