Flooding in an MEDC
Carlisle, Cumbria, 2005
The River Eden runs through North Cumbria and reaches the sea near Carlisle.
- The drainage basin of the River Eden is very large so it catches a large volume of water, leading to a high river discharge.
- Some parts of the basin have steep sides, so water runs quickly down to the river.
- There are many streams that drain quickly into the river, making the lag time short.
On 8th January 2005, the River Eden flooded Carlisle.
The flood return interval (RI) of such a large flood is about 200 years.
1. There was heavy rainfall on the 6th January, for 36 hours. 200mm of rainfall was recorded, which was the equivalent of four months rain.
2. Rain fell on saturated ground so the water didn't soak into the ground but ran straight off into the river.
3. This caused a very high peak discharge (over 1520 cumecs), compared to an average discharge of 52 cumecs.
1. Carlisle is a large built-up area, with impermeable concrete and tarmac surfaces, and little soil or vegetation. This meant there was little infiltration of rainfall and high surface run off, which increased discharge.
2. Drains and sewage systems overflowed in some areas - becoming a source of flooding themselves. 25% of the flooding problems were associated with overflowing drains.
- Three people died in the floods.
- Over 3,000 people were made homeless for up to a year and thousands of personal possessions were damaged. Living in temporary accomodation disrupted lives in many ways, eg. travel arrangements were disrupted, people were separated from community networks and friends, and they had problems receiving post.
- Children lost out on education as four schools were severely flooded. Newman Catholic School didn't re-open until Easter.
- There was an increase in stress related illness following the floods.
- The flooding increased river bank erosion in some areas.
- Rivers were polluted with rubbish and sewage.
- It took about a year to repair the damage to homes and repairs cost over £100 million.
- 350 businesses had to shut down as there was no electricity, telephone service or transport. Trade activities from Carlisle railway station were suspended.
- United Biscuits, the largest employer in Carlisle, was flooded with 3m of water that caused over £5 million damage. 33 out of 1100 employees lost their jobs.
- 70,000 addresses had no power. The sewage works, police station, fire station and council offices were severely flooded.
- 80 buses (most of the public transport fleet) were destroyed. Many roads and bridges were damaged, eg. Warwick Road.
Flooding in an LEDC
South Asia, 2007
Parts of South Asia flood most years, usually in late summer. This is because:
- South Asia has a monsoon climate - 80% of rain falls in just four months.
- Much of South Asia is low-lying land, particularly Bangladesh where 90% of land is less than 10m above sea level.
- Melting snow and ice from the Himalayas in the late summer months increase the Brahmaputra River discharge.
In July and August 2007, the flooding was particularly severe in Bangladesh and India.
1. The monsoon came suddenly after a very dry, early summer.
2. There was heavy rainfall - Assam had a record 169.5mm in 24 hours on 22nd July, and 900mm in total for July.
3. The long duration of heavy rainfall completely saturated the soil, increasing surface runoff and increasing discharge.
4. The peak discharge of the River Gagnes and Brahmaputra coincided, which increased the river discharge downstream.
1. Deforestation in Nepal and the Himalayas meant less rainfall was intercepted, which increased discharge.
2. The growth of urban areas, due to migration, also increased surface runoff.
3. Collapse of old earth dams in Madhya Pradesh, India, caused further flooding.
- Over 2,000 people died. The death toll was high for many reasons, eg. many people were reluctant to evacuate (as they'd have to leave their land and livestock unattended) and many children drowned because they couldn't swim. Poor transport links meant evacuation was slow.
- As wells became polluted with sewage, there was a lack of clean drinking water. Over 100,000 people caught water-borne diseases (eg. dysentery and diarrhoea).
- An estimated 25 million people were made homeless.
- 112,000 houses were destroyed in India, as porous mud bricks became saturated by floodwater.
- Dhaka (Bangladesh's capital) was inundated, especially the poorer districts and shanty towns near the river.
- Children lost out on education as 4,000 schools were affected and 44 schools were totally destroyed.
- The cost of the flood was estimated US $1 billion, including damage to crops and property.
- Factories were closed around Dhaka, due to flood damage and loss of raw materials (eg. rice). Many of the poorest workers became unemployed.
- There was widespread loss of livestock (eg. cattle). Since 80% of Bangladeshis rely on agriculture, many lost their livlihoods.
- 550,000 hectares of land couldn't be planted with rice at peak time, because of flooded fields. A lower rice crop meant the world price of basmati rice rose by 10%.
- 10,000km of roads were destroyed. Landslides blocked roads in the highlands of Nepal and Assam.
- Debt increased, both individually (eg. farmers borrowed money for food and seeds) and nationally (eg. governments imported food and medicine).
- The flood deposited fertile silt on the flood plain.
- Rivers were polluted with sewage.
Hard engineering is used on the Yangtze to control flooding.
- The Yangtze River flows through China. At 6,380km long, it's the third longest river in the world.
- Seasonal flooding is common around the Yangtze - China has a rainy season that lasts from about June until August and the huge increase in river discharge during this time often causes flooding. Flooding causes huge problemsas there's lots of farmland and loads of major cities next to the river, eg. Whuan and Nanjing.
