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Managing Flooding
Dams
3 Gorges
Straightening Meanders
Flood walls
Soft engineering

7. Managing flooding by rgamesby  
Think about it
Rivers are managed in a huge variety ways and for a variety of different reasons.  We use rivers for collecting water for drinking, industry and farming, and we manage then to prevent damage caused by deposition, erosion and flooding.  Management can be split into 2 areas - HARD and SOFT ENGINEERING.

Hard engineering involves the building of entirely ARTIFICIAL structures using various materials such as rock, concrete and steel to reduce or stop the impact of river processes.
One way in which we manage rivers is to build huge concrete and steel structures called dams.  These dams block rivers and cause the water carried by rivers to back up and flood the valley upstream of the dam.  This water can then be used or released through the dam to produce Hydro Electric Power (HEP).  By building a dam it allows engineers to control the flow of a river - this can therefore be used to prevent flooding during high rainfall periods.  Newcastle is protected in part by the
Kielder Water dam, and China has just completed construction of the World's largest dam at 3 Gorges.  This dam has reduced the risk of flooding downstream of the dam from one in 10 years to one in a 100.  Recent flooding prior to dam construction affected millions of people and ruined farmland because of clay that was deposited on the fields.  the dam will protect over 25,000ha of farmland.  The dam has a huge series of locks running up one side to aid navigation, and will generate huge amounts of electricity. However, 1.4 million people had to be displaced (moved) to make way for the 600km lake that has formed behind the dam, there are expected to be problems with the huge amounts of sediment that will be deposited behind the dam and waste has been a problem.  The city of Chongqing puts around 1 billion tonnes of untreated waste into the lake very year.


A satellite view of the lake behind 3 Gorges Dam

A satellite image of the lake behind 3 Gorges

The Dam at 3 gorges

The dam at 3 Gorges

A map of the location of 3 gorges dam


A map of the 3 Gorges location

Find out how the River Skerne has been restored in Darlington

Watch the video below and read the information about 3 Gorges dam and consider if it should have been constructed or not. The Times online article will help as will this Wikipedia article


Try the diamond ranking exercise below


River straightening
Floodwalls in the USA

Flood walls in the USA, note the height of the water relative to the land behind the floodwall.
Straightening Rivers and floodwalls
Another way that rivers can be managed is to straighten meanders.  This involves digging a straighter shorter channel in areas where rivers meander.  The logic behind this is to speed up water flow in flood prone areas, and stop water from "hanging around".  This has occurred on the Mississippi river and in York, but can cause flooding in downstream areas -  a knock on consequence.

A final hard engineering scheme is to build flood walls or embankments in flood risk areas.  These strategies involve raising the banks of the river so that it can hold more water - thus reducing the risk of a flood.  Morpeth has flood walls that will now be improved following flooding there, whilst it was the failure of floodwalls in New Orleans that caused so much damage during Hurricane Katrina.
 
Soft Engineering
SOFT ENGINEERING is a contrasting approach.  It involves managing a river using natural materials and mimicking natural processes to protect more vulnerable areas.  One technique used is to encourage the growth of reed beds which are allowed to flood and slow down river water.    Another technique is known as floodplain zoning.  Here, the areas closest to rivers are only used for low cost uses, such as playing fields and grazing.  Higher cost land uses are kept away from the river on higher land.  The image below breaks the floodplain down into zones of different risk - the high risk zones would be avoided for high cost activities.

Similarly, afforestation can occur where trees are planted near to the river. This means greater interception of rainwater and lower river discharge. This is a relatively low cost option, which enhances the environmental quality of the drainage basin. Another approach is a flood warning system.  In the UK if you live in a flood risk zone your details are kept on a database.  In the event of a flood the Environment Agency will contact homeowners at risk via text message, phone call or a visit from a flood warden. 

River restoration is another approach.  Here, river engineers seek to restore rivers to the "natural" state by taking out previous hard engineering and replacing this with what should have been there in the first place!  This involves putting swamps back in, reinstating meanders, encouraging deposition by using sediment traps and wooden groynes in the channel, adding reed beds etc. 

 A great example can be viewed on the video below:
 

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