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1 River environments
2 Coastal environments
3 Hazardous environments
4 Economic activity and energy
5 Ecosystems and rural environments
6 Urban environments
7 Fragile environments
8 Globalisation and Migration
9 Development and human welfare
1.9. Flood defence
9. What causes floods? How can we control flooding?
[Case study of a flood defence scheme]
Why do rivers flood?
Floods are natural events. They mainly happen when the river
, (that is the area of land that feeds water into the river and the streams that flow into the main river) receives greater than usual amounts of water (for example through rainfall or melting snow). The river cannot cope and this extra water causes the level of the water in the river to rise and a flood to take place. This flooding may take place at any point along the river course and not necessarily at the place where the extra water has entered.
Burt extra water in terms of heavy rainfall will not necessarily cause a river to flood..
Think back to the hydrograph
(and then click and look!) What caused some river to flood more than others?
A river was more likely to flood if:
It has a narrow steep sided valley rather than a wider, flat one. Why?
There was grass or ploughed fields on the banks rather than trees. Why?
It flowed through a town rather than through the countryside. Why
The underlying rock was impermeable rather than permeable. Why?
The soil was thin and stony rather than deep. Why?
I hope you can see that most of these causes of the greater likelihood to flood are down to a common idea – infiltration or lack of it. If the water can soak in more easily, it is less likely to reach the river in sufficient quantities to cause it flood. Think why the water is more likely to cause run-off in the flooding scenarios.
The Impacts of floods
Short term Impacts
Loss of life
Destruction of property
Loss of communication
Fresh water pollution
Loss of power
Long term Impacts
Replacing what is lost or damaged
In HICs insurance covers much of it – but not all as in the case of Hurricane Katrina, many poorer people could not afford insurance.
Governments have to have the funds to rebuild the infrastructure- e.g. roads, water treatment etc
In LICs most people loose everything. Crop destruction can lead to famine.
The governments cannot easily rebuild the infra- structure. What they can takes a long time
How can we control flooding?
Flood management techniques can be divided into
'Hard' options tend to be more expensive and have a greater impact on the river and the surrounding landscape. 'Soft' options are more ecologically sensitive.
Here are the main flood management techniques:
Hard engineering options:
Dam and reservoir construction
Dams are often built along the course of a river in order to control the amount of discharge. Water is held back by the dam and released in a controlled way. This controls flooding.
Water is usually stored in a
behind the dam. This water can then be used to generate hydroelectric power or for recreation purposes.
As we saw at the Three Gorges Dam, building a dam can be very expensive, and sediment is often trapped behind the wall of the dam, leading to erosion further downstream. Settlements and agricultural land may be lost when the river valley is flooded to form a reservoir.
The River Tees, the other river we have been using as an example, has a reservoir called Cow Green.
(Map/ pictures) It helps to stop heavy winter upland rain flooding the lower parts of the river and it supplies water in the Summer to other parts of the North-East which lack water then.
Building embankments (or artificial levees)
are raised banks along the river and they effectively make the river deeper so it can hold more water. They’re expensive and they don’t look natural but they do protect the land around them
are more solid versions of embankments that are built around housing and factories. However unsightly, they are effective. They are often lined with stone or concrete. Often, to make them more attractive and also to restrict bank erosion they built at low angle – these are
. However revetment using fibre matting and naturally growing plants such as willows, qualify as soft engineering.
Another way of stopping erosion and (also stopping landslides from hillsides onto roads) are
. These are wire containers holding lumps of rock. They are much lower in cost as they can be assembled offsite and put in place with little work. There is also no waste. (pic revetment) (pic gabions)
Straightening and dredging:
Straightening the river is also called channelizing a river. It allows it to carry more water, so it doesn’t build up and is less likely to flood. It alters the ecosystem and the natural look of the river, and because the water can pass through more quickly may lead to flooding further down stream
Diversion channels and basins:
..Overflow channels which take surplus water out of a river in times of flood. It is effective if there is a spare piece of land which is not in use.
semi circular, covered smooth channels increase velocity and gets water away from urban areas as quickly as possible. These were used Boscastle to get the R Jordan into the R Valency prior to the major flood there in 2004. However, because of the restricted dimensions, there was a back-up at the start of the culvert that then flooded the whole area around the culvert entrance. Subsequently the culvert was replaced and enlarged.
Soft engineering options:
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.
(also called ecological flooding or wash lands)
The river is allowed to flood naturally in places on the flood plain where development has been restricted and its only use maybe for instance playing fields. This is to prevent flooding in other areas - for example, near settlements.
Planning (or land use zoning) :
Local authorities and the national government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding and the risk of damage to property. There can be resistance to development restrictions in areas where there is a shortage of housing. Also follow the widespread urban floods, some local authorities have restricted the way opeople use their front gardens for parking. They have to use methods that allow the rain to soak into the ground, instead of concrete or tarmac which funnels all the rain into the drainage system
Enforcing planning regulations and controls may be harder in LEDCs.
.Warnings are issued by the Environment Agency so that people can put sandbags by their homes, take furniture upstairs or even evacuate the area.
