Why do rivers flood

Ever since there have been rivers they have flooded. This is due to the fact that the normal bank full flow level is only large enough to accommodate water up to the bank full level. This means that any increase in the discharge results in the normal bank level not being able to accommodate the increase and hence spilling over onto the surrounding floodplain.

The increase in discharge can be due to a combination of two factors; human and natural. Natural factors include those such as heavy rainfall, melting snow, as well as prolonged rainfall. Human factors are more subtle and do not relate to the amount of water usually, excepting the effect of buildings creating heat islands which increase the amount of water in the atmosphere and hence the amount available for precipitation. They instead however affect the rate of discharge by controlling factors such as the shape of the channel (by introducing stone dykes), the bed-load (by dredging) and the flow rate of the river (by building dams) and by constructing levees.

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On the Mississippi a number of large engineering works have taken place in order to try and control the flooding of the floodplain. This was started because the shipping that navigated the Mississippi needed a minimum of 9 feet in order that they did not run aground. It was expensive enough to run a shipping company on the Mississippi, with vessels only lasting around 18 months due to the treacherous storms and frequent flooding of the river.

In order to deepen the river, wing dykes made of rock were built that extended into the channel. The idea behind these was to force the river to scour the riverbed and deposit the material behind the dykes; hence artificially creating riffles and pools.

It then became apparent that in order to speed the passages of vessels using the Mississippi and in order to try and make the river more efficient, it was necessary to truncate the large meanders in the lower course of the river. Around 100 miles were removed from the river amounting to about 5% of the mighty 2000-mile long river.

This itself brought another problem, i.e. with a reduction in length of such magnitude as this, with no control on the discharge rate, the river would easily burst its banks no matter how efficient it tries to be. Fortunately the engineers had foreseen this outcome and 200 dams had been built on the tributaries to the Mississippi and so the amount of water that is let into the rivers and streams can be controlled.

As well as building dams to control the level of the water, the engineers also built raised banks or levees at strategic points along the river to stop the river from flowing out onto the floodplain.

During 1993 there was a prolonged period of rainfall and the Mississippi flooded and burst its banks. At the time there was criticism focussed on the original engineers (US Army corps of Engineers) for having increased the severity of the flood by building the levees. The majority of this criticism came from environmentalists, although one newspaper asked a research student to collect data and perform an independent investigation. He concluded that the introduction of levees caused the river to flood more frequently upstream and that when and if the levees broke the associated flooding would also be worsened.

The floods are worsened because the water level behind the levee is higher than it would normally be without it, hence when it breaks it has more potential energy to flow out with and so it causes more damage than if it had risen slowly.

The engineers are now starting to return to a natural way of thinking. Rather than trying to work against nature by building levees and truncating the channel, they are instead allowing wetland areas to flourish and marshes to be created that are allowed to flood and hence control the river level naturally.

In Bangladesh they are about to attempt to tame one of the most revered rivers in India; the Jamuna or Brahmaputra. The Brahmaputra is a tributary of the Ganges and runs to the south until it confluences with the Ganges.

During the monsoon season 20% of the country can be underwater sometimes, under extreme conditions the entire country can be submerged.

The defences that are intended against flooding are similar in design to those used for the Mississippi. 8,000 kilometres of levees will be built and maintained, no mean feat in itself given that all the labour will be manual with almost all the earth required being moved by hand. The maintenance costs will also be enormous, for the Mississippi the price tag was nearly $2million a year to maintain. The price will be similar for Bangladesh however their labour is manual whereas in the USA almost all of the work was automated.

The river is also intended to be narrowed. At points the river is 14 kilometres wide and it is hoped that this can reduced to 4 kilometres. Not only will this allow land to be reclaimed for an ever increasing population but will make the river more efficient so that it reaches its base level faster.

But why try and control floods? The main reason that a country wants to try and control rivers flooding is economic in a country like the USA and for survival in Bangladesh. Along the banks of the Mississippi River are industries that help the economy. The nation protects these so that its economy does not falter.

Another factor also has to be taken into account and this is the cost/benefit analysis. For the USA the taming of the Mississippi did not cost a great deal in economic percentage terms; the benefits when it works however are enormous. For Bangladesh the price of taming the Brahmaputra is, in percentage terms, far greater and so they must ensure that in the long run they do not create more problems for themselves than they solve.