Flooding 101: Everything You Need to Know

Quick Key Facts

• Flooding occurs when water submerges land that is typically dry.
• There was a flood every eight of 10 days in the U.S. between 2000 and 2021, on average.
• Attempting to drive or walk in floodwaters are the No. 1 and 2 leading causes of flood-related drownings in the U.S.
• Flash flooding is the most dangerous kind of flooding. It is usually triggered by heavy rainfall in around six hours.
• Just six inches of fast-moving flood water can knock adults off their feet and 12 inches can sweep away cars.
• Flooding killed 7,398 people in 2022. 
• The world’s large dams store a combined 1,679 to 1,991 cubic feet of water.
• Flood risk in the U.S. is expected to increase by more than 25% by 2050, disproportionately impacting Black communities. 
• The atmosphere can hold about 7 percent more moisture per degree Celsius that it warms. 
• Hurricane Harvey was made more than three times wetter by the climate crisis.

What Is ‘Flooding’?

Flooding occurs when water submerges land that is typically dry. That water can come from a variety of sources, such as rainfall, a storm surge or high tide, a river overflowing its banks, a dam bursting or snow or ice melting. Some types of flooding can be beneficial for people and ecosystems. When rivers gradually overflow their banks during the wet season and spread into their natural flood plains, this can create habitat for animals and make the soil more fertile for agriculture. The three most ancient human civilizations — ancient Egypt, ancient Mesopotamia, and the Indus River Civilization — all grew up around floodplains. However, flooding can also be extremely devastating. What many calculate was the deadliest natural disaster on record was a flood: the 1931 Yellow River or Huang He flood in China that killed 850,000 to 4 million people and displaced 80 million. 

Floods are the most common type of extreme weather event in the U.S.: Between 2000 and 2021, a flood occurred in the country every eight out of 10 days, on average. They are also the second most widely dispersed weather disaster worldwide after wildfires. In the U.S., they are the most expensive natural disaster and the second deadliest after heat waves. Floods can be dangerous because people often overestimate their ability to drive or walk through floodwaters. In the U.S., in fact, driving or walking in floodwater are the No. 1 and 2 highest causes of flood drownings. As flood risk increases due to the climate crisis, it is more important than ever for individuals and societies to be aware of the dangers of flooding and take steps to protect themselves. 

What Are the Main Types of Flooding?

There are several types of flooding. Which one you have to look out for will often depend on where you live and what time of year it is.

River Flooding

River, also known as riverine or fluvial flooding, occurs when the amount of water entering a river or stream exceeds its existing channel, causing it to overflow its banks. These types of floods are the most common, and are part of the natural water cycle of many river ecosystems. Rivers can overflow because of heavy rainfall from a tropical storm or a series of thunderstorms, the combination of rainfall and snowmelt in the spring, when melting ice forms a natural dam or if a human-made dam or levee breaks. The devastating Yellow River flood is an example of this type of flooding. The disaster was caused by a trifecta of snow melt, heavy rains and seven tropical-style storms in a row and flooded an area of dry land bigger than England. Another historic riverine flood was the Mississippi flood of 1927, when heavy spring rain caused the mighty river to overflow its banks from Illinois to Louisiana. At its worst point, the flood created a temporary sea 75 miles wide and forced a million people — or around one percent of the country’s population at the time — to flee their homes.

Pluvial Flooding

Pluvial flooding occurs when excess rainfall generates flood conditions on its own without adding to a body of water that then overflows. These types of floods can happen anywhere, even if there is no river or lake nearby. 

Surface Water Flooding

Surface water flooding is a type of pluvial flooding that occurs when rainfall overwhelms a drainage or sewer system. When this happens in a city, it is called urban flooding. The amount of concrete in cities means that rainfall is not absorbed and so runs off into storm drains that can be more quickly overwhelmed. This type of flooding tends to be gradual and shallow — around three feet deep — so it is less dangerous than other types of floods but can still cause damage.

