Wednesday, 4 May 2011

Flooding case studies

I am getting the impression that many of you are still struggling when it comes to case studies and which ones it is we need to know and so I thought that I would try and summarise each one over the next week - starting off with the rivers ones....

So, according to the revision guide we need to know about flooding in Bangladesh and Gloucestershire, hard-engineering in the Mississippi and a soft engineering case study (either the river Quaggy or river Rhine). Firstly, I thought I would start off with the flooding case studies so here goes......


Physical causes:-
  • most of the country is a huge floodplain and the deltas of the river Ganges and Brahmaputra. 70% of the land is less than 1m above sea level whilst rivers and lakes cover 10% of the land
  • they experience heavy precipitation due to seasonal monsoon rains, glacier melt, storm surges and cyclones
Human causes:-
  • the building of the Farraka Dam in India in 1971 is blamed for the raising of the river bed of the Hooghly River, a tributary of the Ganges. During the dry season the dam reduces the discharge of the river, thereby encouraging sedimentation on the river bed and increasing the risk of flooding
  • urbanisation - recent development schemes involving the construction of networks of roads and embankments have added obstacles to the free drainage of water from the land and reduce the amount of permeable surfaces
  • climate change - some people link global warming to rising sea levels. The Bangladesh floods in 1998 were notable for their long duration of 56 days. This was blamed, by some, on higher sea levels which increased the time it took for water in infiltrate on the floodplain. An increase in global temperatures was also believed to have caused the execptional high levels of precipitation experienced in the Himalayas during that same year
  • deforestation - the Ganges and Brahmaputra's sources are located in Nepal and Tibet where, in recent years, their rapidly increasing populations have caused the removal of vast areas of forest to provide grazing land and a fuel source (it is estimated that 50% of the forest cover in Nepal that was present in the 1950's has been cut down). Vegetation plays a big role in the hydrology of the upland drainage basins due to the fact that it absorbs water from the ground, binds soil particles and reduces the impact of rain on the ground. Overall the forest cover slows the journey of water to the river channels and therefore reduces the risk of flooding. The removal of forests has reduced interception and increased landslides, surface runoff and soil erosion. The silt and soil which is eroded is deposited in the river channel, causing the raising of the river bed and reducing the capacity of the river. It has been estimated that soil is being lost 400 times faster in deforested areas and is responsible for raising the river bed of the Brahmaputra by 5cm a year.
Social Impacts:-
  • over 1000 people were killed whilst millions were displaced
  • in Assam, in the north-east, more than 1 million people lost their homes and in the Nalbari district 240 villages were submerged
  • an embankment protecing Sandwip, a large coastal isalnd, was breached by the high tide and marooned 1,200 families
  • 46 out of the 64 districts flooded
  • livestock and crops were lost
  • 7000 people had to find shelter in the Government relief camps
  • transport links and infrastructure were severly damaged

Economic Impacts:-
  • crops sumerged meant loss of income from agriculture
  • industrial processes had to be halted due to structural damage and lack of transport for workers
  • cost the country $1 billion
Environmental Impacts:-
  • as the waters receded, brown fields of rotting crops, villages buried in sand and silt and wrecked roads and bridges were left behind
  • risk of food shortages as millions of hectares of agricultural land was underwater
  • large amounts of farmland washed away
  • acute shortages of drinking water and dry food
  • respiratory infections affected large numbers of people along with outbreaks of cholera and other diseases that spread easily in water
  • landslides

