Hydrological change (Boscastle)

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Boscastle

OVERVIEW

Date: 16th August 2004.

Type of flooding: Pluvial (flash) flooding, short lagtime.

Location/Geology: The village is coastal, and stands at the bottom of a very steep and narrow valley at the confluence of the rivers Vallency and Jorden.

PHYSICAL CAUSES

185m of rain fell on Bodmin Moor and the surrounding hills. Steep valley sides led to accelerated surface run off, resulting in high discharge in the R.Vallency. River water Volume increased dramatically by over 2m in 1 hr. Previous stormy weather in the SW meant that the soil was already saturated, therefore less inflitration and greater surface run off into the R.channel. The intense storm event was caused by a combination of localised high temeperatures, moist sea winds and unabsorbed surface water. This resulted in the formation of thunderclouds over 10km high

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Boscastle

HUMAN CAUSES

Urbanistation of the valley where the village is situated meant there was a lot of impermiable surafaces such as concrete, leading to incresed surface run off. Small and ageing drainage systems could not cope with the volume of water. Culverts were not built to handle the increased capacity of water. Low bridges meant that debris and vegatation became stuck and blocked the river channel, the small culvert where the R.Jorden tributary meats the R.Vallency also became blocked with debris leading to both rivers bursting their banks. Deforestation in and around Boscastle meant less interception, accelerating surface run off. Natural channel had been walled so the river couldn't adjust to the sudden increase in water.

 

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Boscastle

MANAGEMENT STRATEGIES

Total Cost: £10 million.

  • (HARD) A new, raised bridge across the widened R.Vallency.
  • (HARD) Straigtened and deepened river beds by 2m.
  • (HARD) Strengthened 200m length of embankments with gabions.
  • (HARD/SOFT) Car park raised 1m, and left as a blue corridor that can flood if necissary. 
  • (HARD) emergancy relief culvert installed.
  • (SOFT) Afforestation in the upper catchment slopes. 

SUMMARY

More hard engineering was used rather than soft. Hard engineering is effective and lasts a long time, however it requires a lot of expensive maintinence. It is not always asthetically pleasing, however looking at it from a social aspect it would make people feel more secure. It does alter the natural processes of the river and can be damaging to wildlife/the environment.

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Tewkesbury

OVERVIEW

Date: 20th July 2007.

Type of flooding: Fluvial (following a long period of steady rainfall over a large area).

Location/Geology: The town is located in Gloucestershire at the confluence of the river Severn and Avon, and is sorrounded by a large flood plain.

PHYSICAL CAUSES: In July 2007 England and Wales recieved twice the average precipitation for the month, with Tewkesbury recieving over 240mm of rainfall, which is around 531% of average rainfall. A slow-moving area of low pressure and associated frontal system caused severe storms to culminate in the lower severn catchment. This was because the polar front jet stream was travelling south of the UK instead of north. There was also abnormally high North Atlantic sea surface temperatures, which increased evaporation and cloud formation, resulting in heavy and prolonged rainfall. There was the equivilent of almost 2 months rain in 1 day, with Tewkesbury recieving 90mm on 20th. The excessive rainfall meant that the soil, ground and river channel had insufficient capacity to store excess water, causing rivers to become overwhelmed, and to flood. 

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Tewkesbury

IMPACTS

  • 10,000 starnded on the M5 - Demographic.
  • 500 people spent the night at Gloucester railway station as the network failed Demographic.
  • Properties were flooded - Social.
  • All 4 access roads to Tewkesbury became impassable, only remaining access route was by foot on top of an old raileay embankment - Social/Demographic.
  • Tewkesbury town centre was flooded and cut off for several days, destroying businesses - Economic.
  • Water treatment works was inundated, leaving 350,000 people without water for over 2 weeks - Social.
  • Castle Meads electricity substation was shut down as a precaution, leaving 42,000 people without power in Gloucester for up to 24 hours. Despite the erection of temporary defences at Walham substation, north of Gloucester to hold back water and prevent 500,000 people from loosing power - Social/Economic. 
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Bangladesh (LEDC)

Year: 1998.

CAUSES

  • Much of Bangladesh consists of the floodplain of the Ganges and Brahmaputra rivers.
  • Over half of the country is below 6m above sea level; water can spread over vast distances once the river has burst its banks.
  • Monsson climate with most places recieving around 1800 and 2600 mm of rain per year. (London revieves just 600mm) 80% of the total rainfall is concentrated over 4/5 months.
  • Heavy rain coincides with high temperatures. High temperatures melt ice and snow in the Himalayers, adding to the discharge in the Ganges and Brahmaputra rivers. 
  • Global warming is causing glaciers to melt in the Himalayers.
  • Monthly high spring tides and tropical storms can prevent water flood water from escaping into the sea.
  • Deposition of silt in the R.channels will cause higher R.beds leading to more frequent flooding.
  • Human mismanagement; Urbanisation of floodplains & deforestation in upper course.
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Bangladesh (LEDC)

EFFECTS

  • Most parts without power for up to 2 weeks.
  • Hospitals were full if people suffering from dysentry and cholera, especially in parts of Dhaka, including the airport where the land was under 2m of water.
  • Delivering overseas aid (medical supplies and food) became imposiible; On 26th August 1998, 2 weeks before the third peak and a month before the flood waters receded urgent supplies were requested.
  • Normal annual food defecit is around 2.1 millionmetric tonnes. Estimated food loss in 1998 is 2.2 million metric tonnes. Resultant food shortage for the year was expected to be 4.3 million metric tonnes!
  • Loss of infrastructure and 1 million homes.
  • Damage estimated at US$1bn.
  • GDP per capita was $1,500.
  • over 1000 desths! amd millions homeless.
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Mississippi (MEDC)

Year: 1993

CAUSES

  • Heavy rain during April 1993 saturated the upper Mississippi basin.
  • Thunderstorms in June caused massive amounts of rain that filled up local reservoirs that would normally hold floodwater and slowely release it into the river.
  • In July one storm gave 180mm of rain in just a few hours.
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Mississippi (MEDC)

EFFECTS

  • 32 Deaths.
  • 36,000 people lost their homes.
  • Many people were evacuated.
  • infrastructure was underwater & 6 million acres of farmland.
  • Millions of tonnes of silt was deposited in the flood zone; big clear up!
  • Estimated US$10-12bn to repair flood damage.
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Mississippi (MEDC)

MANAGEMENT

  • 300 new dams; The Missouri river is now a 1600km chain of 105 reservoirs.
  • Stronger Levees; reinforced by concrete mattresses, each measuring 25 by 8 m. (St Louis Levee is now 18km long and 16m high)
  • Straightened R.channels; blasting through the necks of meanders to make the river straighter and shorter to make them more efficient.
  • Afforestation in the upper R.basin.
  • Diversionary spillways are new channels designed to take floodwater away from the river at times of high capacity.
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Bangladesh (LEDC)

MANAGEMENT

SHORT TERM

  • The government distributed money, water and purification tablets, sanitation services and 400 tonnes of rice.
  • Aid agencies provided boats for rescue efforts, medicines & set up medical centres, clean water, distributed fodder for livestock & food and plastic sheeting.

LONG TERM

  • July 1987; the World Bank prepared an action plan for flood control involving the completion of 3500km of embankments that include compartments for flood water.
  • Building 7 huge dams & 12-15 floodwater storage basins to hold diverted water from rivers.
  • Pumping water out of the ground in the Himalayers during the dry season in order to create underground storage for the monsoon rains.
  • Afforestation in Nepal and Tibet.
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