Flash Flooding

A flash flood occurs when a lot of water moves on to dry land very quickly. Typically, this happens because of heavy rainfall in an area during a less than six-hour time span. Flash floods can be either fluvial or pluvial, with the mass of water overflowing an existing stream or river or filling a dry street, canyon or creek bed. They can also be caused by a dam failure or the sudden removal of a natural river blockage like an ice jam. Debris are often caught up in the fast-moving torrent, creating another hazard. Because they move so quickly and are hard to predict, flash floods are the most dangerous kind of flooding. One example was the Johnstown Flood of 1889, when heavy rain burst a dam near the town of Johnstown, Pennsylvania, driving a wall of water towards the town that was as high as 40 feet. The disaster killed more than 2,000 people, with some bodies swept as far as Cincinnati. 

Coastal Flooding

Coastal flooding occurs when ocean water overrides the shoreline beyond the usual high-tide mark. This can happen because of a tide 1.6 to 2.1 feet or more above the average high tide — a type of flooding referred to as nuisance or sunny day flooding — or because of a storm surge or tsunami. A storm surge is caused by a tropical storm or hurricane, while a tsunami is a large wave displaced by an underwater earthquake or volcanic eruption. Storm surges are one of the most deadly and damaging impacts of a hurricane. A storm surge was one of the main factors behind Hurricane Katrina’s more than 1,800 death toll and why Hurricane Sandy had such a large impact on New York and New Jersey. The deadliest tsunami in history was the Indian Ocean tsunami of 2004, which was triggered by a 9.1 magnitude earthquake off the coast of Sumatra in Indonesia. Near the epicenter, in northern Sumatra, the flooding reached three miles from the coast. Around 25 million people died in the disaster.

What Are the Impacts of Flooding?

A flood can devastate a community within hours, but its effects and dangers can linger long after the initial deluge. 

On Human Health

Floods can be deadly during the first rush of water as people are swept away. Adults can be knocked off their feet by just six inches of fast-moving water, while 12 inches can sweep away most cars and two feet can carry off a truck or SUV. People may also be killed or injured by debris or electric shocks when flooding damages power infrastructure. In the longer term, floods leave behind damaged buildings and roadways, and people can injure themselves on hazards like broken glass or exposed nails when they return to clean up. Flood waters can also spread chemical contaminants like pesticides or fuel or raw sewage that can harm human health and cause outbreaks of water-borne diseases like cholera or typhoid, especially if communities have to wait a long time for access to clean water. Flooded debris can also be an ideal growing environment for unhealthy molds. Flooding killed an estimated 102 people in the U.S. in 2022 and 7,398 people worldwide. Around two-thirds of those deaths were in Asia, at 4,755, with the next highest amount in Africa at 1,768.

On Human Infrastructure

Floodwaters can carry buildings and bridges off their foundations and wash away roads or other transportation infrastructure. This damage forces many people to flee their homes. The historic monsoon flooding in Pakistan in 2022, which drowned a third of the country underwater, displaced almost 8 million people. Flooding can also knock out power or internet and phone service, making it harder for people to stay safe or resume work in the aftermath. The excess water can also drown crops, harming agriculture. In Pakistan, again, the 2022 floods rendered almost half of its cotton crop useless. Floods can also disrupt other industries like fishing, healthcare and tourism. In 2022, U.S. businesses lost a combined total of 3.1 million days of operation because of flooding. This means that floods can have major economic impacts on affected countries, some of which may take years to recover. The global economy lost a total of $82 billion in 2021 because of flooding.

On Wildlife and Natural Ecosystems

Smaller, seasonal floods are a natural and essential part of many river-based ecosystems. During the rainy season, these floods create habitat in the form of seasonal wetlands and are an important part of the lifecycle of certain species like willow and sturgeon. They also benefit human communities by fertilizing the soil by spreading river sediment, refilling groundwater deposits and preventing larger, more destructive floods. However, flooding can disrupt wildlife and ecosystems as well. Flooding brought on by record rainfall in Vermont in summer 2023 washed beavers out of their dams, some of whom were then struck by cars. Floods can erode river banks and dump more sediment into the water, where the nutrients it contains can encourage harmful algae blooms. Through a process called sedimentation, the debris can harm riparian ecosystems by clogging streams and suffocating wildlife. 

