Floods: Causes, Effects & Climate

Why are floods occurring more frequently? What is a 1,000-year flood? What are the causes and most dangerous effects of flooding? Is climate change responsible? We answer these question and lots more. Learn how heavier rainfall and rising sea levels are endangering coastal towns and floodplains. We also examine flash floods, as well as flood defense and management systems.
Floods in Minnesota, USA.
Floods in Minnesota, USA. Photo: USG. Public Domain

When disaster strikes, it often manifests itself through water. According to the United Nations, extreme weather events such as flooding, storms, heavy rainfall, heatwaves, droughts, landslides and tsunamis are all becoming more frequent and more intense. 1 The impact of floods in particular is expected to increase dramatically in the coming decades (Willner et al., 2018, Winsemius et al., 2016), exacerbated by the impact of climate change on the water cycle (more clouds and precipitation) and rising sea levels (higher storm surges).

What Are Floods?

A flood is a natural process where an area of land which is usually dry, becomes submerged in water. It can occur from a range of sources at any time impacting people, property, infrastructure, economies and the environment. Some floods occur suddenly, and recede just as quickly. Others take days or even months to recede and for the land to dry out.

The study of floods is a discipline of hydrology.

Floods are a natural part of the water cycle, and humans have always had to contend with them. But in recent decades, the nature of disastrous floods has changed, with flash floods, acute riverine and coastal flooding increasing in frequency.

According to a report by the United Nations, flooding accounted for 47 percent of all weather-related disasters between 1995 and 2015. It affected 2.3 billion people, the vast majority of whom (95 percent) live in Asia. 2 Asia is particularly prone to flooding due it’s large and varied landmass – including lots of floodplains, deltas and river basins – which is home to big population densities.

The report also showed that the total number of weather related disasters had doubled since the period 1985-1994. While scientists cannot calculate what percentage of this rise is down to climate change, it does signify extreme weather events are becoming much more frequent and we can expect worse flooding in the decades ahead.

To compound the problem, the continued population growth on planet Earth will put more and more people in harm’s way, while uncontrolled building on storm-prone coastal areas and floodplains will increase people’s vulnerability.

While less developed nations are most at risk, Western nations are also suffering. In the United States, as global warming continues to exacerbate sea level rise and extreme weather, it is expected that floodplains will grow by about 45 percent by 2100. 3

What Are Floodplains?

Floodplains are fields or low-lying areas of land which are prone to flooding. For example, the lands next to rivers, which are only covered by water during floods, are floodplains.

In their natural condition floodplains are an important part of a river ecosystem: they filter and store water and help sustain high biological diversity.

Did You Know?

The hydrosphere is the water (H2O) component of Planet Earth. It includes: (1) All water vapor in the atmosphere. (2) All liquid freshwater and seawater, in streams and rivers and oceans. (3) All ice and snow in the cryosphere, held in ice sheets, ice caps, glaciers and in frozen ground called permafrost.

What Is A 1,000-Year Flood?

A 1,000-year flood is a statistical way of judging the magnitude of a flood. It means that a flood has a 1 in 1,000 chance of occurring in any given year.

It is possible for a 1,000-year flood to occur more than once in a thousand years. The 1,000-year recurrence interval means that a flood of that magnitude has a 0.1 percent chance of happening every year. In other words, each year begins with the same 0.1 percent statistical likelihood. For example, Houston, Texas suffered two 1,000-year floods within the space of 2 years, thanks to hurricanes Harvey and Imelda. 4

Another common question: is it possible to have a 1,000-year rainfall, that does not result in a 1,000 year flood?

Yes, is the answer. It comes down to a number of factors such as how dry the ground is. If the ground is already saturated then much of the water will gush into streams, causing floods. If the ground is dry, it will soak up more of the rain, so the runoff will be less severe.

Note: A flood can also be referred to as a 100-year flood, or a 50-year flood.

