Environment
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Accessed 19 Mar. 2024.
Flooding 2023. Geography, Environment. Retrieved 19 March 2024, from
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Geography, Environment, s.v. "Flooding," accessed March 19, 2024.
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Flooding
Flooding occurs whenever an area that is usually dry land becomes submerged under water. Some floods happen suddenly and recede quickly. Others take days or even months to build and recede. When floods happen in an area where many people live, they can be devastating. The sheer volume of water is sometimes enough to carry away cars, trees, houses, bridges—even people. Heavy rain for a short period or light rain for many days and weeks can both result in floods. Rivers may overflow their banks when there is more water upstream than usual. Floods are often the result of catastrophic events such as a storm surge, tsunami, hurricane or a collapsing dam.
Conditions for flooding
Floods happen on flat or low-lying areas when rainwater falls, or snow melts more rapidly than the water can either infiltrate (soak in) or run off (flow across) the ground. This may happen if the underlying rock is impermeable—lacking pores, tiny holes that allow water through. It may also occur if the soil is frozen solid, already saturated with water (particularly where the water table is already high) and can absorb no more, or is so hard and dry that it acts like impermeable rock.
In the countryside, areas that have been cleared of woodland or forest are also more likely to flood. This is because trees provide a natural "umbrella", slowing the rate at which water reaches the ground and thus giving it more time to infiltrate.
The risk of flood is much higher in towns and cities because they have many more impermeable surfaces: concrete, tarmac, paving or roof tiles. Drains and sewers take water directly to the rivers running through the town or city, making them more likely to flood.
A steep-sided valley may act as a funnel, channelling large volumes of water rapidly down a narrow cleft in high ground. Villages or towns located in such valleys can be overwhelmed by such flooding, as happened in Boscastle, Cornwall, following eight hours of heavy rainfall on 16th August 2004.
River floods
A river floods when water flows over its banks and spreads out across the surrounding land. Floods occur in all types of river channels, from the smallest streams to the world's largest rivers. This is a natural process: the land to either side of the river channel is called a floodplain. It stretches from the riverbanks to the base of the enclosing valley slopes. When a river breaks its banks, sediments (sand, silt and clay) in its waters are deposited on the floodplain, forming alluvium. Layers of alluvium gradually build up to create an alluvial plain.
Despite the likelihood of frequent flooding, many villages, towns and cities have been built on floodplains. This is because of the rich fertility of floodplain land for farming; it is easier to build on flat land than on slopes; water sources are easily available; convenient river, road and railway links across level land allow easy passage through upland terrain.
Some of the worst natural disasters in history—of any kind—measured by numbers of deaths have been river floods, most severely on the Huang He (Yellow River) in China. Past floods have caused horrific death counts: up to 3.7 million people in 1931, up to 2 miilion in 1887 and 700,000 in 1938. Such devastating floods have earned the river the nickname: "China's Sorrow".
When Cyclone Idai made landfall near the Mozambique port city of Beira on 15th March 2019, the torrential rains it brought with it resulted in catastrophic floods. The Buzi and Pungwe rivers in central Mozambique overtopped their banks, causing whole villages to disappear beneath their surging waters. In some places the water was 6 metres (20 feet) deep. More than 1300 people died, with many more missing. The disaster greatly affected the lives of more than 3 million people in southeastern Africa.
Levees
When rivers that carry large quantities of fine sediments (silt, sand and mud) suspended in their waters flood, most of these sediments tend to settle close to the banks either side of the channel. If the river floods frequently, the sediments build up into rounded ridges, rising higher than the floodplain beyond. These ridges are called levees. Where sediments also build up on the river bed, the river channel may eventually rise above the surrounding floodplain, penned in only by the levees running either side of it.
Artificial levees—manmade ridges made of rubble and earth—are often built along the banks of some rivers, both to prevent the surrounding land being flooded and to allow the river to be more easily navigable by boats and ships. Artificial levees are also called dykes or embankments. Examples of rivers that have artificial levees include the Mississippi and Sacramento in the United States, and the Po, Rhine, Meuse, Rhône, Loire and Vistula in Europe.
Slow-rising floods
Slow-rising floods occur as a result of rivers flooding over a period of days or weeks. These types of flood most commonly occur on rivers with large catchment (drainage) basins. The increase in flow may be due to continuous rainfall over a long period, or melting snow in mountain streams. Sea levels rising due to higher-than-normal tides also cause slow-rising floods on low-lying coasts. While such slow flooding rarely causes immediate loss of life, it can still damage buildings and roads and spread water-borne diseases. In tropical or subtropical areas, slow-rising floodwaters greatly increase the risk of snake bites, as snakes are carried into villages and towns.
Flash floods
Rapid flooding events, often called flash floods, occur within a very short time—hours or even minutes. They are usually the result of heavy rain, typically during a thunderstorm or hurricane, a collapsing dam or rapid snow melt. Flash floods are the most destructive kind of flood and can be fatal, since people are usually taken by surprise.
Flash floods usually occur on smaller streams—often those with steep-sided valleys—or rivers that flow for much of their length over impermeable ground. Flash floods commonly occur in normally-dry channels found in deserts or semi-deserts. These are known as wadis in North Africa and Western Asia, or arroyos in the southwestern United States. People could be swept away in their vehicles if they tried to cross this terrain during a flash flood.
In spring 2019, flash flooding affected large areas of Iran, hit by heavy rains. Normally dry river beds became raging torrents, killing at least 70 people, many caught driving on instantly flooded roads. About 1900 cities and villages along with more than a third of Iran's road network were damaged.