What Causes Flooding?

Floods have both immediate causes — such as storms or dam bursts — and long-term influences that can make a flood’s impact worse. 

Weather

In the short term, floods are often caused by heavy rainfall, either in a short burst of intense rain or over an extended period of time. When the ground or existing bodies of water can no longer absorb the excess water, it has nowhere to go but dry land. There are certain weather events that are especially associated with flooding. Tropical storms — which typically hit the U.S. in the summer and autumn — can cause flooding both by triggering a storm surge and by dumping rain as they make landfall. Atmospheric rivers — narrow bands of moisture that travel through the atmosphere — are another type of storm associated with flooding. These are more common in California, the Pacific Northwest and Alaska during winter. A series of intense atmospheric rivers drenched California in late 2022 and early 2023.

Seasonal Changes

Another major cause of flooding is related to the seasonal forming and melting of ice and snow. As snow melts, it saturates the soil much like rainfall. If it overwhelms the carrying capacity of the soil, as well as nearby waterways, flooding occurs. Ice jams take place in winter and spring when cold temperatures freeze a river. The jam can cause flooding upstream as the flow of water backs up until it bursts or melts, suddenly causing flash flooding downstream. Snowmelt and ice jams can fuel each other, as runoff from melting snow can put more pressure on river ice, creating a jam.

Dam Failures

Dams and levees keep massive amounts of water at bay. By one estimate, the world’s large dams store a combined 1,679 to 1,991 cubic feet of water. If they all burst, they would release enough water to cover around 80 percent of Canada under three feet. While this is unlikely to happen, a single dam break can devastate a community. Sometimes, there is a weather-related trigger to a dam or levee failure. This is what happened during Hurricane Katrina in 2005, when water overtopped or caused levees and stormwalls to fail in more than 50 areas, flooding more than 80 percent of New Orleans. In another, more recent example, heavy rainfall in Libya in September 2023 caused two dams to fail, unleashing a wall of water that killed more than 11,000 people. At other times, a dam break can occur purely because of a structural or engineering error. This is a growing concern as the tens of thousands of large dams built in the 20th century continue to age. The UN estimates that by 2050, the majority of the world’s population will live below one of these dams.

Land Use Change

As industrial capitalism has transformed the planet, it has made flooding more likely and dangerous in many ways. One major way is through land-use change. Deforestation and the clearing of natural landscapes gets rid of vegetation that would otherwise help slow and channel the rush of precipitation. Urbanization and other forms of development then replace that vegetation, and the soil beneath it, with concrete roads, sidewalks or parking lots that are less porous to rainwater. Urban drainage systems also channel this water more rapidly into streams, increasing flooding. At the same time, more and more human developments are expanding into natural floodplains and wetlands. Since 1985, overall human settlements have increased by 85 percent while developments in the locations most at-risk for flooding have increased by 122 percent.

The Climate Crisis

These changes are occurring at the same time as the burning of fossil fuels warming the atmosphere, increasing the risk of flooding in many ways, from more intense precipitation, to sea level rise, to more rapid snow and ice melt. While the overall number of floods is not necessarily increasing, it’s likely that the most extreme river floods are becoming more common, even as more moderate river floods decrease. In the U.S., the Climate Science Special Report found that flooding was becoming more common along the Mississippi River, in the Midwest, and in the Northeast. In its most recent Sixth Assessment Synthesis Report, the Intergovernmental Panel on Climate Change (IPCC) said that as the climate continues to warm in the near future, the panel had high confidence that flooding would increase in coastal and low-lying areas and that there would be more floods related to mountain glacier melt. In addition, it had medium confidence that higher precipitation would lead to more local floods. Flood risk in the U.S. is expected to increase by more than 25% by 2050.