Types of Floods

There are three major types of floods: (1) Flash floods (2) Rapid onset floods and (3) Slow onset floods.

Floods in Baton Rouge caused by strong El Niño events.
Rapid onset floods in Baton Rouge, 2016. A slow moving system dumped more than 20 inches of rain over 3 days. It was deemed a 500-year flood and caused catastrophic damage. Image: Flickr/U.S. Coast Guard Petty Officer 1st Class Melissa Leake. CC BY 2.0

Flash Floods

This kind of flood begins within 6 hours of heavy rainfall, although typically it occurs within 3 hours or even sooner. Flash floods are caused by heavy rainfall from slow moving thunderstorms, but can also occur due to a mudslide or when a dam or embankment breeches. Flash floods are the most dangerous type of flood because people are taken by surprise and do not have time to prepare.

Rapid Onset Floods

Rapid onset floods are similar to flash floods – they just take slightly longer to develop. People have a little more warning to evacuate or take action. This type of flood lasts a maximum of two days.

Slow Onset Floods

Slow onset flooding develops slowly and can last days or weeks. Typically, it occurs when a river breaks its banks. The flooding tends to occur in floodplains, fields and low-lying areas prone to flooding, and can spread over many miles.

This type of flood is the most likely to cause disease and malnutrition in people. For example, in rural India, children raised in households exposed to recurrent flooding were found to be more underweight and stunted than those living in non-flooded villages.

Also, children exposed to floods in their first year of life suffered the highest levels of chronic malnutrition due to interrupted food supplies. 5

Different Types of Flooded Areas

River Flooding

River flooding occurs when a stream or river overflows its natural banks and saturates normally dry land. It can result from heavy rainfall or rapidly melting snow. According to one study, nearly 41 million Americans are at risk of river flooding, which is nearly three times previous estimates by FEMA. 6

Coastal Flooding

Coastal flooding can occur because of tides or storm surges. However as sea levels are rising, the impact of tidal or wave surges is worsening.

Rising sea levels is a well-accepted outcome of climate change. In fact, it is possibly one of the worst possible effects of global warming on the oceans, because it can make entire cities and low-lying coastal communities uninhabitable.

Much of the focus has been on how much levels are rising, but the biggest danger to life comes from storm surges.

During hurricanes and other powerful storms, strong winds can push a wall of water from the ocean onto land. This advancing surge combines with the normal tides and can increase the water level by 30 feet or more. 7

The double whammy: Higher sea levels combined with storm surges means flood heights will increase. This means that weaker storms in the future will be able to produce the equivalent flooding of a more powerful storm today.

Urban Flooding

Urban flooding is caused by excess water runoff, because the water has nowhere to go. While urban flooding can be connected to storms and hurricanes, more routinely it occurs because rainfall overwhelms the capacity of urban drainage systems. Green infrastructure seen in ‘sponge cities‘ in The Netherlands, Germany and China, can help absorb rainfall, preventing water from overwhelming pipe networks and pooling in concrete squares, streets or basements.

As climate-related flooding intensifies, green infrastructure is bound to become a key element in climate change adaptation policies around the world.

Additional Terms To Know

Fluvial Flooding: Flooding from streams and rivers. Also known as riverine flooding.
Pluvial Flooding: Flooding from intense rainfall events.
Groundwater Flooding: Typically, turloughs which is a type of disappearing lake found mostly in Ireland.
Blocked Culverts: Culverts are small channels used to divert or drain water from the land above it.

How Does Climate Change Cause Floods?

It is not always possible to establish a clear link between global warming and extreme weather events. (See, for instance: Tornadoes.) However, according to the Director of the United Nations Environment Programme’s (UNEP), “it is clear that we need to be prepared to face more intense and more frequent extreme hydro-meteorological events due to climate change.” 8

Few episodes of flooding are direct consequences of climate change. But since a warming planet boosts the amount of water vapor in the air, and causes sea levels to rise, and warms the ocean – thus fuelling hurricanes and cyclones – its indirect effect on floods is huge.