1.11 General revision
to find an animation that looks at both hydrographs and flood defences
Case study: Water use and Flood defences along the Tees
The River Tees has a long history
of flooding. The first documented flood was at Croft on the lower Tees in 1356. Many of the previous floods were precipitated by snow melt. But, the Environment Agency suggest that this will be less of a problem in the futures. However, intense rainfall is more likely to be a problem so flash floods will become more frequent
The Tees valley is also home to a large population and many industries, all requiring a reliable water supply.
The river is managed to provide a water supply and to control flooding. In recent years there have also been developments to increase its potential for recreation and tourism.
Cow Green reservoir was built in 1970 to provide water for the growing industries on Teesside. It is a regulating reservoir, storing water in times of plenty and releasing enough for the needs of industry in times of low flow. In times of severe summer droughts water can be added to the River Tees via a tunnel which connects it to the River Tyne and Kielder reservoir.
Lower Tees Valley- Management: (a)The Tees Barrage (a man made barrier across a river)
The aim of the Tees Barrage was to improve the water quality and recreational value of 22km of the lower Tees. The barrage was completed in 1995 and cost £54 million. The 22km stretch of river between Yarm and Stockton is now kept permanently at high tide. The water is fresher and cleaner as it does not mix with the tidal salt water in the lower estuary. The barrage also reduces the risk of flooding at very high tides or during a storm surge. The barrage has acted as a catalyst for £500 million of investment in offices, housing, educational, leisure and shopping facilities.
The lower stretches of the Tees estuary are dredged (sediment is extracted) periodically to improve navigation by maintaining a deep-water channel. There has also been some dredging in the upper parts of the estuary to reduce the flood risk - it increased the capacity of the channel.
Cutting of meanders
: In 1810, the Tees Navigation Company cut across the neck of the Mandale Loop, a large meander near Stockton. The new route shortened the river by 4km. Other stretches of the river have been artificially straightened. This allows the water to move faster along the channel (less energy lost with the river banks), reducing the flood risk.
(d) Yarm's flood defence scheme
a question about a town on the flood control scheme has been asked in the past
Yarm, a historic market town and once an inland port, is located on the inside bend of a large meander.
Yarm is particularly prone to flooding. The most recent serious flood was in January 1995. Since then a new flood defence scheme costing £2.1 million has been built which reinforced concrete walls with flood gates for access by people and vehicles, installed earth embankments to contain the river (levees of a type) and also put gabions (baskets filled with stones) to protect the walls and embankments from erosion.
The scheme also incorporates features designed to reduce the visual impact of the walls and to enhance the environment. There are fishing platforms, new street lighting and a comprehensive planting scheme.
English Heritage approved all building materials used so that they were in keeping with the existing architecture of the town.
The picture on the left shows Yarm in flood and those below are 2 views of the new improved defence wall, now somewhat higher! The water tight doors in the wall give people access to the walk along the river in less dangerous times
Improved flood warning systems.
These have better liaison with the Meteorological Office, police and other emergency services.
Finally, new development discouraged building on low-lying and flood-prone land is discouraged - an example of land-use zonation (land is used for activities not damaged by inundation e.g. playing fields, parks, urban forests/walks etc)
From the Northern Echo, first published Friday 29th Nov 2002.
It was designed to withstand once in 100 year floods - but two years after Yarm's £2.1m flood defence scheme was opened it was realized the walls had not been built high enough.
The floods of 1995 turned Yarm High Street into a canal with water up to 18 inches deep. After running out of sand - 40 tons of it, shovelled into 1,500 bags - Stockton Council provided anxious residents with ramparts made from 500 bags of rock salt.
Now a further £2m has been spent on building up the defence walls. The Environment Agency says the original scheme completed in 1993, protected the town from flooding several times.
It says knowledge gained following the 1995 flood showed that the original height of the flood walls gave a lower than desirable standard of protection. The upgrade has given the walls another half metre in height.
Jo Turnbull, chairwoman of the Northumbria Regional Flood Defence Committee, said: "This improved scheme is a major step forward to providing better flood protection for residents and businesses in Yarm.
"Although we can never prevent flooding, the work will reduce the frequency of flooding and members of my committee recognised the need by agreeing to help fund the project.''
Construction work on the original scheme, opened by the town's then Tory MP, Tim Devlin, was not helped by a sub contractor going bust. The hydraulic modelled defences had been designed to withstand severe floods which return every 100 years. The floods which occurred in 1995 - just two years after the scheme was completed - were said to happen only every 50 years. Looped by the River Tees,
Yarm has always been prone to flooding. In the great floods of 1881 boats were actually rowed up and down the High Street.
If you really want up-to-date stuff about the River Tees flood control plans, this is it! I have been plaguing the relevant authorities for info, which has been promised on several websites - I gather that this has not yet been published, but they sent it to me anyway!
Tees CFMP summary.pdf
It is a long document but the section on Middle Tees from page 29 is highly instructive!
If you are fed up with Yarm on the River Tees, this one is a brilliant alternative by Ines
(i) Name the town and the river.