Environmental Injustice

When floods do happen, they don’t impact everyone equally. Communities that are poorer or otherwise marginalized tend to suffer more. In U.S. cities, low-income communities or communities of color tend to be hardest hit during flooding, since they tend to live in lower-lying areas with less vegetation to act as a buffer. Poorer urban residents are more likely to rely on public transportation and so cannot evacuate as easily as wealthier people with private vehicles. Hurricane Katrina is one example: the storm did much more damage to Black neighborhoods in New Orleans than white ones, partly due to historical patterns of discrimination that relegated Black residents to lower, swampier areas farther from public transit access. The climate crisis is set to only exacerbate these differences — Black Americans will be disproportionately impacted by greater flood risk by 2050, one study found. Globally, many low-lying or island nations that released relatively few greenhouse gases into the atmosphere are nevertheless extremely vulnerable to climate-fueled flooding. In Bangladesh, for example, a low-elevation country located on a river delta, almost 60 percent of the population is at risk from flooding, more than any other country in the world. Climate change is expected to increase the flow of its rivers by 36 percent in a high-emissions future and 16 percent in a low-emissions future by 2070 to 2099. Yet currently it only contributes 0.4 percent to global greenhouse gas emissions. 

How Exactly Does the Climate Crisis Increase Flood Risk?

The relationship between increased global temperatures and increased flooding isn’t as straightforward as the relationship between global heating and more frequent and extreme heat waves. Instead, the warmer atmosphere interacts with the climate system in many different ways to exacerbate the risk of different types of floods.

Extreme Precipitation

The atmosphere can hold about 7 percent more moisture per degree Celsius that it warms. Since global temperatures have risen on average 1.1 degrees Celsius above the 1850 to 1900 average, the air would in theory be 7.7 percent wetter now. This means that, when any weather system forms, whether it be a major tropical cyclone or small local thunderstorm, it has more moisture available to it. These extreme rainfall events increase the risk of flash floods that contain more water and form in even less time. During September of 2023, for example, record rainfall brought intense flooding to New York City. A study found that the climate crisis made the type of storm behind this event 10 to 20 percent wetter than it would have been in the 20th century.

Extreme Storms

The climate crisis is also increasing the chance that tropical storms intensify into more powerful Category 4 and 5 hurricanes. Stronger storms tend to hold more moisture. A 2018 study found that global heating made Hurricanes Katrina, Irma and Maria five to 10 percent wetter than they would have been without it, and that future warming would have made them 15 to 35 percent wetter still. Another example is 2017’s Hurricane Harvey, which dumped as much as 40 inches of rain on Houston in three days, making it the wettest storm in the U.S. in almost 70 years. That rainfall was made at least three times more likely by the climate crisis, and, if the world warms two degrees Celsius by 2100, similar deluges could become three times more likely again.

Sea Level Rise

The ocean is around seven to eight inches higher than it was at the start of the 20th century, as higher temperatures have melted glaciers and ice sheets. Sea level along coastal areas is expected to rise at least another eight inches by 2050 and potentially 20 to 40 inches by 2100. Certain areas, such as the U.S. Gulf Coast or New York City, are sinking at the same time through a process known as subsidence, increasing the impact of rising tides. Sea level rise can increase coastal flooding in two major ways. First, it can lead to more extreme storm surges, because the inrushing water starts from a higher point. But even without a storm, it can lead to higher high tides, increasing the frequency of nuisance or sunny day flooding. In the U.S., the frequency of high-tide flooding has more than doubled since 2000, and some places could see these floods 25 to 75 days a year by 2050. Sea level rise is especially a problem for low-lying islands like Tuvalu or the Maldives. By 2050, some of these islands could become uninhabitable as higher tides increase nuisance flooding and seep into the groundwater, contaminating it via a process called saltwater intrusion. 

Glacial and Snow Melt

The climate crisis can also increase flood risk by accelerating the melting of mountain snowpack and glaciers. Earlier snowmelt is increasing runoff and flood risk in the U.S. West, for example. Another hazard is the melting of alpine glaciers. The World Glacier Monitoring Service found that their reference sample had lost a combined almost 82 feet of water between 1970 and 2020. As they melt, these glaciers form lakes, which have increased their number, volume and area by around 50 percent each since 1990. This is a problem, because these lakes are susceptible to something called a glacial lake outburst flood, which can be triggered by factors like dam breaches, earthquakes, heavy rains or avalanches. A study published in early 2023 warned that 15 million people were at risk from these types of floods, especially in the Himalayas and the Andes. Eight months later, a glacial lake breached a dam in the Indian Himalayas after heavy rains and a nearby earthquake, killing at least 41 and displacing thousands. 