Rising Temperatures Mean More Rainfall

Rising temperatures on Earth generally cause wet regions to become wetter and dry regions to become drier. 9

So how does temperature affect rainfall? As air warms it can hold more moisture – 7 percent more moisture for every 1 degree Celsius rise in temperature. So, if the planet heats up by 4 degrees Celsius, which it may do by 2100, then there will be around 28 percent more water vapor in the atmosphere. That’s a lot of rainfall!

But rain doesn’t fall evenly across the planet, some areas get more and others less. In general, if you live in a place where rainfall is traditionally high (like Ireland), it will rain more and there will be higher risk of flooding. If you live in a drier region, it will become even drier and there may be higher risk of drought and water stress.

Rising Sea Levels

Sea level rise is due entirely to man-made global warming. Scenarios examined by experts from the U.S. National Oceanic and Atmospheric Administration (NOAA) suggest that a rise of 200-270 cm (6.5 to 9 feet) this century, is plausible. 10

As we saw, an even greater concern is the combination of higher sea level and storm surge. Not least because about 600 million people live in low elevation coastal zones, generating about US$1 trillion of global wealth. So, the environmental and socio-economic impacts associated with increased coastal flooding can be massive. 11

More Frequent Storms and Hurricanes

Climate change is also increasing the frequency of hurricanes, and this trend is expected to continue. Hurricane Harvey in 2017, a category 4 storm, was America’s wettest storm in nearly 70 years. It moved slowly – a consequence of weakened atmospheric currents from a warmer atmosphere – and as a consequence dumped more rain. 12

What Causes Flooding?

In addition to higher sea levels, storm surges and excess rainfall, floods are also caused by:

Shifting Weather Patterns

Every 5 years or so, a change in the winds in the equatorial Pacific causes a shift to cooler than normal ocean temperatures – known as La Niña, or warmer than normal – known as El Niño. The cool and warm phases, of this weather system, together known as the El Niño Southern Oscillation (ENSO), affect rainfall patterns worldwide.

One comprehensive study showed that the world experiences more rainfall in La Niña years than in El Niño years. For example, parts of Australia see flood risk increase by more than 50 per cent in La Niña years. 13

The Indian Ocean Dipole (IOD), sometimes called the Indian Niño, is a similar system that affects the westerly and easterly edges of the Indian Ocean. A ‘positive’ dipole is associated with higher rainfall and floods in East Africa and droughts in south-east Asia and Australia. A ‘negative’ dipole has the opposite effect.

The IOD and ENSO are two of several naturally-occurring weather cycles that play an important role in Earth’s climate system around the equator and in the subtropical southern hemisphere. Others include the travelling Madden-Julian Oscillation in the Indo-Pacific, and the Southern Annular Mode in the Southern Ocean.

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Human Changes To The Environment

This is the Anthropocene Epoch, a time where the biosphere is dominated by human behavior. Our actions have resulted in the extinction of whole species, and have upset the natural rhythm of our environment.

Changes in land-use, such as the removal of wetlands, peat and bog lands have increased flooding in some locations. So too has the removal of vegetation through deforestation, and alteration to waterway courses with levees (dyke, embankment and floodbanks).

It is worth mentioning here – that within river systems, flooding is the natural way for the system to discharge the water arising from occasional large rainfall events. There is no problem until people decide to live on these natural flood plains and then realize that protection against inundation is needed.

River Deltas

A river delta is where a river mouth enters a body of water such as an ocean or lake. The biggest deltas in the world include the Mississippi, Amazon, Nile, Amazon, Ganges, Yangtze, Yellow River and Indus.

