Flooding on the Mississippi
The Mississippi river is situated in the USA and flows through ten states; Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Arkansas, Tennessee, Louisiana, Mississippi - Flooding on the Mississippi introduction. In total the Mississippi river covers 3733km. The drainage basin conditions of this river vary from day to day. That is why this river is very unpredictable.
Causes of the 1993 flood:
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1. Starting as early as the fall of 1992, heavier-than-usual rain and melting snow saturated the soil with moisture. When seasonal rains and snowmelt came in spring 1993, the water ran off into streams because it could not soak into the ground. So streams were already swollen when a rain-producing weather system stalled over the Midwest in June 1993, and unusually heavy rains fell in many places – often twice the normal summer amount. The rains lasted through August.
2. The waters overwhelmed the normal river channel, so serious flooding hit Illinois, Iowa, Minnesota, Missouri, and other states in the Missouri and upper Mississippi basins, an area 500 miles long and 200 miles wide.
3. Urbanisation on the flood plain was also a major factor as grass and tress have been covered over and concrete and tarmac. Replacing a permeable surface with an impermeable one has many effects on the river. Movement (transfer) of water into the soil from the surface (infiltration) is reduced dramatically along with throughflow, whilst surface run off is increased. This is because when trees were on the flood plain they had four jobs which helped keep the possibility of flooding to it’s minimum
a) the leaves intercept rainfall and so delay run-off into the river.
b) Evapotranspiration ( the loss of moisture from water surfaces and the soil and vegetation) reduces the amount of water reaching the river.
c) Roots delay the throughflow of water to the river.
d) Roots take up waster and so reduce the amount entering the river.
4. Also, drains and gutters can help towards a flood even though they are constructed to remove surface water. They may decrease the time taken by rainwater to reach the river but increases the chances of flash floods. Small streams are forced to travel along culverts or underground pipes. Drains and underground pipes may not be large enough to cope with rainwater falling during thunderstorms.
5. The collapsing of levees and dykes can also be considered as a cause of the great flood. Due to the quantity of thunder storms the level of the Mississippi was bound to rise. It did just that by mid-july the level of the river had reached an all time high. Levees surrounding towns were but under tremendous pressure and they collapsed. With nothing to hold the river back it was certain that another flood was about to hit America because of the Mississippi river.
Consequences of the 1993 flood:
1. Dams and reservoirs were used to reduce the flood risk in the Mississippi basin. They used the Missouri river to build six huge dams. These made a chain of 105 reservoirs which prevented flooding, provided a water supply and hydro-electricity. They were built because they took excess water out of the Mississippi river. If they were not built then much more water would have entered the Mississippi, making the floods of 1993 even worse.
2. Afforestation. This is the opposite to the cause that I mentioned earlier on. This means planting trees and leaving permeable surfaces on the surface of the flood plain because trees delay run-off and reduce the amount of water reaching the river. Also the permeable surface lets the water be sunk in whilst tarmac and concrete are impermeable so the water cannot sink in.
3. Diversionary spillways are overflow channels. They can take surplus water during times of flood. There is a well known diversionary spillway called the bonnet carre floodway which begins near new Orleans. In times of flood like the 1993 one it diverts excess water from the Mississippi along a 9km spillway, through 350 reservoirs. This spillway has dramatically reduced the flood risk at new Orleans and baton rouge. So more of these along the course of the Mississippi could be a lot of help to reduce the risk of a flood.
4. Also, the Tennessee valley authority (TVA) helped by making the course of the river straighter and shorter. By making the course straighter the increase in speed made the water leave the basin faster. They achieved this by cutting through the neck of several large meanders. This would also occur naturally but they did it quicker than it would have done because it takes years and years to cut completely through a neck because of disposition and erosion. This made it shorter by almost 300km.
5. Strengthening the levees also made a big impact to keep the river contained. Levees used to be made up of soil covered by willow. This made them vulnerable to erosion. When they straightened the river they made a new type of levee which consisted of layers of concrete mattresses. The layers were built up until the bank was covered from the deepest point of the river above the flood level.
Management techniques used on the Mississippi:
In its natural state a river can be managed or controlled to make them more useful and less disruptive to human activity.
1. Dams or weirs can be built to control the flow, store water or extract energy from the river.
2. Levees may be built to prevent run-off excess river water in times of flood
3. Canals can be built to connect rivers to one another for water transfer or navigation.
4. River courses may be modified to improve navigation, or straightened to increase the flow rate.
5. These were also mentioned in the consequences because they are both needs of management and consequences. They are needed as they have used a management technique as ship navigation.
Success of the management techniques:
The given management techniques were combating all the causes of the flood. So I think that really the management techniques should have been a success.