The short term response to the floods:-
  • Bangladesh Government - distibuted money and 400 tonnes of rice and provided relief supplie of freshwater, water purification tablets, sanitation services. Also appealed for national unity and calm in the wake of the disaster and the general strike which took place in response to the flooding and accusations that the government failed to get basic goods to the people affected.
  • Governments of other countries - the UK sent steel bridge materials and 100,000 million tonnes of wheat. Canada provided 12,500 million tonnes of wheat and money for medicine, watertablets, house repair materials, sanitatin and rehabilitation of farming and fishing. Egypt sent money for medicines whilst Saudi Arabia sent two cargo planes with food, medicines, blankets and tents.
  • The Disaster Forum (a network of aid agencies)- provided boats to rescue people and take them and their belongings to high ground. Supplied medicines to treat and prevent the spread of disease. Medecins Sans Frontieres used 6 mobile teams in boats to travel around one region in which people were literally living in the water. Supplied clean drinking water by digging and repairing wells. Monitored the health situation and set up a medical treatment centre. Distributed fodder for lifestock. Distributed food, plastic sheeting and water purification tablets. Planned a rehabilitation programme to repair and construct housing and sanitation.
The long term responses:-
  • The flood action plan was created to try and reduce the severity of the damaged caused by future floods.
  • Taming the Brahmaputra is being considered and so far the possible methods to do this include:
    • Narrowing the channel by 4km
    • Building 8000km of levee embankments
    • Building wing dykes to trap sediment - at present, modelling suggests that it would cost $5 million to build one dyke (which is 8 times the cost of that to build one in the Mississippi) and a further $10 million to cover the start up costs. This is all before you consider the annual maintainence costs aswell.
  • However, this would:
    • Force millions out of there homes
    • Distrupt fisheries
    • Change agricultural patterns and irrigation patterns - with 80% of the population dependent on agriculture this would be a big problem
    • As mentioned above, very costly due to the fact that all the materials would need to be imported due to the lack of resources on Bangladesh


Physical causes:-
  • weather conditions throughout the summer were far from the norm. The jet stream, which influences the path taken by the low-pressure weather systems in the north Atlantic, had followed an abnormally southerly path. This meant the usual anticyclonic weather conditions, influenced by the high pressure cell in the Azores, did not materialise
  • rainfall totals May-July were highest on record since 1766 with July being the wettest July on record
  • flood risk in summer is usually reduced by dry soil conditions. However, in this case there had been early summer rainfall so soils were already close to field capacity and this, accompanied with the higher than normal groundwater levels, meant there was little infiltration capacity
  • torrential rain - 78mm fell over 12 hours
Human causes:-
  • many housing developments were either built on floodplains or encroached on the river banks
  • urbanization = increase in impermeable surfaces which reduces infiltration and thereby increases surface runoff
  • removal of vegetation reduces interception store and, because there is nothing to trap the movement of sediment, raise the level of the river bed
  • gutters and drainpipes, which do the equivalent of the natural processes of throughfall, stemflow and leaf drip etc, are much more efficient flows and so the water enters the channel quicker

Social Impacts:-
  • 13 people died
  • 2000 people had to stay in rest centres
  • electricity had to be turned off which left 42,000 homes in Gloucester withotu power
  • 3966 homes flooded with 1300 experiencing major loss of possessions
  • 1950 people had to be rescued
  • 350,000 people left without clean water
Economic Impacts:-
  • Total cost = £6 billion
  • 10,000 motorists stranded on the M5 with 500 people stranded at the railway
  • £2.5 million to repair highway damage
  • 20 schools badly damaged
  • 500 businesses flooded and over 7,500 temporarily closed
  • £2 million to repair community buildings
  • Large increase in insurance premiums for those living near rivers

Environmental impacts:-
  • flooding of water treatment plants resulted in water pollution
  • loss of crops and damage to agricultural land
  • much of the mand was under 3ft of water and once this disappeared it left behind muds and sands which damaged the land
Responses to the flooding:-
  • rest centres set up by the County Council
  • Army distributed water bottles and 23 bowser tanks were used to supply drinking water
  • Gloucester fire and rescue service attened 1,800 calls in 18 hours
  • RAF rescued those trapped in buildings and cars
River managment techniques implemented as a response to the floods:-
  • Early warning systems improved
  • River flood defences anaylsed
  • Analysis was conducted of the areas prone to flooding
  • Sandbags prepared for expected flash floods
  • 73,00 people (additional) singed up to flood warning systems
  • 8500km of flood defences inspected
So, there are the two flooding case studies - sorry alot of it is in note form but I think this is the basics of what we need to know - either later tonight or tomorrow I will try and write about hard engineering in the Mississippi and soft engineering in the River Quaggy and River Rhine.......

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