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Case Study- Lynmouth 1952 - The village clings to the side of the Valenay
Valley. Storm Duration:
Characteristics of Basin: Factors that Contributed to Flooding: - constant shower for 2 hours
- High drainage density. - Steepness of valley side and long profile. - Total duration was 5 hours in Boscastle.
- Impermeable rocks. - Small size of catchment area. Flood:
- Valley sides and gradient are steep. - Relative absence of thick vegetation cover.
- Small catchment basin. - Peak flow at 140 cubic metres.
- Impermeable Devonian slates. - 77 cubic metres has an annual chance of
- Narrow valley. - Abundant drainage density / surface 1:100.
drainage. - Water speed in excess of 4m/sec more
- Funnelling of water into steep sided gorges. than sufficient to cause structural damage.
- Bridges trapped boulders and trees which - 2 valleys meet, formed by the Rivers - 2m tonnes of water.
caused a temporary dam which than broke
Valenay and Jordan.
to release a 12m wave.
- High rainfall. Immediate Impact:
- river had been diverted and its channel was - 104 cars swept out to sea.
narrower due to tourist accommodation and - High water table. - Bridges swept away.
amenities - Antecedent moisture conditions. - Museum of Witchcraft was severely
Physical Activity: Immediate Cause of Flood: - Boulders and up-rooted trees s=thrown
- Rained 12/14 days, ground became - severe convective storm (unlike Lynmouth onto streets.
saturated. which was frontal in origin) - Housing torn in half.
- Heavy thunderstorms and frontal rains. - Low pressure. - Huge build-up of silts, muds and other
- Over 200mm of rain in the last 14 hours. - Broad SW airflow covered peninsula - Rocks and paths washed away.
tropical maritime. - Economically damaging.
Impacts: - Slower clouds developed as temperature
- 34 dead, 1,000 homeless, 90 houses and rises, the trigger temperature to start What to Do:
hotels destroyed 130 cars and 19 boats convection. Cloud topped up to 1,200m - DON'T build housing estates on flood
lost. cumulonimbus clouds. plains.
- Different appearance, to ensure the safety - Strong winds aloft allowed sea air to move - Early warning system.
of inhabitants then to recapture its former away from the storm centre while new - Management of landscape.
character. growth started by the surface.
- Flood protection and improvement.
- Lose of life and property. - Sea breezes along coast join force until
- Inhabitants moving out?? moist unstable air to create a line of slow Bridgend:
- River Ogmore and other rivers deepened to
West Lyn River changed direction to an older river - Steep topography caused by orographic hold more water.
course to form a triangle of destruction. effect.
- Bridges over rivers are raised to allow more
Case Study- North Cornwall Rainfall/Flood Statistics: water content.
Boscastle August 16 2004. - 200mm in Atterham in 24 hours. - Hasn't flooded for 30 years.
- 175mm at Lensnewth in 24 hours. - River Ogmore only floods non-essential
- The only natural harbour for 30km along the - Peak intensities were at 30mm per hour. areas such as Newbridge Fields.
North Cornwall Coast. - 4 out of 10 nearest gauges recorded less - River is straightened to get water away
from valuable areas as quickly as possible.
- The Ria with long, narrow, steep valley than 3mm in total localised event.
sides and deep waters. - Probability each year of the heaviest hours - Bridgend is on a narrow flood plain, so
of rainfall is 1:400 water will cover Bridgend if the river floods.
- For over 100 years the village was a
- A wall has been built along rivers as well as
thriving part, but for company railways led - Extreme in accordance with Environmental
Agency grassy banks to protect valuable areas.
to its decline as a trading part.