Dam and levee failure
Flooding is especially dangerous when triggered by a collapsing dam or broken levee. Vast amounts of water contained behind the dam or levee are suddenly released, surging across adjoining land and devastating anything in their path—often within minutes, or even seconds, of the collapse. When a dam at an iron ore mine near Brumadinho, Minas Gerais, Brazil, collapsed on 25th January 2019, the resulting mudflow killed at least 248 people.
After Hurricane Katrina hit Louisiana in 2005, the breaching of the Mississippi River Gulf Outlet levees surrounding New Orleans—specially built to protect the city from flooding—made the flooding far worse than it would have been otherwise.
Storm surges
In some parts of the world, low-lying coastal plains—particularly if they are also in regions where hurricanes or monsoons occur—are vulnerable to flooding. This is because high winds over the sea may cause sea levels to rise and sea water to spread inland. Known as storm surges, these can cause extreme flooding in coastal areas, particularly when they coincide with normal high tide. Storm tides, as these are called, can reach up to several metres high. Flooding in rivers and estuaries may be particularly severe if storm surges collide with channels already heavily swollen with rainwater or snowmelt.
Tsunamis, caused by earthquakes under the seabed (sometimes hundreds of kilometres away), may also result in severe coastal flooding. A series of powerful waves surge ashore at high speed, inundating low-lying areas.
Global warming
A rise in sea levels across the world—a consequence of global warming leading to the melting polar ice sheets—threatens to flood coastal areas, where many of the world's largest cities are located, permanently. Coastal flooding from storm surges will become more frequent as sea levels rise. This could be as much as 2.4 metres (8 feet) by 2100 if, as many scientists predict, the Antarctic ice shelves break up, leading to loss of ice from the West Antarctica ice sheet. The climate emergency is also likely to increase in the intensity and frequency of storms and hurricanes, which in turn make river flooding more frequent and storm surges more powerful.
Effects of flooding
Rapid flooding may lead to loss of life as well as damage to buildings and other structures, including bridges, roads and canals, making it difficult to get aid or health treatment to those affected. Floods—both rapid and slow—may also damage power transmission, power stations, water treatment plants (causing loss of drinking water and water contamination) and sewage disposal facilities. In some countries, lack of clean water combined with human sewage in the flood waters raises the risk of waterborne diseases such as typhoid and cholera.
Floodwaters often inundate farmland, making the land unworkable and preventing crops from being planted or harvested. This can lead to shortages of food, both for humans and farm animals.
Sometimes floodwater carries industrial chemicals or mining waste and pollutes the ground it flows over. In October 2010, a dam collapse near Kolontár in Hungary caused major flooding. The dam contained waste from a nearby bauxite factory. A river of toxic red mud flowed across a wide area, killing fish and other wildlife and contaminating the topsoil in farmland.
Bangladesh
Bangladesh is frequently affected by flooding. After a tropical cyclone forced a storm surge inland, flooding took the lives of 140,000 people in 2001. About 9 million were made homeless and 1000 died from drowning or waterborne diseases in the 2007 floods.
Bangladesh is a low-lying country crossed by wide rivers flowing into the Bay of Bengal. It is highly vulnerable to tropical cyclones. At certain times of the year, the rivers are swollen by heavy monsoon rains and meltwater from the Himalayas. Water levels are even higher as a result of both accelerating deforestation in highland valleys upstream and glaciers melting more rapidly than ever before due to global warming.
Flood prevention
We cannot stop heavy rains falling, sea water surging inland or rivers from bursting their banks. In an emergency, a wall of sand bags can hold back floodwaters for a short time, but only larger-scale construction projects can prevent floodwaters from causing harm or damage in the longer term.
Dredging makes a river deeper so that it can hold more water (this may reduce the flood risk in built-up areas, but it can increase the risk downstream). Flood relief channels, or spillways, take floodwater from a built-up area to a place where it can be absorbed or re-enter the river further downstream. Retention basins can hold extra water during times of flooding: the water drains into the basin first, diverting it away from homes and so giving people more time to evacuate. Levees, embankments built along a river's banks, can prevent river water from overflowing (although flooding could be more severe if the levees are breached).
The Thames Barrier, completed in 1982, prevents London from being flooded when there are exceptionally high tides and storm surges moving in from the North Sea. Its rotating barrier gates can be closed during these times. At low tide the gates are opened to restore the River Thames' flow to the sea.
Benefits of flooding
Floods—when they are not too severe—can also bring benefits. When a river is in flood, it spreads fertile muds across its floodplain, which increases nutrients in the soil. Floodwaters also provide much-needed water in arid and semi-arid regions where annual rainfall is low or irregular. It may also drown pests in the farmland crops or soil.
Periodic flooding was essential to the survival of farming communities in ancient civilizations that grew up along major rivers, including the Tigris-Euphrates in the Middle East, the Nile in Egypt, the Indus and Ganges in South Asia and the Yellow River in China.
Amazon flooding
Floodplain forests make up between 3 and 4% of the Amazon Basin land area. When rivers flood them during the rainy season, richly fertile silt brought down from the Andes Mountains is spread across them. Known as the varzea, flooded forests extend about 20 kilometres (12 miles) from the river banks. Waters may rise between 7.5 and 15 metres (25–50 feet). Aquatic wildlife, including fish, otters, capybaras, caimans and anacondas, swim among the tree trunks during the floods.
Consultant: Ian Fairchild