One study found that river deltas occupy half a percent of Planet Earth’s land surface, yet are home to 339 million people – 4.5 percent of the global population. 14

In 2007 the Intergovernmental Panel on Climate Change report concluded that many river deltas areas are at risk from sea level rise. Other studies show that human activities are even causing deltas to sink due to the construction of upstream reservoirs and dams. 15

Ruptured Dams

Ruptured dams or levees can cause sudden flooding. Dams are man-made constructions primarily used for hydropower or electricity generation. Sometimes too much water is held up in the dam, causing it to rupture or spill over.

For example, in 2018, following a period of heavy rain, the Xe-Pian Xe-Namnoy hydroelectric power plant in Laos collapsed, sending a wall of water into neighboring villages. At least 49 people were killed, and 7,000 more lost their homes.

In 2006, after 40 days of heavy rainfall, rising water finally overflowed Hawaii’s Ka Loko Dam. It sent 400 million gallons of water downstream, reaching 20 feet in height, destroying whatever was in its path, including trees, homes and vehicles.

Ice and Snowmelts

In colder climates, melting snow runoff into streams and river is a part of the water cycle and a major component of the global movement of water. Snowpacks, or mountain snow fields, act as natural reservoirs in the western United States, storing precipitation until the warmer season when it begins to melt. As much as 75 percent of water supplies in those states come from snowmelt. 16 Excess snowmelt, has a potential to cause flooding and rapid snowmelt can also trigger mud and landslides. In alpine regions like Switzerland, this is a particular concern.

What Are The Effects Of Flooding?

Health Impacts

Flood impacts people and communities directly by causing physical injury, illness and stress. Flooding brings contamination and disease. For example, floodwaters can carry raw sewage, leaked toxic chemicals, and runoff from hazardous waste sites. Drinking water can be polluted, causing eye, ear, skin and gastrointestinal infections. When the waters recede mold and bacteria may remain, increasing the rates of respiratory illnesses like asthma. Flooding can also trigger severe stress and even mental health problems, due to the loss of a home, job or business.

What To Do After A Flood: Health Guidelines
What to do after a flood, health guidelines. Image: World Health Organization (WHO).

Economic Cost

The cost of flooding has been escalating rapidly, due in part to human-caused climate change. A recent study from Stanford University found that of the $199 billion in U.S. flood damage from 1988 to 2017, more than one-third is directly related to intensifying precipitation. 17

In 2020 storms and floods cost the world billions. Six of the ten most costly events took place in Asia, five of them associated with an unusually rainy monsoon.

Cyclone Amphan, which struck the Bay of Bengal caused $13 billion in damages. Floods in China cost $32 billion and in India $10 billion.

In Europe, Storm Ciara which hit the UK and Ireland cost $2.7 billion.

And in Africa, huge locust swarms ravaged crops across several countries, causing an estimated $8.5 billion in damages. The outbreak has been linked to wetter conditions brought about by unusual rains, fueled by climate change. 18

Damage To Infrastructure

Floods can damage roads, railway networks and shipping ports, with significant impacts on regional and national economies.

Agriculture

Flooding damages crops and causes loss of livestock. Crop losses can be aggravated by delays in harvesting due to waterlogged soils.

Environmental Damage

Floods and storm surges can cause significant stress and damage to important marine ecosystems like coral reefs, as well as coastal ecosystems that harbor valuable blue carbon reserves in seagrass meadows, kelp forests and mangrove swamps.

Types of Flood Defenses

Many of the impacts of flood are preventable since flooding – unlike most types of weather-related disasters – lends itself to primary prevention through affordable technologies such as dams and dykes. For this reason, flood defenses are top of the list on most countries climate change adaptation plan.

Dams and Reservoirs

Dams are barriers, usually constructed across rivers, to hold back and contain water in a lake or reservoir. Dams help prevent flooding by storing all or a portion of flood waters in the reservoir, and then releasing it slowly later.

Typically, dams have a multiple of purposes and flood management may be required only for a few days or weeks in any particular year. Dams are also used for hydroelectricity generation, irrigation and drinking water. Examples include the hydropower Hoover Dam in the United States and the Three Gorges Dam in China.

