Friday, 6 May 2011

River management case studies

HARD Engineering in the Mississppi
  • The US needed to prevent the yearly floods and tame the river to make it navigable in order to develop
  • Before management schemes were implemented the river constantly shifted its channel and eroded its banks
  • They used stone dykes to trap sediment and provoke the river to erode vertically so that the channel was deep enough for paddle steam boats to use
  • More wing dykes were constructed along with reserviors, levees and channel straightening, channelisation (concrete matressing) and dregding were also used -----> this all made the river faster as they increased the gradient along the rivers long profile
  • All of this management, like all river management in the America, was completed by the US Army Corps of Engineers and costs $180 million a year in maintainence as the force of the water sweeps away thousands of dollars worth of management each year. It is hard to manage rivers as they constantly change (in a state of dynamic equilbrium) and so management techiniques are based on guess work and trialed in labs. Some people, though, think that management of the Mississippi has made the floods worse......
    • 1993 = 3 months of torrential rain -----> defences were not designed for the such large size of flood that occured and the local people chose not to pay for the levees to be heightened. The levees failed. However, many think that if the levees didn't fail then the flooding would have been worse as they are believed to constrict water movement, block up the channel and increase pressure.
    • Floodplain development has raised the flood risk as concrete increases surface runoff by reducing infiltration. Also the removal of vegetation reduces the interception store and, because there is nothing to trap the sediment, can raise the level of the river bank. Drains etc, which are designed to imitate the natural processes like throughflow,are a lot more efficient and so the water enters the channel quicker. Therefore scientists conclude that floodplains should not be built on as they are a natural flood defence that is supposed to flood.
SOFT Engineering in the River Rhine
  • The high flooding of the River Rhine in 1993 and 1995 , in combination with the growing awareness of global climate change, made the public and respective authorities realise that constantly raising the height of levees and dykes, for example, is neither economically or environementally sustainable and that, instead, it is more appropriate to allow the river more room so that it can deal with a higher discharge at a lower water level. This reflects a new philosophy that we should adapt to the shape and behaviour of the river basins nto alter them to suit us. This has been approached by:
    • Landuse change and relocation of habitats - not allowing building developments to be constructed on flood plains as they are supposed to flood
    • Floodplain land use zoning - land is being zoned for uses that will not be damaged by winter floods like forests and parks etc.
    • Afforestation - the planting of trees has increased the interception store, prevented the net movement of sediment and so reduced the amount of water and sediment reaching the river
    • Room for the River scheme which includes:-
      • An increase in water meadows which can be allowed to flood when necessary. The sealing of the soil surface with tarmac or concrete in vulnerable areas is being limited to slow the water run off into the rivers
      • Ground coverage of vegetation with woodlands and grasslands is being increased
      • The use of fertilisers on soil is being carefully monitored because these affect the soil structure and its ability to retain water
      • To allow more space for trees on the floodplain, metres of silt accumulated over many years has been stripped and deep trenches constructed
  • All of these soft engineering methods have increased the time taken for water to enter the channel, reduced the amount of water that does enter the channel, created a channel that has a larger cross section and so can accomodate a larger volume of water and moved people away from the most vulnerable areas - remeber that disasters, like a flood, only occur when people come in close contact with a risk!
  • Some hard management options are still being used though like the building of flood relief channels to siphon off the Rhine flood water when the delta becomes overloaded, making the course of the river straighter and shorter and increasing the height of some of the levees.
SOFT Engineering in the River Quaggy
  • The River Quaggy runs through southeast London and since the 1960's it has been heavily managed by building artifical channels and culverts to divert the flow beneath the surface as it passed through Greenwich.
  • The areas of Lewisham and Greenwich have become more densely populated and the flood risk has increased, due to the continued development, and so more is needed to be done to protect the surroundign area. Further widening and deepening of the channel were considered but instead teh Environment Agency decided a softer option was most appropriate. A solution was proposed by the local residents, who formed the Quaggy Waterways Action Group, that would improve the local environment whilst also provided protection against floods.
  • The plan was to bring the river back above ground once again , cutting a new channel through Sutcliffe Park, and creating a new multi-functional open space. This method improves both the flood management and quality of the park. A culvert did remain to take soem of the excess water, during times of flood, underground but a new lake was built to allow the are to deal with the majority of the excess water when the river floods.
  • The park itself was lowered and shaped to create a new floodplain where watercould naturally collect, instead of rushing downstream through the previous artifical channels to flood Lewishantown centre. The parks flood storage capacity is equivalent to 35 Olympic swimming pools, has reduced the risk of flooding for 600 homes and businesses in the local area and created a diverse environment for wildlife
  • By reducing the river to a more natural course and including a flood storage area, the scheme has created a wetland environment with reedbeds, wildflower meadows and trees. This scheme won the Natural Environment category in the 2007 Waterways Renaissance Awards and the Living Wetland Award.
This is the remainder of the river case studies we need to know for the exam. I don't know if any of you feel the same, but after going through the mock yesterday, I realised that it is so much easier to do well if you know the case studies really well. So next I think I will go through the population case studies as there are rather a lot of them - the population policies and migration case studies are already on here........

6 comments:

  1. Hi

    This is an excellent blog - thank you very much!

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    1. Thanks - it is always nice to hear that people find this blog helpful!!!

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  2. I love your blog it's really helping with my revision :D Thanks

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    1. Glad to hear you are finding this useful - if there is ever anything you would like me to write about just let me know! Good luck in your exams :)

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  3. If possible, could you write about more igcse geography case studies?

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