Controlling floods in Urban areas Back to the Tees!

2 What happens when there is flooding? Flooding greatly increases the river’s energy so it can do more work. The deeper and faster flowing river can carry more load – see the tree trunk about a month ago in the River Wye! Most rivers turn brown because of the large amount of sediment carried in suspension. What does that mean?

3 What happens when there is flooding? The amount of erosion carried out by hydraulic action and abrasion is greatly increased. How does this happen? In the lowlands many features are formed during flooding:  Levees  Flood plain deposition  Ox-bow lakes

4 Damage caused by flooding Short term Short term Loss of life Destruction of property Crop damage Loss of communication Fresh water pollution Loss of power

5 Damage caused by flooding Long term Long term Replacing what is lost or damaged In MEDCs insurance covers much of it – but not all as in the case of Hurricane Katrina, many poorer people could not afford insurance. Governments have the funds to rebuild the infrastructure- e.g. roads, water treatment etc In LEDCs 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

6 Causes of Floods A Flood can occur when a river exceeds its bank full stage and water will subsequently inundate the adjacent surrounding area. Heavy rainfall - long periods of heavy rainfall will lead to an increase in surface runoff and increase in river level. Snow melt - water in storage is often freed by Spring melts increasing surface runoff. Deforestation - cutting down of trees leads to a reduction in interception rates and an increase in surface runoff. This may also lead to rapid erosion rates due to a lack of stability in the soil subsurface. Urbanisation - tarmac and concrete surfaces are impermeable and lead to an increase in surface runoff. 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. For example, in a city that is totally served by storm drains and where 60% of the land surface is covered by roads and buildings, floods are almost six times more numerous than before urbanization

7 When we talked about the hydrograph – what other issues may mean that rivers are more liable to flood? – besides these? Heavy rainfall? Snow melt? Urbanisation De-forestation

8 Flood Control and Management There are a number of ways managing floods: 1. Afforestation - planting trees increases interception rates and reduces surface runoff. 2. Dams and Reservoirs - these hold back and regulate the flow of river water. Can be used as fresh water supply and generation of HEP. 3. Diversion Channels and basin - overflow channels which take surplus water out of a river in times of flood. 4. Channel Straitening and Dredging - smoothens the channel to increase the speed (velocity) of the river and get water out of the drainage basin as quickly as possible.

9 Flood Control and Management There are a number of ways managing floods: 5. Artificial Levees - makes river banks higher therefore holding more water. 6. Culverts - semi circular, smooth channels increase velocity and gets water away from urban areas as quickly as possible. 7. Revetments, Channel Walls, gabions - strengthen river banks from erosion using large lumps of stone – see over 8. Restricted use of flood-plains - legislation, higher selective insurance premiums/refusal to insure particular locations. 9.Co-ordinated flood warning and emergence reaction procedures e.g. Environment Agency Flood watch

10 Revetments, Gabions and Culverts

11 River Tees - Management The River Tees has a long history of flooding. The first documented flood was at Croft on the lower Tees in Many of the previous floods were precipitated by snow melt. However, 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.

12 River Tees - Management 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.

13 Lower Tees Valley- Management 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.

14 Lower Tees Valley- Management Dredging 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.

15 Lower Tees Valley- Management 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.

16 Lower Tees Valley- Management Yarm's flood defence scheme 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 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.  earth embankments to contain the river.  gabions (baskets filled with stones) to protect the walls and embankments from erosion.

17 Lower Tees Valley- Management 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.

18 Lower Tees Valley- Management 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)

Ganges/Brahmaputra drainage Basin

20 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 m 3 /sec, with a maximum during flood of 80,984 m 3 /sec and 6,041 m 3 /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

21 Bangladesh is … is 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

22 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!!!

23 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

24 The Human Causes of the Floods 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.

25 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 bronchitus 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