Diversion canals

These are man-made channels which are used to divert water along a specified route. They are also used for river transport and recreation. An aqueduct is another type of diversion canal. Water flow into diversion channels is regulated by gates.

Floodplains and groundwater replenishment

Floodplains provide a range of important ecosystem services: flood control, water and nutrient retention, carbon sequestration and diversity of wetland and aquatic habitats. Floodplains are under pressure, and for this reason the European Union has been driving a number of restoration projects through their LIFE+ programme.

River Defenses

River defenses include levees, bunds and weirs:

Levees (also called embankments and dykes in some countries) are constructed mainly from earth and used to confine stream flow within the stream, or to prevent flooding due to sea waves or tides.

Bunds are a form of enclosure that can provide emergency containment to stop hazardous materials such as flammable or toxic liquids leaking onto the ground or into surface water.

Weirs, also called a low head dam, is a barrier across the width of a river that changes the height of the river level. A weir is also used to control the flow of water from lakes, ponds and reservoirs.

Coastal Defenses

These include groynes, revetments, sea walls and gabions.

Groynes are walls of concrete, stone or timber that extend out from beaches, acting as barriers.

Sea walls are structures that are usually incorporated into a promenades, and are built to limit erosion caused by wave attack and wave overtopping.

Revetments are onshore sloped structures used to limit the energy of the waves as they break and so reduce their erosive power.

Gabions are steel mesh cages that are filled with rocks and concrete and sometimes aggregate, and used to stabilize vulnerable areas.

Retention Ponds

Retention ponds are ponds with the capacity to store extra surface runoff during rainfall events. They consist of a permanent pond area with landscaped banks and surroundings. They can also help with protecting biodiversity and aquatic habitats and filtration of pollutants.

Movable Gates and Barriers

The Thames Barrier is one of the largest movable flood barriers in the world. It is a retractable barrier system that is designed to prevent the floodplain of Greater London from being flooded by exceptionally high tides and storm surges moving up from the North Sea.

The Thames Barrier has been closed 195 times since it became operational in 1982 (correct as of January 2021). Of these closures, 107 were to protect against tidal flooding and 88 were to protect against combined tidal/fluvial flooding.

Temporary Flood Defenses

These are defences that can be brought to specific problem areas as required. They include sand bags, plastic or metal barriers and pumps.

The Shift From Flood Control To Flood Management

As the Earth is becoming increasingly over-populated, this has forced more people to move closer to flood prone areas. This increases flood risks and costs.

Flood management has largely relied on building structural solutions, like dams, reservoirs and embankments, to protect socio-economic activities in flood plains. But this has come at a cost, resulting in loss of environmental habitats and biological diversity. Furthermore, the physical presence of permanent flood structures has the potential to create a false sense of security, leading to inappropriate land use in the protected areas.

The need for sustainable development has highlighted the negative consequences of flood defenses, and is leading to a shift from flood control to flood management.

Flood management essentially encompasses non-structural measures for reducing flood risk. Non-structural measures play an important role in not only reducing the consequences of flood, but also on mitigating the impact on the environment.

Flood management methods include:

Better Flood Forecasting and Warning

Flood forecasting systems provide a basis for early warnings. The goal is to create an accurate forecasting system, based on local conditions, so that flood warnings can be given in a timelier manner. This gives authorities extra time to take preventive and emergency measures.

One response may be the opening and closing of gates from various flood management systems, or the anticipatory release of water reservoirs to increase storage capacity. People at risk may receive flood warnings by text message, or sirens and bells can go off in the town or village.

Improved flood forecasting now regularly leads to warnings which have generated significant savings.

Emergency Preparedness, Response and Recovery

This is perhaps the most important element in flood management. Communities at risk of flooding should be regularly reminded of the dangers and understand their role in responding to emergency situations. This ensures better coordinated evacuation, and helps maintain healthy and hygienic conditions after floods. For example, in the devastation after floods, little attention may be paid to dumping of rubbish. That rubbish can end up in waterways or water supplies impacting wildlife and human life.

