Events leading up to the flood
Heavy rain in April 1993 saturated the upper Mississippi basin. Thunderstorms throughout June caused huge surface run-off and lash floods. During July the thunderstorms increased in severity with one giving 180mm of rain in jut a few hours. By mid-July the level of the Mississippi had reached an all time high. Levees surrounding towns were put under a tremendous pressure from the weight of the water, in many places they collapsed. Away from the towns the river spread across its flood plain to a width of 25km. An area larger than the size of the British Isles was affected by the flooding. The Mississippi proved that it had nit been tamed, as it claimed 43 lives and caused billions of pounds worth of damage.
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(Above) The Mississippi and
Missouri Rivers as they
Approach their confluence
Above St Louis. The purple
shows the flooded area.
(Above right) the Mississippi
flood before the flood. (Right)
the Mississippi during the
flood.
Effects of the Flood
The effects of the flood did not end when the river level began to fall. It took several months for the water to drain off the land. Although the land was covered in fertile silt, the ground was to wet for planting crops. The contents of houses and factories, even if not the buildings themselves, were ruined. Clearing up operations took months and cost millions. Where sewage had been washed into water ways there was a risk of disease. Stagnant water attracted mosquitoes and rats. Insurance claims were high and numerous.
How might the flood risk be reduced?
1. The Missouri river much of the Missouri, which is longer than the Mississippi, appears on a modern atlas map to be a series of long lakes. Six huge dams have created a 1600km chain of 105 reservoirs which, apart from preventing flooding, provide a water supply and hydro-electricity. If thee dams had not been built then much more water would have been added to the Mississippi, making the 1993 flood even worse
2.Afforistation Vast forests have been planted in the basin to delay run-off and reduce the amount of water reaching the rivers.
3.Diversionary Slipway These are overflow channels which can take surplus water during times of flood. The Bonnet Carre Floodway begins 50km north of New Orleans. In times of flooding it diverts excess water from the Mississippi along a 9km slipway, through 350 small reservoirs, into Lake Pontchartrain, and eventually into the Gulf of Mexico. This has greatly reduced the flood risk at New Orleans and Baton Rouge.
4.Making the course straighter and shorter This method was aimed at trying to get rid of flood water from the river basin as quickly as possible. It was achieved by cutting through narrow necks of several large meanders. Between 1934 and 1945 a 530km stretch of river was shortened by almost 300km. by shortening the distance the gradient and therefore the speed of the river is increased.
5.Strenghtening the levees Levees used to consist only of soil covered by bundles of willow and were venerable to erosion by the river. Now a specially designed barge backs away from the shore laying concrete mattresses, each mattress measuring 25 metres by 8 metres. The process is repeated until the bank from the deepest point of the river to above the flood level.