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The Mississippi Flood 1993 Case Study

An example of an exam question, and how to structure your answer because there is a lot of information. This is very full on and very long, in an exam, you may want to cut short your answer. But this has everything, is very detailed, so that you have an example of a complete answer. Make it your own words and make it more concise. That's your job now!

For a named flood defence scheme, describe and explain how it helps to reduce the flood risk. (9m)

Name of flood defence scheme: Mississippi River Flood Defence Scheme, USA

The Missouri River is a main tributary to the Mississippi River, 6 huge dams have been built on it, creating a 1600km chain of 105 reservoirs, controlling the amount of water added to the Mississippi River. Thus reducing the risk of flooding. The Tennessee is another major tributary, 9 dams have been built on its river such as Kentucky and Nick-a-Jack, and 10 have been built on its tributaries. The Tennessee Valley authorities (TVA) have also been responsbile for planting many trees. Afforestation has occurred in the upper Mississippi drainage basin system, delaying surface run-off by interception by vegetation. Trees also absorb water, their roots delay throughflow and run-off too. All this reduces the amount of water reaching the river and delays it as well. This gives the Mississippi more time to transport flood water away. The Bonnet Carré floodway has been constructed to divert excess water from the Mississippi. It begins 50km North of New Orleans and diverts excess water along a 9km spillway through 350 small bays to Lake Pontchartrain, and eventually into the Gulf of Mexico. The Mississippi River has been straightened and shortened. From 1934-1945, a 530km stretch of river has been shortened by almost 300km. It cuts through the meanders making it straighter and shorter. this increases the gradient and therefore the speed, so flood water can be transported away faster. Levées have been strengthened and heightened. Instead of soil covered by bundles of willow, which is very vulnerable to erosion, it is now reinforced concrete. A special barge backs away from the shore leaving 25m x 8m concrete mattresses. This is repeated until the deepest part of the bank is covered to above flood level. For example in St. Louis where the confluence of the Missouri and the Mississippi is, it has a levée made of reinforced concrete. It is 15.8m high. The flood level of 1993 was 15.05m. This levée protects St. Louis from the Mississippi flooding. 

An update:

To help you formulate your own answer, I thought this might help:

River management/Flood Defence scheme: Mississippi, USA (reduced flood risk)

1. Dams and reservoirs

The Missouri River (tributary): 

  • 6 huge dams built on it,
  • creating 1600km chain of 
  • 105 reservoirs that prevent flooding, provide water supply, and HEP.

-these dams reduce water reaching/added to the Mississippi, without them, 1993 flood would be even worse.

1b. Dams and reservoirs 2

The Tennessee River (tributary): 

  • Tennessee Valley Authorities (TVA) set up in 1930s have many functions, one of which is to control flooding of this river. 
  • 9 reservoirs on the main river (e.g. Kentucky, Nick-a-Jack, Wilson, Wheeler..)
  • 10 reservoirs on its tributariers

Dams hold back water in times of flood, and release it when river levels are lower. 

Success-1957, instead of river reaching dangerous peak of 16.5m, dams and reservoirs limited the level to harmless 9.8m. 

Note: The TVA is a multipurpose scheme which:

  • controls flooding
  • provides a water supply
  • produces hydro-electricity
  • improves navigation
  • increases afforestation
  • reduces soil erosion
  • encourages industry
  • encourages tourism

2. Afforestation 

TVA also responsible for planting many trees. Trees delay run-off and reduce amount of water reaching the river, as its roots absorb some. 

3. Diversionary Spillways

These are overflow channels-they take surplus water during times of flood.

The Bonnet Carré Floodway begins 50km north of New Orleans.

In times of flood, it diverts excess water away from the Mississippi through a 9km long spillway, through 350 small bays/reservoirs into Lake Pontchartrain, where the water eventually ends up in the Gulf of Mexico.

This has greatly reduced flood risk in New Orleans and Baton Rouge. 

4. Straightening and shortening course

By cutting through narrow necks of several large meanders. 

Between 1934 and 1945, a 530km stretch of river was shortened by almost 300km

By decreasing the distance, the gradient increases, hence the speed increases--so flood water is transported away quicker. 

5. Strengthening the levées

Levées used to be made of soil covered by bundles of willow-but this was prone to erosion.

Now, specially designed barges back away from shore laying concrete mattresses measuring 25m x 8m. The process is repeated until the bank is covered from the deepest point of the river to above the flood level. 

St. Louis levée is made of reinforced concrete (steel bars inside) 15.8m high (and 18km long) and protects St. Louis, because that is where the confluence of the Missouri and the Mississippi is. (1993 flood peak was 15.05m, but the levée withstood the water's weight.)

Тогда почему бы не вызвать службу безопасности, которая могла бы его задержать. - Пока рано, - сказал Стратмор.  - Если служба безопасности обнаружит затянувшуюся надолго работу «ТРАНСТЕКСТА», перед нами возникнет целый ряд новых проблем.