Shanghai and the Huangpu River.
(ii) Explain why building has often taken place on the flood plain in the past.
People used to build on flood plains in the past because they would have easy and direct access to the water for irrigating their crops. Also when the river flooded all of the rich sediment would be deposited on the land next to it and this would make the soil very fertile for growing crops. Flood plains are flat and it is easier to build on flat land. Also being by the river means it’s easy to transport building materials.
(iii) Explain how your chosen town is protected against flooding.
The Huangpu River is in Shanghai, which is a city of 20 million people located by the sea. It is only 3 metres above sea level. From 1960 onwards storm surges caused by typhoons have increased and often cause flooding in Shanghai. Some of the flood prevention measures include building a concrete flood wall, 6.8 metres high, which is designed to protect against a flood with a top height of 6.7 metres for a 1 thousand year period of time. A flood barrier built where the Suzhou Creek empties into the Huangpu River is raised and lowered twice a day according to the tides and weather. Along the main city stretch of the river, called the Bund, a levee of 6.9 metres high was built. There are other levees all along the costal area and these have to be raised every time the sea level rises. More levees are going to be built. The most recent measure is the construction of the Houtan wetlands park Apart from providing a nice walkway by the river it protects against flooding because terraces were created along the river bank to slow the runoff into the Huangpu. Finally the city’s storm drainage system has been upgraded recently and more monitoring sites have been added to keep a closer eye on the water levels
Follow this link to find out the current state of play with regards the depth of water in the River tees at Yarm
Comparison between how HICs and LICs cope with floods
As can be seen from the example on the River Tees, where there is a known problem, some measures can are taken to reduce the impact in HICs. If floods do occur, there are plenty of emergency services to help and water and power are restored as quickly as possible as cost is not an issue. There are well defined plans and exercises are regularly carried out by local authorities and emergency services to test whether their plans do work.
However, in areas such Bangladesh where the Ganges and Brahmaputra join the situation is somewhat different. As you will see under threat is huge, the chance of flood is very high and the funds to put in soft or hard engineering solutions are not there.
Ganges/Brahmaputra drainage Basin
The drainage basin covers an area of 1,664,700 sq km and the combined length of the two major rivers exceeds 3,900 km.
Their average annual combined discharge into the Bay of Bengal is approximately 29,692 m3/sec, with a maximum during flood of 80,984 m3/sec and 6,041 m3/sec during low water periods.
The major floods occur during the months from June through September. The channels of both rivers are extremely unstable and bank lines can migrate as much as 400 m in a single season.
Sediment load is extremely high, with suspended sediment load during flood stage reaching as high as 13 million tons per day.
Most of the land in the alluvial valley is cultivated in rice and jute and population density more than 1,000 persons per km².
one of the world's most densely populated countries!
has a population of 125m inhabitants
is one of the poorest countries in the world with a GNP of $200 per head
has three of the world's most powerful rivers passing through its country - The Ganges, the Meghna & the Brahmaputra
contains virtually no raw materials or rock
experiences floods and tropical storms every year
The Physical Causes of the Floods
Most of the country consists of a huge flood plain and delta
70% of the total area is less than 1 metre above sea level
10% of the land area is made up of Lakes and Rivers
Snowmelt from the Himalayas takes place in late spring & summer
Bangladesh experiences heavy monsoon rains, especially over the highlands
Tropical storms bring heavy rains and coastal flooding
The main cause was the above average & long period of heavy rain which caused all 3 rivers to have their peak flow at the same time!!!
The Human Causes of the Floods
Deforestation in Nepal and the Himalayas increases run off and adds to deposition and flooding downstream
Urbanisation of the flood plain has increased magnitude & frequency of floods the building of dams in India has increased the problem of sedimentation in Bangladesh
Global warming is blamed for sea level rise, increased snow melt & increased rainfall in the region
Poorly maintained embankments (levees) leak & collapse in times of high discharge
Excessive Development: Rapid population growth creates extra pressure on the land of already overcrowded Bangladesh.
Agricultural lands give way to housing developments and roads.
This rapid development and urbanization must have aggravated the flooding problem in Bangladesh
Prior to urbanization there is a greater lag time between intense rainfall and peak stream flow.
After urbanization the lag time is shortened, peak flow is greatly increased, and total run-off is compressed into a shorter time interval – favourable conditions for intense flooding.
The Effects of the 1998 Floods
Over 57% of the land area was flooded
Over 1300 people were killed
7 million homes were destroyed
25 million people were made homeless
There was a serious shortage of drinking water & dry food
Diseases spread such as bronchitis and cholera/diarrhoea
As the waters receded - it left fields of rotting crops, wrecked roads and bridges and destroyed villages
2 million tonnes of rice was destroyed
1/2 million cattle and poultry were lost
Overall the floods cost the country almost $1 billion
Without hammering the point too severely, it is obvious that lack of preparation, paucity of preventive infrastructure and huge problems and little money all make the impacts on LICs far more severe and long lasting that those on HICs.
about the Cumbria floods and see how we respond to emergencies
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