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Rivers: Case study of the Mississippi Floods, 1993

Use our dedicated case study to learn in-depth about the Mississippi Floods, April to October 1993.

Introduction

The Mississippi river drains an area of nearly 3 million square kilometres and is the fifth largest river basin in the world. It provides a good case study as there are a range of human and physical causes, huge impacts of flooding and some good examples of effective action taken as a result. The river floodplain is up to 200km wide in the middle stages of the river’s course and as you would expect, there has been a lot of development of the area with much urbanisation taking place. The river has been managed for many years by hard and soft engineering techniques, channel straightening has been used extensively and has reduce the length of the river by 150 miles for example.

The flood was one of the worst in American history with dams bursting, levees being breached and the river remaining at a height above flood level for nearly 150 days. In total, nearly 78,000 square kilometres of land were flooded.

Causes

Physical causes:

High rainfall towards the back end of 1992 meant that the soil in the basin was holding more water than usual and in many parts the ground was completely saturated. The soil’s capacity to hold water was therefore very low. There was more snow that usual in the winter from later 1992 to 1993, this built up stores of water that potentially would enter the river during the snowmelt season. From the late spring there were a large number of storms and the area suffered persistent rainfall. As much of the ground was saturated and the water table was high, water entered the river channels very quickly as surface run-off or overland flow. There were huge amounts of rainfall in 24 hour periods across the basin, up to 180mm was not uncommon. Many areas of the basin experienced up to 6 times the usual amount of precipitation. Heavy rainfall in the highest reaches of the snow covered areas of the basin accelerated snowmelt and this had a huge impact on the volume of water entering the channel.

Human Causes:

Shortening of the river’s channel by 150 miles over many years had a cumulative effect of pushing water downstream more quickly, especially during times of peak flow. Extensive dredging had taken place to keep the channel sediment free to improve navigation, the river is a significant part of the transport infrastructure of the USA. 75% of the wetland areas of the river basin had been urbanised impacting on the time taken for water to enter the river system. The wetlands ability to act as a store and release the water slowly into streams and rivers was virtually destroyed. Once flood warning were issued and areas were advised to be evacuated, many people thought that they were not at risk because there were so many protection measures in place. Once the river did flood then there was more damage and danger to people who had stayed in their homes and businesses.

Impact

Socio-economic

The flood cost an estimated US $15 billion dollars in damages and over 800,000km2 were affected. 30,000 people had no clean water supply until it could be restored by engineers, 72,000 homes were flooded and close to 48,000 were evacuated from their homes. 45 people died as a direct result of the flood.

Flood defences were destroyed completely or damaged. Farmers lost an estimated US $2.5 billion of crops. Much of the flooded farmland was rendered useless for years after the event as it was so badly affected. Infrastructure was seriously affected with roads, bridges, railway and of course river traffic affected. This had a knock on effect to the economy as close to 10,000 were made jobless as a result of their place of work being destroyed.

Environmental

There was a huge loss of natural habitat and wetland areas which were important breeding grounds for bird and animal life having a long term effect on the ecosystem and biodiversity. The habitat for fauna that lives in the water improved due to increased areas of protection and shelter (much like a shipwreck in the sea provides a haven for marine life). Wildlife did have to contend with contaminated water though as chemicals from farms, stores and industry were dissolved into the floodwater. This also affected human drinking water supply.

After the initial flooding there was a lot of standing and stagnant water which attracted insects including mosquitoes, and rats were a problem in some areas.

Flood management since 1993

Since the flooding subsided the US government prioritised the management of the area affected and much engineering has taken place to try and reduce the risk of future flooding. The 6 very large dams built along the Mississippi’s main channel have been added to by building more on the tributaries. There are 9 dams on the Tennessee (a major tributary of the Mississippi) and a further ten on its own tributaries.

A series of 105 reservoirs have been linked to control the risk of flooding by controlling the flow of water through them.

Afforestation has been prominent in some areas to slow down the rate at which precipitation or snowmelt enters the channel system.

Although straightening and shortening of the river added to the problems of the 1993 flooding, more of this has been done and engineers have learned from previous experience. Levees have been reinforced with concrete rather than more natural and weaker methods (soil for example) and raised to more than the level of the 1993 flood waters. These levees provide protection for 1000’s of settlements along the Mississippi’s course.