- Five major floods have happened over the last century - in 1931, 1935, 1949, 1954 and 1998.
- The flood of 1954 covered 193,000km squared of land and killed 33,169 people. Over 18 million people had to move. It covered the city of Wuhan for over 3 months.
- The flood in 1998 killed around 3,000 people and made 14 million people homeless.
- Flood protection is mostly done through hard engineering defences.
- There are many dams on the Yangtze that reduce flooding (46 are planned or under construction at the moment). The biggest of these is the Three Gorges Dam:
1. Work began on the 101m high Three Gorges Dam in 1994.
2. A resevoir is building up behind the dam (it takes years to build up as the dam is huge). The resevoir catches any flood water, which can then be slowly released over time. The resevoir can store around 22km cubed of flood water.
3. It's also the largest hydroelectric power station in the world. The flow of water turns 26 turbines built into the dam.
4. Locks have been built alongside the dam so ships can get past it.
- There are many levees along the river, eg. there are 3600km of levees along the middle and lower parts of the river.
Effects of the Three Gorges Dam
- It's thought that the dam has reduced major flooding from once every 10 years, to once every 100 years.
- The turbines in the dam produce a lot of electricity - capacity is likely to reach 22.5 gigawatts (enough to supply about 3% of China's huge demand).
- The reduction in flooding has made it much safer to navigate up the Yangtze. River shipping has also increased as ships can now travel up the river as the resevoir is deeper than the old river.
Effects of the Three Gorges Dam
- People had to relocate as the water level in the resevoir had risen. It's thought that between 1.3 and 2 million people in total will have to relocate by the time it's full - 13 cities and 1352 villages will be submerged.
- The resevoir will also flood farmland, 657 factories, 1,300 sites of cultural and historic interest. For example, as the water rises the Temple of Zhang Fei will be submerged.
- A huge amount of sediment is normally carried down the Yangtze River. The dam will trap the sediment, which could lead to failure of the dam and cause catastrophic flooding.
- The dam could destroy habitats and endanger species, eg. the endangered Siberian Crane spends the winter in wetlands below the dam, which are expected to be affected by less flooding. Fewer than 100 Baiji Dolphins are eft in the Yangtze, and the dam could reduce their food supply.
- The Three Gorges Dam doesn't protect everyone - the rising water levels in the resevoir will increase flooding along the tributaries that lead into it, eg. the Daning River. The increased water levels in the tributaries will also increase erosion of riverbanks, causing collapses and landslides.
Problems from Levees
In the 1998 floods many levees broke, which contributed to devastating flooding. After this, many levees were reinforced. They were effective at reducing flooding in the 2002 floods (though these floods were less severe than the 1998 ones).
Soft engineering is used in Abingdon to control flooding.
- Abingdon, a town in South East England, was built on the flood plains of the River Thames and the River Ock.
- 1500 properties in Abingdon have a 1% chance of flooding in a given year.
- Abingdon has had regular floods over the years - in 1947, 1968, 1977, 1979, 1992, 2000 and 2007.
- Intense storms in July 2007 caused particularly bad flash floods. The River Thames and Ock burst their banks, flooding 660 properties in Abingdon. Increased surface run off in built up areas made the flooding even worse.
- Hard engineering defences have been considered but have been rejected for various reasons, eg. a diversion spillway to transport Ock flood water south of Abingdon was too expensive, and flood barriers to protect properties along the Ock would increase flood risk downstream. Flood protection is mostly done through soft engineering defences.
- Gravel soakaways have been built along the A34 road.
- Low-value land is allowed to flood, eg. Tilsley Park sports ground is being considered as an additional flood storage area.
- There are planning restrictions on new housing developments built on the Ock flood plain, stating they must have improved drainage systems.
- Tesco were forced to revise recent extnsion plans - they had to add drainage improvements such a soakaways and permeable tarmac.
- The Environment Agency's Local Flood Warning Plan warns specific areas at risk and provides a 24 hour Floodline.
- There are restictions on land use, eg. planning permission was refused for buildings on the Thames flood plain.
- There's detailed advice on the internet about reducing flood damage, eg. by raising cupbords and using water-resistant plaster.
- Local voluntary flood wardens communicate advice and warnings.
- Improvements have been made to riparian buffers along smaller rivers. Planting trees reduces the volume of water reaching the Thames and Ock rivers where the flood problem is greater.
Soft engineering reduces damage but floods still happen.
- It's difficult to measure the success of flood defences because it's hard to figure out if any reduction in flooding was because of the success of new defences, or because the weather conditions were less severe.
- Several flood warnings were issued by the Environmental Agency in 2008.
- The Ock flood plain, which has developments on, didn't get flooded. The Thames flood plain did get flooded but it's largely clear of development, due to land use management and planning restrictions.
- The 2008 floods did less damage than in previous years, with minimal cost, little disruption to community services, no lives lost and only a few injuries. However, flooding does still happen in Abingdon.