More Land Use Regulations

Ideally authorities would restrict occupation of flood plains for socio-economic development. But this is not always possible. But the introduction of land use regulations can help. For example, by introducing a tree planting scheme, encouraging the growth of certain crops, or introducing minor engineering works like contour bunds, check dams and trenches.

Living With Floods

In Asia, there is an age-old holistic practice of living with floods. It may be seen by some as negative or fatalistic thinking, but indigenous inhabitants understand that is not possible to eliminate all risk of flood, and that floods can even provide benefits to local ecosystems. This forms the basis of flood ‘management’ traditionally in many villages in Bihar, West Bengal and Bangladesh.

Better Water Storage Management

Storage of water in reservoirs alters the water quality. In reservoirs, anaerobic producing algal plants tend to dominate, which produce methane, a potent type of greenhouse gas. If there is an excess of nutrients in the water, such as nitrogen and phosphorus, then eutrophication occurs. This leads to less oxygen in the water to support aquatic life. A better design and operation of existing and proposed dams could keep the environmental impacts to a minimum.

Further Reading & Resources

The Associated Programme on Flood Management (APFM): A joint initiative of the World Meteorological Organization (WMO) and the Global Water Partnership (GWP), promotes the concept of Integrated Flood Management (IFM).

The growth mode of built-up land in floodplains and its impacts on flood vulnerability. Science of The Total Environment, Volume 700. Yapan Han et al. 2020.

A Data-Driven Probabilistic Rainfall-Inundation Model for Flash-Flood Warnings. This study develops a data-driven, probabilistic rainfall-inundation model for flash-flood warnings. Applying a modified support vector machine (SVM) to limited flood information, a model which is superior to the Boolean functions of the traditional rainfall-flood threshold method. Tsung-Yi Pan et al. November 2019

Climate change attribution and the economic costs of extreme weather events. A study on damages from extreme rainfall and drought. This paper attempts to quantify recent costs related to extreme weather due to human interference in the climate system, focusing on economic costs arising from droughts and floods in New Zealand during the decade 2007–2017. D.J., Rosier, S.M., Noy, I. et al.

References

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  4. “Tropical Storm Imelda Brings Houston Area A Second 1,000-Year Flood In Just Two Years” Forbes 2019. []
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  6. “Estimates of present and future flood risk in the conterminous United States.” Oliver E J Wing et al. 2018. []
  7. “Storm Surges” National Ocean Service (NOAA) []
  8. “How climate change is making record-breaking floods the new normal.” UN Environment Programme. 2020. []
  9. “Drought and Climate Change.” Center for Climate and Energy Solutions. []
  10. Global and Regional Sea Level Rise Scenarios for the United States. NOAA Technical Report NOS CO-OPS 083. National Oceanic and Atmospheric Administration. January 2017. []
  11. “Projections of global-scale extreme sea levels and resulting episodic coastal flooding over the 21st Century.” Sci Rep 10, 11629 (2020). Kirezci, E., Young, I.R., Ranasinghe, R. et al. []
  12. “Did Climate Change “Cause” Harvey?” OnEarth. []
  13. “New study maps countries most at risk from El Niño flooding.” Oct 2014 []
  14. “Coastal flooding will disproportionately impact residents of river deltas, study finds”. Oceanographic Magazine []
  15. “World’s River Deltas Sinking Due To Human Activity, Says New Study.” University of Colorado at Boulder. 2009 []
  16. “Snowmelt Runoff and the Water Cycle.” USGS Science for a Changing World. []
  17. “Contribution of historical precipitation change to US flood damages.” Frances V. Davenport et al. January 2021. []
  18. “Counting the cost 2020: A year of climate breakdown.” December 2020. Christian Aid. []
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