What Can Be Done About Flooding?

While flooding may seem overwhelming, especially in the context of the climate crisis, there are things that people can do to prepare for potential floods, protect themselves and their communities and reduce risk.

On an Individual Level

The U.S. government has clear guidelines for how to stay safe during a flood. First, you can check your risk through the FEMA Flood Map Service Center. Then you should make an evacuation plan for everyone in your household, including pets, and prepare supplies like non-perishable food and a gallon of water per person or animal per day for three days. If your area is under a flood warning, you should find a safe place to shelter and evacuate if told to do so. It’s important to avoid flood water either on foot or in a car. Even if you don’t get swept away, it could contain dangerous debris, downed power lines, chemical contaminants or disease. Don’t cross a bridge over raging torrents, as the bridge could be swept away without warning. If you end up stranded in a vehicle, stay put. If water starts to enter the car, get on to the roof. If you are in a building that is flooding, move to a higher level but don’t enter a closed attic. Once the flood is over, it’s also important to be careful during the cleanup process. Don’t return home before advised, wear personal protective gear and don’t enter moldy buildings if you have a respiratory or immune condition. 

On a Community Level

As flood risks increase around the world, towns and cities are taking steps to adapt. According to the C40 group of cities committed to climate action, the best ways urban areas can reduce risk are the following:

1. Stop developing on wetlands or floodplains;

2. Restore natural floodplains and give space for vegetation around riverbeds;

3. Replace impermeable paving materials with more permeable ones;

4. Build small reservoirs to collect rain;

5. Build larger rain-storage tanks underground;

6. Find ways to stop trash from clogging sewers;

7. Fit buildings with rainwater tanks and green walls and roofs to help siphon water;

8. Make buildings safer in especially at risk areas by raising foundations or moving essential functions to higher floors; and

9. Work with regional partners to reduce risks outside the city, such as coastal storm surges.

A major way to reduce large scale risks is by restoring ecosystems that naturally control floods, like wetlands or mangroves. The Billion Oyster Project in New York City is working to restore the Big Apple’s once-famous but now depleted oyster reefs, in part to help protect the city from flooding and sea level rise. It’s also important for cities to study where exactly they are vulnerable and to create early warning systems for residents. 

On a Global Level

The IPCC’s Sixth Assessment Report made clear that every degree of warming increases future climate risks, including flooding, and every degree avoided helps to make them less severe. Limiting warming to 1.5 degrees above pre-industrial levels instead of two degrees could prevent an extra four inches of sea level rise by 2100, for example. However, in order to have any hope of reaching the 1.5 goal, world leaders must act to phase out fossil fuels as soon as possible to halve emissions by 2030 and reach net-zero by 2050. Since deforestation and land-use change contribute both to flood risk and to the climate crisis, preventing the continued destruction development of natural ecosystems can prevent dangerous flooding on both counts.

Takeaway

2023 made the risk of climate-fueled flooding extremely apparent. Over just 12 days in September, 10 countries and territories endured extreme inundations. One of these events was the deadly dam collapse in Libya that swamped as much as a fourth of the coastal city of Derna. Scientists calculated that the rainfall that triggered the event was made as much as 50 times more likely by the climate crisis. Derna’s muddy wreckage offers one picture of the future if world leaders decide to keep pumping fossil fuels into the air and fail to repair or adapt infrastructure to account for the increased risk.

But there are glimpses of other possible futures. In Seoul, urban planners decided to take down a 10-lane highway and allow the Cheonggyecheon Stream beneath it to flow in the open again. This provides protection against a one-in-200-year flooding event, but it had other benefits as well, creating a new park drawing more than 50,000 people a day, increasing biodiversity in the area by 639 percent and curbing small particulate matter air pollution by 35 percent. If we work together with natural ecosystems and cycles instead of fighting them, we can create a safer and more vibrant water system for human and non-human life to enjoy.