The 1993 Mississippi floods were caused by hard river engineering
In 1993 the Mississippi River, whose mouth is located near New Orleans in South of the USA, flooded causing $15 mil worth of damage. Around 50 people were killed in the disaster as 18 thousand square metres of the States were covered in overland flow in North America’s worst flood. In normal conditions, every spring the river fills up draining half of the country’s water, and floods are common so to aid this, American governments set up various ‘hard engineering’ schemes to control the water.
Examples of this type of engineering feature the construction of artificial levees to prevent the river from overflowing, and the building of over 200 reservoirs in an attempt to hold back large amounts of water. The instalment of wing dykes to improve transport through river dispersion was also used, and the interference of hard engineering has been seen as one of the leading causes for the worsening of the 1993 floods .
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It can be argued that a lot of the hard engineering work done on the Mississippi river actually made the consequences of the flood a lot more severe than if the river had been left to take its natural course. Artificial levees, which make up over 500km of the Mississippi’s banks can be seen to greatly enhance the flood damage rather than reduce it. These are extensions to nature’s raised river banks, which are caused by the sediment deposited by high water levels. Coarser material builds up closer to the river and alluvium, being lighter, is transported further away from the banks.
The water that transported this sediment then gradually infiltrates back into the river in a downwards fashion through the ground, creating a floodplain. Artificial levees on the other hand add to these by building even higher mounds along the river banks, on top of the ones formed naturally by the river. This is done in an attempt to prevent the river from ever overflowing, as it was not expected that the water could ever reach as far up as the artificial levees do.
However the sediment that would naturally be deposited on top of the existing mounds when the river rises now cannot be ‘thrown’ from the river as it never reaches the point where it can overflow. Instead, it collects along the river bed, decreasing the volume of space allowed for water in the channel, so pushing upwards the water levels. In order to ‘solve’ this, further extensions to the levees need to be installed, in order to prevent the water from rising over the top of them and flooding the now urbanised floodplain.
This has the same effect, as sediment continues to build up on the channel floor. Even though small risings in the rivers surface level can be controlled, artificial levees actually increase the risk of much more devastating floods that occur when the man-made mounds are breached. The greater gravitational potential energy that the river has at such a height means that any overflowing of such engineering would cause the levees to crumple under such pressure releasing huge amounts of water in doing so
Diversionary spillways are another example of poor engineering, where a straighter, more direct route to the mouth (or sometimes a new mouth altogether) is created, therefore quickening the river’s discharge rate by removing the hindrance caused by ‘unnecessary’ meanders. In the case of the Mississippi floods, the diversionary spillway went from Baton Rouge, where a lock was put in place to create a new artificial channel all the way down to the coastline.
As the water now flows in a relatively straight line, there is reduced friction along the sides and bed of the channel, increasing the velocity and making the river more efficient. The theory behind this protecting from floods is that there is less of a risk of flooding at the lock due to the faster flowing water. However as soon as the water reaches the natural channel that is uneven by nature, velocity is reduced and efficiency reduces. This means that water collects, presenting an even greater risk of flooding than before, as its banks can be easily broken.
Another school of thought implies that some forms of hard engineering actually do contribute to effective reduction of the effects of flooding, or at least they do not worsen them. Revetments for example, are usually wooden or metal bulky material that is set up in meanders in the river in order to prevent meander migration, so that it does not expand further outwards onto land that is frequently urbanised or containing such a structure as a bridge. When these are installed, the river, as it can no longer erode one side of the river, simply affects the other side of the channel.
This method of hard engineering in fact helps to create a ‘naturally’ straightened river and has no contributing effect to flooding. Another example of a beneficial method of hard engineering is the construction of reservoirs, and there are over 200 of these along the Mississippi. These usually involve the construction of a dam in a steep part of the river in order to hold back large amounts of water. Although flooding above the dam is common due to its build up behind, this is planned for though, and the dam can be partially opened in order to release excess water.
Despite the fact that the reservoir is not directly aimed to prevent flooding, it does not exacerbate floods or flooding effects and can be seen as a successful form of hard engineering. Urbanisation of the flood plain can be seen to have increased the scale of flooding due to there being a lack of open ground. This means that infiltration is practically impossible, due to the concrete foundations and underground systems present in urban areas, this increases the overland flow of water whenever it rains or indeed a minor flood occurs.
It can then be said that human intervention also has an effect on flooding, which is not only a result of hard engineering. Due to the lack of evidence of any other floods that were as bad as the 1993 floods in the area, the effects of the flood had to be guessed, and so hard engineering success could only really be determined after the floods occurred. Also, no two floods are alike, so it is impossible to even make much of a prediction as to how serious they would be. In conclusion it can be said that yes, some forms of hard engineering have exacerbated floods due to poor planning for river response.
Diversionary spillways create a very fast flow of water that is unnatural and therefore disrupts the river when the water rejoins the natural channel. Similarly, artificial levees mean that minor natural floods are not permitted in order for sediment to pile up around the banks, so it builds up at the bed of the river, reducing space in the channel, and therefore encouraging spill over. On the other hand, it can be said that floods are not always the result of hard engineering due to the presence of evidence to suggest that some forms of hard engineering have little effect, and sometimes even a positive one on the floods in comparison.
Other contributing factors have been said to have exacerbated the floods as much as some forms of hard engineering, such as the presence of urbanisation in the river’s floodplain, and the lack of any accurate or existing records of other serious floods allowing less scope for predictions and therefore decent planning. Therefore it should be said that hard engineering in some cases has been known to create bigger flooding problems, however it is not solely to blame for the Mississippi 1993 floods, as the presence of other human intervention can be seen to have also exacerbated the effects of flooding.